Biology (A-Z)

American Concise Encyclopedia

Abiotic factors Physical and chemical conditions that affect the ability of a
given species to live and reproduce in a particular place. Included in the
abiotic factors are temperature, light, water, oxygen, pH (acid-base balance)
of soil, type of substrate, and the availability of minerals. Certain kinds of
plants and animals will flourish in a natural community if the conditions are
present that permit their survival. Species interact to influence the survival
of one another. One important principle of ecology is that no living organism
is independent of other organisms or of the physical environment, if they share
the same community.

Abscisic acid A growth-inhibiting plant hormone produced in the bud that helps
to prepare the plant for winter by directing the leaf primoridia to form scales
and by inhibiting cell division in the vascular cambium.

Abscission layer A layer along which a leaf or fruit naturally separates from
the stem.

Absorption The passage of dissolved materials through the cell membrane, into
and out of the cell.

Acetylcholine A neurotransmitter secreted by motor neurons. When an impulse
arrives at the knobs in the axon branches of a motor neuron, a neurotransmitter
is released from the synaptic vesicles. Neurotransmitters are chemical
substances that diffuse across the synaptic gap and initiate a second impulse
when chemoreceptors on the dendrites are stimulated. Cholinergic cells secrete
the neurotransmitter, acetylcholine. Adrenergic cells secrete the
neurotransmitter, epinephrine.

Acid rain This is rainfall that can be as acid as vinegar. It is formed when
gases of nitrogen oxide and sulfur dioxide are given off into the atmosphere as
by-products of fuel combustion by automobiles, homes, factories, and power
plants. As the fumes are carried into the air by wind currents, they combine
with water vapor molecules and are transformed into microscopic drops of nitric
acid and sulfuric acid. When it rains or snows, the precipitation returns the
acids to earth, sometimes thousands of miles from their origin. Lakes and
streams have become so acidified that the populations of trout, salmon, and
other fish are being destroyed. There is also concern about the possible
effects of acid rain on soil minerals and nitrogen-fixing bacteria.

Acquired characteristics Variations that cannot be passed from parent to
offspring. These variations are not in the individual's gene pool. Lamarck
(1809) advanced the hypothesis that acquired variations are transmitted to the
offspring. According to him, structures that are in use are better developed
and preserved, whereas unused organs eventually disappear. Characteristics thus
acquired through use could be inherited, he thought, by the next generation.
Facts do not seem to support his conclusions.

Acromegaly A condition of overgrowth of the bones in the hands, feet, and jaw
brought on by an oversecretion of somatotropin by the pituitary gland.

Acrosome The forward tip of an animal sperm that contains the enzymes that
help the sperm penetrate the egg. See Sperm.

ACTH (adrenocolticotropic hormone) Stimulates the cortex of the adrenal gland
to produce a number of hormones, including cortisone. In severe cases of
rheumatoid arthritis, it has brought about dramatic relief from disability and
pain.

Actin One of the major proteins of muscle. The other muscle protein is myosin.
Actin makes up the Z band of a sarcomere.

Action potential The graph of an action potential demonstrates the sequence of
events involved in the transmission of a nerve impulse. A threshold value
(Ecrit.) must be attained before depolarization occurs. Once the threshold
value is reached, an impulse is generated throughout the nerve fiber. A nerve
cell will transmit the impulse either totally or not at all. There are no
graded responses. This maximal firing condition is referred to as an "all or
none response."

Activation energy The smallest amount of energy that must be available from an
outside source that enables a chemical reaction to start.

Active site The specific place on the surface of an enzyme where a substrate
attaches by weak chemical bonds and where catalysis occurs.

Active transport The movement of a substance across the cell membrane against
a concentration gradient, that is, from a region of low concentration to a
region of high concentration. Active transport involves the expenditure of
energy.

Adaptation A trait that aids the survival of an individual or a species in a
given environment. An adaptation may be a structural characteristic such as the
hump of a camel, a behavioral characteristic such as the mating call of a bull
frog, or a physiological characteristic controlling some inner workings of
tissue cells. Adaptations permit the survival of species in environments that
sometimes seem forbidding. For example, some bacteria are able to live in hot
springs that have temperatures up to 80 degrees C (175 degrees F). They have
adaptations that permit the carrying out of metabolic functions at extremely
high temperatures.

Adaptive radiation The evolutionary division of a single species into several
species adapted to divergent forms of life. For example: Darwin's finches.

Adenine A nitrogenous base contained in DNA.

ADH (antidiuretic hormone) Also called vasopressin. A hormone secreted by the
hypothalamus that prevents the excretion of urine by stimulating the kidney
nephrons to reabsorb water.

Adrenals Compound glands located at the top of the kidneys. They have an outer
zone known as cortex and an inner one known as medulla. The cortex secretes a
complex of substances appearing to be closely related chemically. Their
combined action controls sodium, potassium, and chloride metabolism, which
affects water balance, causes the change of glycogen to glucose, and influences
sex. Deficiency results in a syndrome known as Addison's disease. The patient
suffering from it experiences a general decline in muscular strength and sexual
activity, a lowering of blood pressure, disturbance of digestion, and a
bronzing of the skin. The best-known cortical secretion is cortisone, which has
been used in treating arthritis and allergies. The medulla secretes a hormone
called epinephrine (adrenaline), which is produced in accelerated quantities
when one is stimulated by anger or fear. Apparently it is the only endocrine
gland directly responding to nervous stimuli. The influence of epinephrine on
the conversion of glycogen to glucose, stimulation of heart muscles, enrichment
of blood supply to muscles, and acceleration of blood coagulation are without
doubt useful to provide more and quick energy or to stop bleeding in times of
stress or injury.

Afferent or sensory neurons Nerve cells that transport impulses from sense
receptors to the central nervous system.

AIDS AIDS is a baffling disease that has recently attracted considerable
attention. The name stands for "acquired immune deficiency syndrome." It refers
to a severe breakdown in the body's immune system. A person with AIDS is
vulnerable to a variety of infections and tumors that would normally be
attacked by the body's white blood cells. As a result, the person becomes weak
and dies. The cause of AIDS is a virus. It was identified in 1984 by Dr. Luc
Montagnier of the Pasteur Institute in Paris, who called it LAV- I, and also
named by Dr. Robert C. Gallo of the National Cancer Institute, HTLV-III. It is
now known as HIV, or human immunodeficiency virus. It is believed that the AIDS
virus kills a specialized type of white blood cell, called the helper T4 cell,
which normally protects the body from infection by destroying foreign
substances that enter it. The disease is believed to be passed through blood
and semen, but not passed by casual contact, such as sneezing or using the same
utensils. In the United States, certain groups of people have the greatest risk
of getting the disease: male homosexuals, intravenous drug users who share
contaminated needles, and babies born to infected mothers. Before 1987,
recipients of contaminated blood transfusions contracted AIDS. In Central
Africa where AIDS is prevalent the disease is also spread by heterosexual
contact. Heterosexual transmission is also becoming more common in the United
States. Considerable research is being conducted to develop a defense against
the disease. In 1986, a drug, azidothymidine (AZT), was found to be effective
against a form of pneumonia common among AIDS patients. It appears to interfere
with reproduction of the virus inside the body cells. In the near future, it is
hoped that research will produce a vaccine against the virus. See also Health
and Medicine.

Air sac See Alveolus (air sac).

Albinism An inherited trait requiring two recessive genes resulting in
individuals without pigment.

Algae Simple photosynthetic organisms. Algae may be composed of a single cell,
a filament of cells, or a flat plate of cells. Cell walls of algae are made of
cellulose. Algae have some characteristics of animal cells. With the exception
of the red algae, most algal cells have centrioles. During cell division, most
algal cells form a cell furrow like those in animal cells.

Alimentary canal See Digestive system.

Allantois An embryonic membranous sac that stores the nitrogenous wastes of
reptiles and birds up to the time of hatching. In mammals, it forms part of the
placenta.

Alleles Two or more genes that have the same positions on homologous
chromosomes. Alleles are separated from each other during meiosis and come
together again at fertilization when homologous alleles are paired, one from
the sperm cell and one from the egg cell. Two or more alleles determine a
trait.

Allergy The sensitivity of some people to substances that are quite harmless
to most other people. The cells of these sensitive people produce antibodies to
ward off whatever substance affects them. The antibodies become attached to the
tissue cells, rendering the person sensitized. Whenever that particular
substance enters the body again, it reacts with the attached antibodies and
damages the cells. These damaged cells prompt certain symptoms such as itching,
sneezing, tearing eyes, red welts, large hives, fever, and a general feeling of
not being well.

Allopatric speciation Speciation brought about when an ancestral population
becomes separated by one or more geographical barriers that prevent dispersal.
An example is Death Valley where each isolated spring has its own water
temperature and salinity and each is a habitat for a different species of
pupfish.

Alternation of generations The sea lettuce Ulva is a green algae that lives in
saltwater. The life cycle of Ulva is described as alternation of generations
because one generation of Ulva is produced sexually by gametes while the next
generation is produced asexually by zoospores. The gametophyte generation is a
haploid thallus from which small, flagellated gametes are released into the
water. They pair off and fuse. Each fused pair of gametes forms a zygote that,
after a short time, becomes a diploid thallus of a new generation of Ulva. The
diploid thallus is the sporophyte generation. It produces haploid zoospores
that develop and grow into a haploid thallus, which is now the gametophyte.
Many other simple organisms also have an alternation of gametophyte and
sporophyte generations.

Alveolus (air sac) A minute air sac in the lungs through which oxygen enters
the bloodstream and through which carbon dioxide and water are excreted from
the bloodstream.

Alzheimer's disease Progressive mental deterioration more often in older
people. Recent research has revealed that nerve cells in parts of the brain are
damaged by an accumulation of amyloid proteins.

Amino acids All proteins are built from small molecular units known as amino
acids. The amino acid molecules link together in a particular way through
peptide bonds. A dipeptide consists of two amino acids. A polypeptide contains
many amino acid molecules. A protein is composed of one or more polypeptide
chains.

Amniocentesis The procedure of amniocentesis in which a small amount of
amniotic fluid is removed from a pregnant woman and is used to study cells of
the embryo. In this way certain chromosomal defects can be determined before
birth.

Amnion See Embryonic membranes.

Amniotic fluid The fluid that surrounds the developing embryo in mammals,
birds, and reptiles.

Amoeba The members of the phylum Sarcodina are described as being amoeboid.
Amoeba proteus is the species most often studied. Species included in the
Sarcodina move by means of pseudopods, flowing extensions of the flexible and
amorphous body. The pseudopods also serve in food-catching. Most of the
sarcodines live in fresh water. A contractile vacuole, an organelle designed to
expel excess water from the protist cell body, plays an important role in
maintaining water balance. Food is temporarily stored in a food vacuole where
it is digested by the action of enzymes.

Amphibian An amphibian must spend part of its life cycle in the water where
its eggs are laid and fertilized. The eggs develop into a larval stage, or
tadpole, that has fish-like characteristics. In tadpoles breathing is by means
of gills, blood is pumped by a two-chambered heart, and swimming is by means of
tail and body movements made possible by muscles in the body wall. Most
amphibians undergo metamorphosis into a lung-breathing adult with a 3-chambered
heart.

Anaerobic Referring to a type of cellular respiration that occurs without
oxygen such as fermentation. Referring also to certain species of bacteria,
such as the tetanus bacilli, that live in an atmosphere devoid of oxygen.

Anaphase A stage in mitosis in which the chromosomes are pulled apart. See
Mitosis.

Angiosperms Flowering plants that form seeds inside ovaries. Examples are
apple, rose, and dandelion.

Animal kingdom All animals belong to the kingdom Animalia, a grouping of 29
phyla. Twenty-eight of these phyla include animals called invertebrates because
they do not have a vertebral column, or true backbone, the 29th phylum includes
the vertebrates, animals with a vertebral column.

Annelid The annelids are segmented worms that live in soil, fresh water, or
the sea. Most of the annelids are free-living, although some of the marine
forms burrow in tubes and some species (class Myzostoma) are parasites on
echinoderms. The body of an annelid is divided into a series of similar
segments and is said to be metamerically segmented. Most annelids have a closed
circulatory system where the blood is contained in vessels. Enlarged muscular
blood vessels function as hearts and pump the blood through the system of
vessels. Annelids may be dioecious (have separate sexes) or hermaphroditic.
Most annelid species go through a ciliated larval stage known as the
trochophore larva. This is a larva of evolutionary importance because the same
type appears in several phyla.

Annual A plant in which the life cycle is completed in a single year or
growing season, such as corn, tomato, and beans.

Anther Pollen producing organ in the flower.

Antibiotic An organic compound that is made and secreted by a living organism
(commonly a mold) and is able to prevent the growth and reproduction of another
species. Example: Penicillin is synthesized and secreted by the mold
Penicillium notatum.

Antibodies The body produces substances known as antibodies to fight
disease-producing agents. Antibody production is a relatively slow process.
First of all, the body cells must recognize the invading agent as "foreign" --
an antigen -- and then produce an antibody that is exactly right to immobilize
the protein invader. Finally, the blood cells must go into full scale
production of this specific antibody. An antibody is specific against "a
particle" type of germ. For example: diphtheria antibodies will not be
effective against scarlet fever antigen.

Anticoagulant A substance that prevents blood from clotting. Heparin is an
anticoagulant.

Anticodon A "triplet" of nucleotides in transfer RNA that is able to pair with
a complementary triplet (a codon) in messenger RNA, thus arranging transfer RNA
to the proper site on the messenger RNA.

Antigen A foreign protein that stimulates the formation of antibodies by the
immune system.

Antitoxin An antibody that works against a specific toxin.

Anus The body opening at the posterior end of the alimentary canal.

Aorta A large artery that carries blood away from the heart. See Heart.

Apical dominance A process in which the terminal bud (top of the stem) in
plants inhibits the growth of buds at the sides of the stem.

Appendix A small protruding pouch without function positioned where the small
intestine joins the large intestine. See Digestive system.

Arachnids The class to which spiders belong. The body of the spider is divided
into the cephalothorax and the abdomen. The cephalothorax has six pairs of
jointed appendages. Spinnerets at the end of the abdomen are projections
through which the spider spins webs. Most spiders breathe by book lungs. Other
members of the class Arachnida are mites, ticks, scorpions, horseshoe crabs,
and harvestmen.

Archegonium The multicellular, water-retaining structure, in which a single
egg is produced and fertilized in the liverworts, mosses, and hornworts.

Archenteron The cavity appearing in the early embryo during the gastrula stage
that ultimately becomes the gut cavity.

Arteriole A small artery.

Artery A blood vessel that transports blood from the heart to the organs and
tissues of the body.

Arthropods The arthropod group is by far the largest group of animals with
respect to the number of species it contains. They share the common
characteristics of having segmented bodies, jointed appendages, and
exoskeletons.

Asexual reproduction Involves only one parent. The parent may divide and
become two new cells, thus obliterating the parent generation. Or the new
individual may arise from a part of the parent cell; in such a case, the parent
remains. Types of asexual reproduction are binary fission, sporulation budding,
regeneration, and parthenogenesis.

Assimilation Involves the changing of certain nutrients into the protoplasm of
cells.

Atoms Elements are made of invisible building blocks called atoms. Each atom
has a central nucleus surrounded by a definite number of moving negatively
charged electrons.

ATP (Adenosine triphosphate) The compound that stores energy produced during
cellular respiration and releases this energy when needed for the cell's work.
Energy produced during cellular respiration is passed on to adenosine
diphosphate (ADP), which then becomes upgraded to ATP.

Atrium The chamber of the heart (also known as the auricle) that receives
blood and passes it to the ventricle for pumping.

Autonomic nervous system See Nervous system, autonomic.

Autosome A chromosome that does not determine sex. Human cells have 22 pairs
of autosomes.

Autotrophs Organisms that are able to change inorganic materials into organic
compounds. Among these are the photosynthetic bacteria and the green plants
that use light energy to produce food. The chemosynthetic bacteria are capable
of oxidizing the inorganic compounds of ammonia, nitrites, sulfur, or hydrogen
gas into high-energy, organic compounds without the need of light energy.

Auxins Plant hormones called auxins are produced by actively growing plant
tissues like the growing tips of roots and branches, developing leaves, or
flowers and fruits. They promote cell enlargement, which is one phase of growth
(the other is cell division).

Axon Extension of the neuron (nerve cell) that can carry impulses; is often
the longest and least branched process of the cyton (cell body) and usually
carries impulses away from the cell body of the neuron.

B cell A white blood cell known as a lymphocyte produced in the bone marrow
that works with T cell lymphocytes to destroy germs.

Back cross (test cross) Plant and animal breeders mate an individual with a
particular dominant trait to an individual that is recessive to determine if
the dominant trait is pure or hybrid. As a result of this mating, if organisms
appear that have the recessive trait, then the breeder knows that the parent
individual is hybrid (heterozygous) for the dominant trait.

Bacteria Bacteria are the smallest living organisms. They range in length from
0.2-7 micrometers; in diameter from 0.2-2 micrometers. As you recall, the unit
used to measure bacteria is the micrometer and is equivalent to 1/1000 of a
millimeter. The smallest cells known are the mycoplasmas, which have only
one-half the DNA of other bacterial cells. The mycoplasmas are bacteria that
live only as parasites on or in the bodies of plants and animals. Despite their
small size bacteria are true cells: they provide their own genetic material
(DNA and RNA) and the necessary cytoplasm for their own reproduction; they have
multienzyme systems to control biochemical activities necessary for the life of
the cell; and they build their own ATP molecules and use the stored energy to
synthesize other organic compounds. Bacterial shapes include spheres (cocci),
rods (bacilli), and spirals (spirilla). See also Eubacteria -- true bacteria.

Bacteriophage A virus that infects a bacterial cell.

Biennial A plant for which the life cycle is two years. Vegetative growth
occurs during the first year; flowering, the second. Beets and carrots are
examples.

Bilateral symmetry Two sided, as demonstrated by animals that have an anterior
(head end) and a posterior (tail end), a right and left side that are similar.
For example: a human being shows bilateral symmetry, as does a horse. If sliced
down the middle, each has two identical halves.

Binary fission A form of asexual reproduction in which a parent organism
divides into two identical daughter cells. The nucleus goes through mitosis.
The cytoplasm divides equally.

Binomial nomenclature The scientific naming of species by a double name. The
first name in the binomial is the genus name. This system was developed by
Carolus Linnaeus (1707-1788) and has become a worldwide standard for
classifying and naming organisms.

The Genus Quercus
Scientific Name Common Name
Quercus alba white oak
Quercus coccinea scarlet oak
Quercus montana chestnut oak
Quercus rubra red oak
Quercus suber cork oak
Quercus virginiana live oak

Biogenesis The doctrine that living things come only from other living things
of like kind.

Biogeochemical cycles Certain compounds cycle through the abiotic portion and
the biotic communities of ecosystems. These compounds contain elements that are
necessary to the biochemical processes that are carried out in living cells.
Among these elements are carbon, hydrogen, oxygen, and nitrogen. In elemental
form, they are useless to cells and must be combined in chemical compounds. Let
us trace the pathways of some of the vital compounds from the earth to living
organisms to the atmosphere and back to earth. This cycle of events is best
described by the term biogeochemical cycles. See figure.

Biology The study of living things; an extensive science including botany,
zoology, bacteriology, genetics, physiology, anatomy, and many others.

Biome A climax community in a broad geographical area having one type of
climate. Examples of biomes are the Taiga, coniferous forests of Canada; the
Desert, regions where the annual rainfall is less than 6.5 centimeters; the
Grasslands, annual rainfall is low and irregular; the Tropical Rain Forest,
characterized by high temperatures and constant rainfall; and the Marine biome,
the sea.

Biomes, world A climax community in a broad geographical area having one type
of climate is known as a biome. The earth is divided into several biomes.

The Tundra. Vast stretches of treeless plains surrounding the Arctic Ocean
where cold is the limiting factor. Plant life consists of lichens, mosses,
grasses, and sedges. Animal life includes the musk ox, caribou, polar bears,
wolves, foxes, and some marine mammals.

The Taiga. The coniferous forests of Canada and Russia where spruce and fir
trees predominate. The kinds of mammals that live in this region are the black
bear, the wolf, lynx, and squirrel.

The Deciduous Forest. These are forests of temperate regions, where
broad-leaved trees that lose their leaves in the winter predominate. The types
of animals that live in these forests are deer, fox, squirrel, skunk,
woodchuck, and raccoon.

The Desert. Deserts form in regions where the annual rainfall is less than 6.5
centimeters and where evaporation of water is high. Creosote, sagebrush, and
cacti are plants adapted for the desert. The animals include lizards, insects,
kangaroo rats, and arachnids.

The Grasslands. These regions have low annual rainfall. Grasslands are located
in regions that are sheltered from moisture-laden rainfall. The animals that
predominate in temperate grasslands, called steppes or prairies, are bobcats,
badgers, hawks, kit foxes, owls, and coyotes. Typical animals of tropical
grasslands or savannas are zebras, giraffes, baboons, and gazelles.

The Tropical Rain Forest. The tropical rain forest is characterized by high
temperatures and constant rainfall. This type of biome is found in Central and
South America, in Southeast Asia, and in West Africa. The trees are tall and
the vegetation is so thick that the forest floor is shaded from light. The
animals of the rain forest include monkeys, lizards, snakes, and birds.

The Sea. Ocean waters cover almost three fourths of the earth's surface and
support the greatest abundance and diversity of organisms in the world.
Averaging 3.5-4.5 kilometers in depth, a marine biome constitutes the thickest
layer of living things in the biosphere. The dominating physical factors
determine the type of living organisms that compose its communities.

Biometrics The science that combines mathematics and statistics needed to deal
with the facts and figures of biology. Biologists handle enormous numbers that
must be organized and simplified so that they become useful in the analysis of
data.

Biotic environment The part of the environment that is living and has some
effect on other living organisms.

Bipedalism The ability to walk on two legs instead of four. Bipedalism has
freed the forearms for doing work.

Birds Birds are terrestrial vertebrates with feathers. Feathers are the
distinctive feature of birds: all birds have them and no other animals are so
covered. The forelimb is modified into wings for flight, leaving the hindlimbs
for walking (bipedal locomotion). Birds are built for flight; special
adaptations in body structure effect lightness in weight, efficiency, and
strength. Not only are the feathers light in weight and easily moved and lifted
by wind, but they also create warmth next to the body. Body heat warms the air
that is in contact with the bird's body. Warm air becomes lighter and rises.
Other adaptations for flight are the compact, but hollow, bones, numerous air
sacs occupying all available body spaces, reduced rectum, loss of teeth, and
feathers replacing a bony tail.

Bivalves Animals, such as clams, that have the body encased within two hinged
shells. Lining the inner surfaces of the shells is a membranous mantle. The
cavity inside of the shells is the mantle cavity.

Blastula An early stage in animal embryology; a hollow ball of cells
surrounding a central cavity. See Cleavage.

Blood A liquid tissue consisting of a liquid medium called plasma and three
kinds of cells: red blood cells (erythrocytes), white blood cells (leucocytes),
and platelets. There are about 25 trillion red blood cells in the human body.
For each 600 red blood cells there is one white blood cell. Blood cells number
in the billions. The red blood cells carry oxygen. The white blood cells
function in immune reactions.

Blood clotting Platelets are the smallest blood particles. When a capillary is
cut, the platelets collect at the site of the injury. There they break into
smaller fragments and initiate the complicated chemical process of blood
clotting in which more than 15 factors, including thromboplastin, calcium
(Ca27), and fibrinogen, are involved in the formation of a clot containing
blood cells in a fibrin meshwork.

Blood types The main types of blood are A, B, AB, and O. Transfusions of blood
are possible only when the blood types are compatible. If the blood types are
not compatible proteins in the plasma will recognize foreign antigens on red
blood cells and respond by causing the cells to agglutinate, or clump, a
condition that causes blockage in small blood vessels and often death. The
following table summarizes the blood proteins involved in blood types.

Proteins of Blood Types
Blood Cell Plasma
Type Antigen Antibody
A A b
B B a
AB AB none
O none a and b

Note: Type AB -- universal recipient
Type O -- universal donor

Bone The living tissue that comprises the skeleton. Bone tissue is made of
cells that are surrounded by hardened calcium phosphate.

Bowman's capsule See Nephron.

Brain The brain and the spinal cord compose the central nervous system. In the
vertebrate body, the organs of the central nervous system are well protected by
being wrapped in connective tissue and enclosed in bone. The brain, covered by
the membranous meninges, rests in the skull cavity where it is enclosed by the
cranium. The spinal cord, also covered by connective tissue, is circled by the
vertebral column. The human brain is divided into many parts, each with special
functions. Among the most important parts are the cerebrum, cerebellum, and
medulla.

Bronchi (Bronchus) Two tubes made of cartilage rings that extend from the
windpipe (trachea) into the lungs (See Lungs).

Bronchioles Small tubes in the lungs that branch off from the bronchi. Each
bronchiole ends in an air sac called the alveolus. See Respiratory system.

Bryophytes The bryophytes are the first green land plants. They are primitive,
small, and inconspicuous. Although multicellular, the tissue differentiation is
quite simple. Bryophyte species have no tissues that are specialized for
water-carrying and no cambium specialized for growing new cells. Bryophyte
species do not have true stems, leaves, or roots. Simple rootlike structures
called rhizoids anchor the plants to the ground and absorb moisture from the
soil.

Budding A form of asexual reproduction in which the parent cell body gives
rise to a bud. The bud has the same number and kind of chromosomes as the
parent cell, but has much less cytoplasm than the parent cell. The bud develops
into a new individual. Example: yeast cells reproduce by budding.

Calorie The amount of heat required to raise the temperature of one gram of
water by one degree Celsius (1 degree C). (A kilocalorie is the amount of heat
needed to raise the temperature of one kilogram of water by 1 degree C.)

Calvin cycle The second major stage of photosynthesis involves reductive
carbon dioxide fixation. Because the cyclic reactions that function at this
time do not require light as a source of energy, the term dark reaction has
been used to designate this phase of photosynthesis. Calvin and his associates
determined the path of carbon in the carbon dioxide by the use of 14C. See
Photosynthesis.

Cambium See Woody stems.

Capillary The smallest blood vessel.

Carbohydrates Are characteristically composed of carbon, hydrogen, and oxygen
in the proportion of CH2O. The hydrogen and oxygen are in the same proportion
in carbohydrates as in water.

Carboxyl group See Amino acids.

Carcinogen Any substance or radiation that causes cancer; asbestos, the
herbicide dioxin, or radiation from radioactive materials, sunlight, or X rays
are examples.

Carnivores Flesh eaters such as snakes, frogs, hawks, and coyotes.

Carrier-facilitated diffusion Transport of a substance across the plasma
membrane by carrier molecules but without energy. This process cannot effect
the net transport of a substance from a region of low concentration to a region
of high concentration. This is a form of passive transport.

Carrying capacity In ecology, the largest number of organisms of a given
species that can be maintained indefinitely in a particular part of the
environment.

Cartilage A specialized type of dense connective tissue not as hard as bone,
in which the cells are contained in a rubbery matrix that is smooth, firm, and
flexible: occurs in joints, at the end of bones, and in the ears, nose, and
windpipe.

Catalyst A chemical substance that speeds up a reaction without itself being
used up in the overall course of the reaction. Enzymes are biological
catalysts.

Cell Each cell is a living unit. Whether living independently as a protist or
confined in a tissue, a cell performs many metabolic functions to sustain life.
Each cell is a biochemical factory using food molecules for energy, repair of
tissues, growth, and ultimately, reproduction. On the chemical level, the cell
carrries out all of the life functions. Living organisms function the way they
do because their cells have the properties of life.

Cell division When a cell reaches a certain size it divides into two new
cells, identical to each other and very similar to the original parent cell.
The new cells are known as daughter cells. The events marking cell division
differ in prokaryotes and eukaryotes. See Mitosis.

Cell membrane The outer boundary of the cell, also called the plasma membrane,
about 10 nanometers in width. The cell membrane controls the movement of
substances into and out of the cell in a process known in general as transport.
Highly selective as to the substances that cross its boundary, the cell
membrane is said to be semi-permeable. Using the concept demonstrated by the
fluid mosaic model, biologists explain how some molecules are able to penetrate
the cell membrane while others cannot. According to this model, the core of the
membrane is made up of phospholipids. Large circular proteins are set into the
membrane; smaller proteins lie on the surface. The proteins and the
phospholipids have special functions and form certain structural pathways that
serve to admit or deny passage to specific molecules.

Cellulose A straight chain polymer of glucose molecules secreted by plants and
used as structural supporting material.

Cell wall A relatively rigid structure composed of cellulose that encloses the
cells of plants. The cell wall gives these cells their shape and limits their
expansion in hypotonic media.

Central nervous system See Brain.

Centrioles Paired structures that lie just outside of the nucleus of nearly
all animal cells and some cells of lower plants. They are absent in cells of
higher plants. Under the light microscope, the centrioles look like two
insignificant granules, but the electron microscope demonstrates that they have
a very intricate structure.

Cerebellum See Brain.

Chemosynthesis The production of high-energy organic compounds from inorganic
raw materials without the aid of light energy by some bacteria that live on
ammonia, nitrogen, and sulfur.

Chitin Tough, nitrogen-containing polysaccharide that is present in the
exoskeletons of insects and in the cell walls of most species of fungi.

Chlorophyll See Chloroplasts.

Chloroplasts A group of structures that has the general name plastid. Plastids
are membrane-bound organelles found only in plant cells. Usually plastids are
spherical bodies that float freely in the cytoplasm, holding pigment molecules
or starch. Chloroplasts contain the green pigment chlorophyll, a substance that
gives plants the green color. Chlorophyll is a special molecule that has the
ability to trap light and to convert it to a form of energy that plants can use
in carrying out the chemical steps of the food-making process known as
photosynthesis. Each chloroplast is surrounded by a double membrane. Inside the
chloroplast are numerous flattened membranous sacs called thylakoids (formerly
called grana). The thylakoids are the structures that contain the chlorophyll
and it is within these sacs that photosynthesis takes place. Stroma is the name
given to the dense ground substance that cushions the thylakoids. Animal cells
do not have chloroplasts and therefore cannot make their own food. The figure
shows the fine structure of a chloroplast.

Chordates Set apart from lower animals by several distinguishing
characteristics in addition to having a notochord. First, all chordate embryos
have the three primary germ layers from which all specialized tissues and
organs develop. Second, chordates are bilaterally symmetrical animals with
anterior-posterior differentiation. Third, the body has a true coelom and a
digestive tract that begins with a mouth and ends with an anus. Other
characteristics that differentiate the chordates from other animals are the
presence of pharyngeal gill slits and the dorsal hollow nerve cord.

Chromatography A technique of separating substances, such as proteins in a
complex liquid, by varying their rates of absorption on media such as filter
paper or in a column of silicia gel.

Chromosomes In the nucleus of the nondividing cell is a tangle of very fine
threads that absorb stain quite readily. In the granular stage these threads
are known as chromatin. The chromatin threads come together, shorten and
thicken forming chromosomes that can be seen quite prominently in the dividing
cell.

Chromosome Numbers of Some
Common Species
Organism Haploid No. Diploid No.
mosquito 3 6
fruit fly 4 8
gall midge 20* 8*
evening primrose 7 14
onion 8 16
corn 10 20
grasshopper (female) 11 22
grasshopper (male) 10 21**
frog 13 26
sunflower 17 34
cat 23 38
human 23 46
plum 24 48
dog 39 78
sugar cane 40 80
goldfish 47 94

* In the fertilized egg of the gall midge, 32 chromosomes become nonfunctional
leaving 8 functional chromosomes.
** The male grasshopper has only one sex chromosome.

Cilia See Flagella and cilia.

Circulation Distribution of blood pumped from the heart, through the arteries,
arterioles, and capillaries to the body tissues. Blood is carried back to the
heart by veins.

Circulatory system The human circulatory system consists of the heart and the
system of blood vessels that transport blood throughout the body. See Heart.

Cistron The genetic unit of function, considered equivalent to a gene. Each
cistron contains the genetic information for a single polypeptide chain.

Classification The design of the classification system is a simple and
practical one that easily lends itself to the addition of new names of
organisms as they are discovered. Each group of organisms within the scheme is
known as a taxon (plural, taxa). The classification groupings are as follows:
kingdom, the largest and most inclusive group, followed by the phylum, class,
order, family, genus, and species.

Cleavage The fertilized egg goes through a series of cell divisions in which
there is no growth in size of the zygote nor separation of the cells.

Climax community A stable ecological community where one or two large trees
predominate.

Clone A population of cells (or whole organisms) that has descended from an
original parent cell, which was stimulated to reproduce by asexual means.

Codominance A form of inheritance in which neither of the allelic genes that
determine a characteristic is dominant over the other. The result of this
inheritance is a blend. When red-flowered primroses are crossed with
white-flowered primroses, the offspring have pink flowers.

Codon A "triplet" of three nucleotides in messenger RNA that directs the order
of a particular amino acid in a protein molecule.

Coenzyme An organic compound, not a protein, that supports the catalytic
activity of an enzyme. Vitamins are coenzymes.

Coenocyte A cell bounded by a single plasma membrane but containing many
nuclei.

Cold-blooded animal Poikilotherm. An animal whose body temperature changes
with the external environment. Examples: fish, amphibians, reptiles.

Commensalism See Nutritional relationships.

Community All of the plant and animal populations living and interacting in a
given environment are known as a community.

Compounds Classified as organic or inorganic. Organic compounds are generally
thought of as most compounds of carbon. Inorganic compounds are compounds made
of other elements and a few carbon compounds similar to earthlike substances,
like calcium carbonate (CaC03). Organic compounds are called organic because of
the original belief that they came from living organisms. Today thousands of
organic compounds not found in nature are being synthesized in laboratories.
Both organic and inorganic compounds are necessary to life.

Conditioned behavior A type of learned response in which a new response
becomes associated with an original stimulus.

Conifers (cone bearers) See Gymnosperms.

Conjugation A form of sexual reproduction that is occasionally demonstrated by
the ciliates. Two organisms will join together at the oral groove. The
micronucleus of each will undergo meiosis, producing several cells. All but two
of these in each organism disintegrate. One of these haploid micronuclei
remains in each cell, while the other migrates into the other cell, fusing with
the stationary gamete. The new nucleus -- which is now diploid and contains a
new genetic combination -- goes through cell division producing a new
macronucleus and a new micronucleus.

Consumers Primary consumers -- herbivores, or plant-eaters. Herbivores come in
all sizes: crickets, leaf cutters, deer, and cattle. The carnivores, or
flesh-eaters, such as snakes, frogs, hawks, and coyotes are secondary consumers
because they feed on the herbivores. The tertiary consumers are those that feed
on the smaller carnivores and herbivores as well. There are also scavengers in
the ecosystem. Earthworms and ants feed on particles of dead organic matter
that have decayed in the soil. Vultures eat the bodies of dead animals.

Contractile vacuole See Paramecium.

Cotyledons The first leaves of a plant; they are often strikingly different
from later leaves. In some plants like the bean, they contain large quantities
of stored food that get the embryo off to a good start until it can shift for
itself. They are the two halves of the bean seed. Cotyledons usually persist
for only a short while after germination.

Crossing over Genes are linked on chromosomes and are inherited in a group on
a particular chromosome. However, linkage groups are broken by crossing over, a
phenomenon that may occur during meiosis when homologous chromosomes are
intertwined during synapsis. It is at this time that chromosomes may exchange
homologous parts and thus assort linkage groups.

Crustacea Derived from the Latin crusta meaning crust, the name Crustacea
describes the lobsters and their relatives aptly. The body is covered by a
tough exoskeleton arranged in the form of arched plates that thin out at the
joints to permit maximum movement. The lobster is representative of this class.

Cyclic AMP (cAMP; cyclic adenosine monophosphate) A compound formed from ATP
that regulates the effects of numerous hormones in animals (second messenger).

Cyclosis The circulation of cytoplasm within cells. This is especially true in
plant cells where there are large vacuoles. Protoplasm flows around the margins
and in cytoplasmic strands that sometimes extend through the vacuoles.
Protoplasm in adjacent cells may flow in opposite directions, or it may reverse
directions in any particular cell. Light and temperature especially influence
this action in plant cells, but the exact cause of movement is unknown.
Circulation results in a thorough mixing of protoplasm and its contents.
Cyclosis of protoplasm in palisade cells of leaves may prevent the overexposure
of chloroplasts to light, since they remain in a position of maximum exposure
only momentarily.

Cytochromes Iron-containing red proteins; molecules of the electron-transfer
machinery in photosynthesis and respiration.

Cytokinesis The division of cytoplasm occurring during the last state
(telophase) of mitosis.

Cyton Nerve cell body. See Nerve cell (neuron).

Cytoplasm The ground substance of the cell that supports all of the cell's
organelles. See Cell.

Cytosine A nitrogen base that pairs with guanine in DNA and RNA.

Dark reaction The stage of photosynthesis in which carbon dioxide fixation
occurs, resulting in the formation of sugar. See Calvin cycle.

Daughter cells See Cell division.

Deciduous forest The forests of the temperate regions are dominated by
broad-leaved trees that lose their leaves in the winter. Examples of the kinds
of trees that compose these hardwood forests are oak, hickory, chestnut, beech,
maples, willows, cottonwood, and sycamore. The types of animals that inhabit
these forests are deer, fox, squirrel, skunk, woodchuck, and raccoon.

Decomposers Decomposers form an important part of ecosystems. Bacteria and
fungi are organisms that break down dead organic matter and release from it
organic compounds and minerals that are returned to the soil. Many of the
materials returned to the soil are used by the producers in the process of
food-making. Without the work of the decomposers the remains of dead plants and
animals would pile up, not only occupying space needed by living organisms, but
also keeping trapped within their dead bodies valuable minerals and compounds.

Dehydration synthesis As the two molecules join, a molecule of water is
produced during the process in addition to the double sugar. A synthesis of
this type is known as dehydration synthesis. Within living cells,
carbohydrates, proteins, and fats are formed by dehydration synthesis.

Dendrite See Nerve cell (neuron).

Denitrifying bacteria Soil bacteria that change nitrates back to atmospheric
nitrogen. The cycle then repeats.

Deoxyribonucleic acid (DNA) DNA molecules are the particular type of nucleic
acid out of which genes are made. Genes are the bearers of hereditary traits
from parent to offspring. See also Replication.

Desert Deserts form in regions where the annual rainfall is less than 6.5
centimeters, or where rain occurs unevenly during the year and the rate of
evaporation is high. The temperature changes drastically from hot days to cold
nights. Plants that survive in the desert have specific adaptations for low
moisture and high temperature. Examples of desert plants are creosote,
sagebrush, cacti, and cheat grass. Examples of desert animals are lizards,
insects, kangaroo rats, and arachnids.

Diaphragm The muscular structure that separates the chest cavity from the
abdominal cavity in mammals; in a microscope, the part that regulates light
entering the lens system.

Dicotyledon (dicot) A type of angiosperm plant in which the embryo plant is
contained between two seed leaves or cotyledons.

Differentiation See Embryo.

Diffusion The movement of molecules from an area of greater concentration to
an area of lesser concentration. Diffusion is a type of passive transport.

Digestion Begins in the mouth. Teeth grind the food while three pairs of
salivary glands pour salivary juice (saliva) into the mouth. Saliva contains
the enzyme salivary amylase (ptyalin), which begins the digestion of starch.
The moistened, chewed food is swallowed and moves through the throat into the
food tube, or esophagus. The esophagus has no digestive function but moves the
food into the stomach by waves of muscle contractions called peristalsis.
Chemical digestion is known as hydrolysis. Extracellular digestion takes place
outside of cells. Intracellular digestion takes place inside of cells within
cell vacuoles.

Digestive system The human digestive system begins with a mouth and ends with
an anus, and is often described as a "tube within a tube." Variously called the
gut, alimentary canal, or the gastrointestinal tract, the digestive system
extends from the lower part of the head region through the entire torso.
Essentially, this system carries out five separate jobs that have to do with
the processing and distribution of nutrients. First, it governs ingestion or
food intake. Second, it transports food to organs for temporary storage. Third,
it controls the mechanical breakdown of food and its chemical digestion. A
fourth function is the absorption of nutrient molecules. Its final piece of
work is the temporary storage and then elimination of waste products.

Dihybrid Mixed genes for two traits. See Heredity, Mendelian.

Dinoflagellates Dinoflagellates are small protists and usually unicellular.
Most of these organisms have two unequal flagella, one extending longitudinally
from the posterior end of the cell, the other encircling the central part of
the cell. Some dinoflagellates extend trichocysts like the paramecium; others
have nematocysts, stinging cells common in the coelenterates. Some species --
Noctiluca, for example -- are bioluminescent, giving off light like a firefly.

Diploid number The full complement of chromosomes in somatic (body) cells
designated by the symbol 2N; also known as the species number or chromosome
number.

Disease A disease is a disorder that prevents the body organs from working as
they should. In general, diseases can be classified as being infectious or
noninfectious. Infectious diseases are caused by organisms that invade the body
and do harm to the cells, tissues, and organs. As a rule, there is disease
specificity when a specific disease-producing organism causes a particular
disease. Disease-producing organisms are said to be pathogens and are described
as being pathogenic. Noninfectious diseases are caused by factors other than
pathogenic organisms. Among the factors that are responsible for noninfectious
diseases are genetic causes, malnutrition, exposure to radiation, emotional
disturbances, organ failure, poisoning, endocrine malfunctioning, and
immunological disorders. Whatever the cause, a disease works counter to the
well being of the diseased organism.

Dominant trait When organisms with contrasting traits are crossed, the trait
that shows up in the F1 generation is called the dominant trait. The trait that
is hidden is called the recessive trait.

Double helix See Deoxyribonucleic acid (DNA).

Droplet infection A common method of passing germs along. Disease germs are
present in droplets ot water that escape from the nose and mouth when sneezing,
coughing, and talking. If these infected droplets are inhaled or taken in by
mouth, the germs then enter the body of another person.

Ear and hearing The human ear is made up of three divisions: the outer ear,
middle ear, and inner ear. The outer ear catches sound waves and transports
them to the eardrum, a membrane that stretches across the outer canal
separating it from the middle ear. Sound waves cause the eardrum to vibrate.
The middle ear contains three very small bones called the hammer, anvil, and
stirrup. These are the smallest bones in the body. These bones accept the
vibrations from the eardrum and transmit them to the oval window, one of two
small membrane-covered openings between the middle ear and the inner ear. The
inner ear, which is entirely encased in bone, has a fluid-filled structure
called the cochlea, so named because it resembles a snail in shape. The cochlea
has numerous canals that are lined with hair cells. The vibrations from the
oval window are transmitted to the hair cells in the cochlea and thence on the
auditory nerve, which conducts the vibrations to the brain. In the brain,
signals are interpreted into sounds.

Echinoderms Spiny-skinned invertebrates that include the starfish, brittle
stars, sand dollars, sea urchins, and sea cucumbers. Although they do not look
very much like vertebrate animals, the development of the echinoderm embryo
strongly resembles that of the chordates in the early stages. The larval stage
is free-swimming and shows bilateral symmetry.

Ecological niche An important concept of ecology is that of the niche. An
ecological niche is a feeding pattern exhibited by species that compose a
community. A niche is a feeding way-of-life in relationship to other organisms.

Ecology The science that studies the interrelationships between living species
and their physical environment. The word "ecology" was coined in 1869 by the
German zoologist Ernst Haeckel to emphasize the importance of the environment
in which living things function. The environment includes living or biotic
factors and nonliving factors referred to as abiotic factors.

Ecosystem The living community and the nonliving environment work together in
a cooperative ecological system known as an ecosystem. An ecosystem has no size
requirement or set boundaries. A forest, pond, and field are examples of
ecosystems. So is an unused city lot, small aquarium, the lawn in front of a
residential dwelling, or a crack in a sidewalk. All of these examples reflect
areas where interaction is taking place between living organisms and the
nonliving environment.

Ectoderm See Embryo.

Effectors The kind of responses that organisms can make is related to their
own body equipment. The responding part is the effector. Most animal effectors
are either muscles or glands. In considering responses one ordinarily thinks
about rapid, visible muscular responses. Glandular responses are much more
subtle.

Embryo The stage in human or animal development following cleavage. During
embryonic development the tissues and organs are being built. When the embryo
takes human or animal form, it is known as a fetus. The process in which
tissues are formed is known as differentiation.

Differentiation of the
Three Primary Germ Layers
Ectoderm Endoderm Mesoderm
skin lining of lungs muscles
nervous system lining of skeleton
digestive system
sense organs pancreas heart
liver blood vessels
respiratory blood
system
ovaries, testes
kidneys

Embryonic membranes The embryo produces several membranes that do not form any
part of the new baby but which are necessary to the development and well being
of the embryo. One of these membranes is the amnion, a waterfilled sac that
completely surrounds and protects the embryo. The water absorbs shocks and
prevents friction that might damage the embryo. The implanted embryo is
attached to the uterus by means of the umbilical cord, a structure that
contains blood vessels that function in carrying nutrients and oxygen to the
embryo and transporting wastes away from the embryo. The umbilical cord
connects with the placenta, a vascularized organ made up of tissues of the
mother and embryo. See Allantois.

Endocrine system Made up of the pituitary gland, thyroid gland, parathyroid
glands, the adrenal gland, the isles of Langerhans in the pancreas, the thymus
gland, the pineal gland, and the gonads -- testes in the male and ovaries in
the female. Certain secretions of the stomach and small intestine are also
hormones and thus part of the endocrine system. Through their secretions the
endocrine glands regulate growth, rate of metabolism, response to stress, blood
pressure, muscle contraction, digestion, immune responses, and the development
and functioning of the reproductive system. Hormones exert their influence by
becoming involved with the genetic machinery of cells and by affecting the
metabolic activities of cells working through the cellular respiration
pathways.

Endoderm See Embryo.

Endoplasmic reticulum Spreading throughout the cytoplasm, extending from the
cell membrane to the membranes of the nucleus is a network of membranes that
form channels, tubes, and flattened sacs; this network is named the endoplasmic
reticulum. One function of the endoplasmic reticulum is the movement of
materials throughout the cytoplasm and to the plasma membrane. The endoplasmic
reticulum has other important functions related to the synthesis of materials
and their packaging and distribution to sites needed.

Enzyme-substrate complex An enzyme affects the rate of reaction of the
substrate molecule that fits the enzyme's activity site. In order for this to
happen, a close physical association must take place between enzyme and
substrate. This association is called the enzyme-substrate complex. See
Substrate.

Enzymes (organic catalysts) A catalyst is a molecule that controls the rate of
a chemical reaction but is itself not used up in the process. Enzymes are
proteins that control the rate of chemical reactions that take place in cells,
tissues, and organs. Each chemical reaction that occurs in a living system
requires the assistance of a specific enzyme (enzyme specificity).

Epinephrine The hormone secreted by the medulla of the adrenal gland; also
called adrenaline. It is secreted as a result of stress and produces effects on
the circulatory system and on glucose mobilization.

Epithelial tissue See Tissues.

Estrus The period of maximum sexual receptivity in the female mammal. Estrus
is also the time of the release of eggs in the female.

Eubacteria -- true bacteria This group, referred to as the true bacteria,
represents a large number of species. All of these bacteria have thick and
rigid cell walls. Some of the species are nonmotile (nonmoving), while others
are motile, using flagella or a sling motion to move from place to place.
Species belonging to the eubacteria are identified by their shapes. The
rod-shaped bacteria are known as bacilli (bacillus, sing.), the round bacteria
as the cocci (coccus, sing.), and the spiral-shaped as spirillae (spirillum,
sing.). Some species typically remain attached: diplococci occur in pairs,
streptococci in chains, and staphylococci in clusters. See also Bacteria.

Eukaryotes Organisms whose cells contain the genetic material (DNA) enclosed
in a nucleus; includes all organisms above the level of bacteria and blue-green
algae.

Evolution Concerns the orderly changes that have shaped the earth and that
have modified the living species that inhabit the earth. Evolution is a fusion
of biological and physical sciences that have provided supporting data that
confirm the fact that over periods of time major changes have occurred in the
interior of the earth and on its surface, accompanied by modifications in
climate. All of the changes in the earth are classified as nonbiological or
inorganic evolution. Changes that have taken place in living organisms are
known as biological or organic evolution.

Evolution, evidences of Evidence that evolution -- gradual change over a
period of time -- has occurred in living things is provided by many sciences
and includes facts from the geologic record, the study of fossils, and evidence
from cell studies, biochemistry, comparative anatomy, and comparative
embryology.

Evolution, theories of Since the eighteenth century several theories have been
proposed to explain evolution. Among these are the use and disuse theory of
Lamarck and the theory of natural selection formulated by Charles Darwin.
Recent advances in genetics cell biology, and functional anatomy and
biochemistry have led to the formulation of a modern theory of evolution based
on Darwin's concept of natural selection.

Excretion Removes waste products of cellular respiration from the body. The
lungs, skin, and kidneys are excretory organs in humans that remove carbon
dioxide, water, and urea from the blood and other body tissues. Guttation is
the excretion of drops of water from plants during periods of high humidity.

Excretory system In human beings, the lungs, skin, and urinary system work to
expel the wastes produced in metabolic activities. The lungs excrete carbon
dioxide and water. The skin expels water and salts from the sweat glands and a
small amount of oil from the sebaceous glands. The urinary system handles the
major work of excretion.

Exocrine glands Glands, such as salivary and sweat glands, from which the
secretions are discharged through ducts directly into an organ or onto the
surface of the body.

Exoskeleton A hard covering on the outside of the animal body. The exoskeleton
of arthropods has the same functions of support as the bony internal skeleton
of vertebrates.

Eye The human eyeball measures about 2.5 centimeters in diameter. Most of the
eyeball rests in the bony eyesocket of the skull. Only about one sixth of the
eye is exposed. External structures associated with the eye are eyelids,
lashes, and eyebrows. A transparent protective membrane, the cornea, covers the
eye. Six small muscles attach the eye to the eyesocket. Secretions from tear
glands help to keep the eye moist. The lens is a transparent body that focuses
light on the light-sensitive retina where images are formed.

Fats Organic compounds that supply cells with energy. Fats are usually
insoluble in water. A fat results from the combination of one glycerol and
three long-chain fatty acids joined through dehydration synthesis. Certain fats
are essential to the structure and function of body cells, to the building of
cell membranes, and to the synthesis of certain hormones. Fats also aid in the
transport of fat-soluble vitamins. Foods rich in fats include butter, bacon,
egg yolk, cream, and certain cheeses.

Fermentation See Anaerobic.

Ferns The fern plant used in flower bouquets is the sporophyte generation. The
sporophyte generation produces asexual spores. The mature fern has true roots,
leaves, and stem. Ferns growing in temperate climates have an underground stem
called a rhizome that grows in a horizontal position. The rhizome not only
stores food materials, but also gives rise to new fern plants that grow along
its length. The stems of tropical species grow upright in a vertical position
and serve as trunks of tree ferns.

Fertilization, birds Fertilization is accomplished during mating at which time
the male and female place their cloacas close together. Sperm swim from the
cloaca of the male into the female cloaca and up into the oviduct.
Fertilization takes place high up in the oviduct before the albumen and the
other surrounding membranes are secreted by the oviduct cells. Most birds lay a
clutch of less than six eggs. However, ducks may lay as many as 15 eggs at one
time. See Allantois.

Fertilization, fish Many sperm never reach the eggs and many fertilized eggs
die before development. Hence there is an overproduction of gametes to ensure
the survival of the species. In a few fish species fertilization is internal
and parental care is given to the fertilized eggs. The stickleback male, for
example, takes care of the fertilized eggs in nests and the male seahorse
carries them around in a brood pouch.

Fertilization, frogs In frogs, fertilization is external. Sperm leave the
testes through tubules called vasa efferentia that communicate with the kidney.
The sperm cells then pass into the Wolffian duct that leads to the cloaca, a
passageway that opens to the outside of the body. In the female, large egg
masses are released into the body cavity from two ovaries, located at the
anterior end of each kidney. Beating cilia sweep the eggs into coiled tubules
known as oviducts where they are propelled to the cloaca and then out of the
body. As the eggs pass through the oviducts they are coated with a thin layer
of jelly-like material. At the time when the female is depositing eggs in the
shallow waters of a pond or brook, the male deposits sperm over them. The sperm
swim to the eggs and as each sperm reaches an egg, it digests its way through
the jelly and into the egg, effecting fertilization. After fertilization the
jelly coating on the eggs swells due to the absorption of large amounts of
water. The swelling of the black jelly causes the eggs to adhere together and
protects them from predation by fish and other animals. The fertilized egg, or
zygote, undergoes cleavage, forming a tadpole.

Flagella and cilia Fine threads of cytoplasm that extend from the surfaces of
some cells. Both of these structures are involved in the locomotion of some
protist species. Cilia are relatively short extensions but appear in great
numbers, usually surrounding the body of the protist. Flagella are much longer
than cilia and appear in fewer numbers. In addition to serving the locomotive
needs of one-celled organisms, flagella and cilia help functions of other types
of cells. Sperm cells of animals and plants are propelled through fluid media
by the whip-like actions of their flagella. Tissue cells of the human windpipe
are lined with cilia that wave back and forth catching dust particles and
pushing them away from the lungs. The microstructure of the flagella and cilia
resembles that of the centrioles.

Flatworms The simplest of the flatworms demonstrate bilateral symmetry. This
phylum represents a step up the evolutionary scale showing a recognizable head
end and definite development of excretory, nervous, and reproductive systems.
Most of the flatworms are hermaphrodites, as well as parasites. Some flatworms
are serious parasites of humans and other animals. Among the flatworms are the
planaria, flukes, and tapeworms.

Flower, parts of The reproductive structure of the angiosperm is the flower
that encloses the male and female sex organs. The green leaflike sepals protect
the flower when in the bud stage. Collectively, sepals are known as the calyx.
Just inside of the calyx are colored petals. All of the petals in a flower are
known as the corolla. The stamens are the male reproductive structures; pollen
grains are produced in the anther. The pistil is in the center of the flower.
The top portion of the pistil is the stigma. The style is the long stalk that
leads to the rounded portion of the pistil called the ovary. Inside of the
ovary are one or more ovules. The pistil and its many parts compose the female
portion of the flower.

Flower, reproduction in Involves the maturation of the pollen grain during
which three nuclei are produced. One is a pollen tube nucleus; the other two
are sperm nuclei. As the sperm nuclei are traveling down the style, each ovule
is going through a maturation process that results in the formation of a viable
egg cell and a double nucleus. The sperm nuclei fertilize the egg cell and also
the double nucleus. The zygote goes through a series of changes that lead to
seed formation.

Fluid Mosaic Model See Cell membrane.

Food chain The flow of energy through an ecosystem can be studied by way of
food chains which show how energy is transferred from one organism to another
through feeding patterns. An example of a food chain on a cultivated field
might be as follows:
Lettuce --> Rabbit --> Snake --> Hawk
The flow of energy in a food chain is in a straight line pattern. Most of the
energy is concentrated in the level of the producer. At each succeeding level
the energy is decreased. However, the feeding relationships among organisms in
an ecosystem are not usually this simple, and, in actuality, are more complex,
forming a food web.

Food pyramid Another way of illustrating energy flow in an ecosystem. The
autotrophs at the base of the pyramid support all of the heterotrophs
(consumers) that exist at each nutritional level and there is a decrease of
available energy at each nutritional level.

Food web Shows that there are several alternative energy pathways in a food
web. It is the alternative pathways that enable an ecosytem to keep its
stability. One species does not eradicate another in the quest for food.

Fossils The preserved remains of plants and animals, usually found in
sedimentary rock. The age of fossils is estimated by the use of carbon dating,
the ratio of radioactive carbon (carbon 14) to nonradioactive carbon (carbon
12). The fossil records contained in the layers of sedimentary rock provide
reliable evidence of change in plant and animal species. The lower down the
rock layer, the older the fossil. Top layers contain more recent fossil remains
of more complex species.

Fruit A ripened (mature) ovary bearing one or more seeds. A simple fruit
develops from a single ovary; examples are tomatoes, plums, and pears. An
aggregate fruit develops from a group of ovaries produced in a single flower;
examples are raspberries and blackberries. A multiple fruit develops from the
ovaries of a cluster of flowers that are carried on the same stalk; examples
are pineapples, figs, and mulberries.

Fungi Eukaryotic, multicellular, and multinucleate organisms. Yeasts are
unicellular forms. The cells of fungi are different from those of other species
because the boundaries separating the cells are either entirely missing or only
partially formed. Thus fungi are primarily coenocytic organisms; this means
that the cells have more than one nucleus in a single mass of cytoplasm.
However, the characteristic that most distinguishes the fungi from other
organisms is their mode of nutrition. Fungi are saprophytes, absorbing organic
nutritive matter from decaying plant and animal bodies.

Gametes Sex cells; male sex cells are called sperm and are produced in gonads
called testes or spermaries. Female sex cells are the eggs or ova and are
produced in female gonads called ovaries. All gametes have the haploid
(monoploid) number of chromosomes.

Gametophyte In alternation of generations, the gametophyte generation produces
haploid gametes. Fusion of the haploid gametes forms a diploid zygote, which
grows into a sporophyte (plant). The sporophyte produces haploid reproductive
spores. Each spore grows into a multicellular haploid plant, the gametophyte.
The cycle repeats.

Gamma globulin A protein in the blood plasma from which antibodies are made.
Antibodies, produced by the lymphocytes in the immune system, are molecules
that inactivate or destroy antigens.

Ganglia (ganglion, sing.) Functional groups of nerve cell bodies that lie
outside of the brain and spinal cord, allowing parts of the nervous system to
coordinate activities without involving the whole system.

Gastrula A stage in embryonic development when the three primary tissue layers
(ectoderm, mesoderm, and endoderm) develop and the primitive gut (archenteron)
forms. See Cleavage.

Gene Around 1911, Thomas Hunt Morgan introduced the tiny fruit fly Drosophila
melanogaster as the new experimental organism for work in the field of
heredity. The experimental work of Morgan resulted in the discovery of the
chromosome as the means by which hereditary traits are transmitted from one
generation to another. Morgan's chromosome theory of inheritance includes the
concept that chromosomes are composed of discrete units called genes. Genes are
the actual carriers of specific traits and move with the chromosomes in mitotic
and meiotic cell divisions. Morgan further proposed that genes control the
development of traits in each organism. When genes change, or mutate, the
traits they control change. Morgan's work was based on the chromosome theory of
inheritance, which began to take shape in 1902 from the work of Walter S.
Sutton.

Genetic code The DNA molecule carries coded instructions for controlling all
functions of the cell. At the present time, scientists know most about the
functions of the genetic code that control protein synthesis. They have
determined that triplet combinations of bases code for each of 20 amino acids.
The coded sequence of amino acids determines the formation of different types
of proteins. The code for proteins is present in messenger RNA molecules that
are complementary to DNA molecules. For example: let us suppose that a portion
of a DNA molecule carries a code such as AAC GGC AAA TTT. Its mRNA complement
would be as follows: UUG CCG UUU AAA.

Genetics The science of heredity.

Genome The complete store of an organism's genetic material, which consists of
genes on chromosomes.

Genotype The genetic makeup of an individual. For a given trait an individual
may have two like genes or two unlike genes. Genotype determines whether the
dominant or recessive trait will show and whether or not the individual is
dominant or recessive for the trait.

Geotropism Different parts of a plant may respond differently to the same
stimulus. Thus the stem and leaves will grow upward, away from gravity; they
show negative geotropism. On the other hand, the roots will grow downward,
toward gravity; they show positive geotropism.

Germs Organisms that invade the body of animals and plants and cause disease.
Organisms that cause disease are bacteria, spirochetes, viruses, and parasitic
worms.

Gibberellin Plant growth substance isolated from the fungus Gibberella
fujikuroi that has the following effects on plants:

1. It causes corn, wheat, and many other plants to grow very rapidly, showing
an increase in height that is three to five times the normal in a short period
of time.

2. It makes dwarf plants that by heredity should always be stunted, such as
dwarf pea or dwarf corn, grow to the size of normal plants.

3. Seeds that are soaked overnight in it germinate ahead of time.

4. Biennial plants such as foxglove and carrot, that flower in the second year
of their life cycle, burst into bloom in only one year.

5. Tomatoes and cucumbers develop from flowers that are not pollinated if the
flower buds are sprayed with it.

6. Garden and house plants, such as geranium and petunias, bloom ahead of time,
and have large flowers.

Glycogen A multibranched glucose storage polysaccharide deposited in the liver
and muscles of animals; also known as "animal starch."

Glycolysis The anaerobic degradation of glucose into pyruvic acid; the initial
stage of cellular respiration. See Anaerobic; Respiration.

Gonads The sex organs of male (testes) and female (ovaries). Sex cells
(gametes) are produced in the sex organs. Male sex cells are called sperm;
female, ova (ovum, sing.)

Grasshopper Belongs to a large group of organisms classified as arthropods.
The name arthropod in literal translation means "jointed foot," a distinctive
characteristic of this group, expressed traditionally as "jointed appendages."
The arthropods are segmented animals protected by an exoskeleton made of
protein and the flexible but tough carbohydrate chitin. The chitinous
exoskeleton is fashioned in articulating plates held together by hinges
covering both the body and the appendages, and attached to muscles that make
possible quick and unencumbered movements.

Greenhouse effect The amount of carbon dioxide in the atmosphere has been
found to be increasing. Much of this is coming from the burning of fossil fuels
(coal, oil, and natural gas) by factories, homes, and automobiles. Since trees
use carbon dioxide in photosynthesis, the cutting down of the world's forests
over the years is preventing some of this C02 from being absorbed. The actual
effects of this increase are not fully known. Some scientists claim that carbon
dioxide, like the glass of a greenhouse, allows visible sunlight to pass
through to the earth. As the earth warms up, it gives off infrared rays. These
are absorbed by the carbon dioxide in the atmosphere, instead of being given
off into space. It is believed that this "greenhouse effect," as it is called,
will cause the earth's atmosphere to eventually warm up. Some of the possible
effects of such an increase in atmospheric temperature might be: spread of
desert areas; reduction of food crop production; warmer climate; melting of
polar ice, with the raising of sea levels and the consequent flooding of
heavily populated areas along the coast.

Growth Describes the increase of cell size and increase of cell numbers. The
latter process occurs when cells divide in response to a sequence of events
known as mitosis.

Guard cells Cells in the epidermis of the leaf that regulate the opening and
closing of the stomates. See Leaf cross section.

Gymnosperms Gymnosperms are cone-bearers. They are woody plants, chiefly
evergreens, with needle-like or scale-like leaves. Cone-bearing plants grow in
many parts of the world including tropical climates. However, most species are
found in the cooler parts of temperate regions. Examples of gymnosperm species
are pines, spruces, firs, cedars, yews, California redwoods, bald cypresses,
and Douglas firs. Most biologists think that the gymnosperms evolved directly
from progymnosperms present during the Devonian period.

Haploid number The organism's sex cells, or gametes -- eggs and sperm, contain
half the species number of chromosomes. This number is called the haploid
number and abbreviated N. Meiosis is the kind of nuclear division that leads to
the formation of sperm and egg cells.

Hardy-Weinberg principle A population includes all members of a species that
live in a given location. Modern geneticists are concerned about the factors in
populations that affect gene frequencies. All of the genes that can be
inherited (heritable genes) in a population are known collectively as the gene
pool. The Hardy-Weinberg principle uses an algebraic equation to compute the
gene frequencies in human populations. The conditions set by the Hardy-Weinberg
principle for determining the stability of a gene pool are as follows: large
populations, random mating, no migration, and no mutation.

Heart The human heart lies in the chest cavity behind the breastbone and
slightly to the left. The heart is a bundle of cardiac muscles specialized for
rhythmic contractions and relaxations known as heartbeat. The rate of average
heartbeat is 72 times per minute. The inside of the heart is divided into four
chambers. The two chambers at the top are the receiving chambers, or the atria.
The lower chambers, the ventricles, are pumping chambers. Each atrium is
separated from the ventricle below by a valve. The atrium and the ventricle on
the right are separated from the left atrium and ventricle by a thick wall of
muscle called the septum.

Hemoglobin An iron-protein complex in red blood cells that functions as an
oxygen carrier.

Heredity, Mendelian Gregor Mendel, an Austrian monk, began the first organized
and mathematical study of how traits are inherited. Using the garden pea as the
test organism, Mendel identified seven different traits that were easily
recognizable in this self-pollinating plant. He called each of these traits
unit characters. Mendel not only identified characteristics that seemed to be
inherited, but for each unit character, he identified an opposite trait. For
example: if the unit character was height, the opposite traits were short and
tall. If the unit character was seed coat color, the opposite traits were
yellow and green. Mendel formulated three major laws or principles of
inheritance.

The Law of Dominance: If two organisms that exhibit contrasting traits are
crossed, the trait that shows up in the first filial generation (F1) is the
dominant trait. For example: when a pure-bred tall pea plant is crossed with a
short pea plant, all of the offspring will be tall. The offspring will not be
pure tall, however, and are therefore known as hybrids. The factor for
shortness is hidden. We say today that the phenotype of the F1 plants is tall.
A phenotype refers to the traits that we can see. The genotypes of genetic
makeup of these plants is said to be hybrid or heterozygous, meaning mixed.

The Law of Segregation: When hybrids are crossed, the recessive trait
segregates out at a ratio of three individuals with the dominant trait to one
individual with the recessive trait. The 3:1 ratio is known as the phenotypic
ratio, because it refers to the traits that can be seen and not those factors
hidden in the germplasm. The hybrid cross is also known as the F1 cross and the
offspring produced by this cross are known as the second filial generation or
F2. In terms of modern knowledge, the F2 generation also produces another type
of ratio called the genotypic ratio which refers to gene makeup. The genotypic
ratio is 1:2:1, translated into 1 homozygous dominant (pure) individual: 2
heterozygous (hybrid) individuals: 1 homozygous recessive. Only the homozygous
recessive shows the recessive trait.

The Law of Independent Assortment: Mendel believed that each trait is inherited
independently of others and remains unaltered throughout all generations. We
now know that Mendel's "factors" are genes that are linked together on
chromosomes and that if genes are on the same chromosome, they are inherited
together.

Hermaphrodite An animal that has both male and female reproductive organs and
produces both eggs and sperm. Lifestyles such as burrowing (earthworms), living
attached to objects (barnacles), or living within the body of another organism
(tapeworms) make it difficult for such organisms to meet with a member of the
opposite sex. Hermaphroditism solves the problem of reproduction. Most
hermaphrodites mate with another member of the same species each donating sperm
to fertilize the eggs of the other. Sequential hermaphroditism is a
reproductive pattern exhibited in some reef fish that are able to change their
sex from male to female or vice versa as conditions warrant.

Heterotroph An organism that cannot synthesize its food from inorganic
materials such as carbon dioxide and water. A heterotroph must obtain its
nutrition by the intake of preformed organic molecules. All animals are
heterotrophs.

Heterotroph-autotroph hypothesis The first organisms on earth were probably
heterotrophs utilizing the organic pool for nutrition. As time went on, the
early heterotrophs must have faced a serious crisis because oxygen from
photodissociation (atmospheric reactions) began to increase in concentration.
This changed the atmosphere. Oxygen and hydrogen peroxide (H2O2) destroyed many
heterotrophs. Eventually, photosynthetic organisms -- autotrophs such as
blue-green algae -- evolved, increasing the oxygen content of the atmosphere
and thereby threatening the continued existence of the heterotrophs. An ozone
layer in the atmosphere developed from the high concentration of oxygen, which
further diminished the organic compounds available to the beleaguered
heterotrophs. Some heterotrophs developed pathways for utilizing oxygen in
energy production (aerobic respiration). The carbohydrates produced by
autotrophs and the oxygen of the atmosphere supplied the new heterotrophs with
the nutritive materials necessary for survival (heterotroph-autotroph
hypothesis). Thus the stage was set for the development of life on the scale
that is known today.

Homeostasis The term used to describe the stable internal environment of the
cell and the organism as a whole. Homeostasis is a condition necessary for
life. The internal chemical balance of the cell must be maintained at a steady
state to promote innumerable biochemical activities that foster the production
and use of energy. The concept of homeostasis was initially developed by the
nineteenth century physiologist Claude Bernard.

Homologous structures A comparative study of the bone structures and body
systems of animals from the various phyla reveals a great deal of similarity. A
comparative study of the skeletal systems of vertebrates shows that many of the
bones are very much alike. Much of our evidence for evolution comes from a
study of homologous structures. Homologous structures are bones that look alike
and have the same evolutionary origin although they may be used for different
purposes. The flipper of a whale, the arm of a human, and the wing of a bird
are homologous structures having the same evolutionary origin and maintaining
similarity of structure.

Hormones Secretions from the endocrine glands that regulate the activities of
body organs. Examples: growth hormone regulates growth of the long bones;
thyroxin regulates body metabolism; adrenaline controls the release of sugar
from the liver. Hormones are secreted directly into the bloodstream, which then
carries them to their target organs.

Hybrid An organism having mixed genes for a trait.

Hydra A freshwater coelenterate that is representative of a genus of the same
name. Hydra is a polyp and has no medusa form in its life history. In length,
each hydra is about 12 millimeters and has about eight tentacles that surround
the mouth-anus. Hydras move about by somersaulting, end over end. The animal's
locomotion is made possible by cells that have contractile fibers called
myonemes. These epitheliomuscular cells have locomotor and sensory functions.
Reproduction in hydra is sexual and asexual. A single organism produces both
egg and sperm, which are discharged into the water where fertilization takes
place. Asexual reproduction occurs by budding.

Hydrolysis See Digestion.

Hypothalamus The region of the brain that controls body temperature,
osmoregulatory activities, maturity, thirst, hunger, and sex drive. The
hypothalamus is also the region where the nervous and hormonal systems
interact.

Immunity Ability to resist the attack of a particular disease-producing
organism. Immunity to one kind of disease germ does not automatically make a
person immune to other types of disease germs. Active immunity is brought about
by antibody production by a person's own body cells. Active immunity can be
stimulated in either of two ways; by getting the disease and recovering from it
or by being immunized against the disease. Immunization that produces active
immunity involves the injection of weakened disease agents that stimulate
antibody production but produce only mild symptoms or none at all. Active
immunity is longlasting because the body cells continue to produce the
antibodies. An injection of gamma globulins can give a person temporary
immunity against certain specific diseases. This means that a person has
borrowed antibodies in the blood and not those made by his (her) own cells.
This kind of immunity is called passive immunity. It lasts only as long as the
antibodies last; when they are used up, the immunity ceases.

Infectious diseases Caused by pathogens (germs). These pathogenic
microorganisms include certain bacteria, protozoans, spirochetes, richettsias,
mycoplasmas, and fungi. Parasitic worms and viruses also often produce disease
in humans. Most infectious diseases are contagious -- capable of being passed
from one person to another by means of body contact or by droplet infection.

Ingestion The taking in or procuring of food. Digestion refers to the chemical
changes that take place in the body by which nutrient molecules are converted
to forms usable by cells.

Inheritance, intermediate Geneticists have discovered that in many cases a
trait is not controlled by a single gene, but rather by the cooperative action
of two or more genes. There are many instances in which Mendel's "law of
dominance" does not hold true. A case in point is what was once called blending
inheritance, or incomplete dominance. It is now known as codominance. When
red-flowered evening primroses are crossed with white-flowered primroses, the
hybrids are pink. Neither red nor white color is dominant and therefore the
result is a blend. In sweet peas, the expression of red or white flowers is
dependent upon two genes: a (C) gene for color and an (R) gene for enzyme. If C
and R are inherited together, the flower color is red. If the dominant C is
missing, the flower is white and if the dominant R is missing, the color is
white. Therefore white flowers are the result of several different genotypes:
ccrr, ccRR, CCrr, Ccrr.

Invertebrates Animals without backbones. About 90 percent of all animal
species are invertebrates. Just like all members of the kingdom Animalia,
invertebrates are multicellular. They are composed of cells that lack walls.
Most invertebrates are capable of locomotion and have specialized cells with
contractile proteins that facilitate movement. However, the adult forms of some
lower invertebrate species are sessile, belonging to a group of filter feeders.
These animals use cilia, flagella, tentacles, or gills to sweep smaller
organisms from the currents of water that flow over or through their bodies
into the digestive cavities. Some of the lower invertebrates reproduce
vegetatively by budding. Other invertebrate species reproduce sexually,
utilizing sperm and egg. Still other species reproduce asexually by
parthenogenesis in which an unfertilized egg develops into a complete
individual. Some invertebrates have marvelous powers of regeneration, the
growing back of lost parts or the production of a new individual from an
aggregate of cells or from a piece broken off from the parent organism.
Examples of invertebrates are sponges, jelly fish, worms, clams, starfish,
insects, crabs.

Involuntary muscle See Muscle, smooth.

Karyotype A technique of producing a photograph of matched chromosome pairs
developed in 1956 by Joe Hin Tjio and Albert Levan.

Kidneys Paired organs of the human urinary system located dorsally in the
abdomen. The kidneys reabsorb water, sodium, glucose, and some proteins. Excess
water and the protein waste, urea, pass into the collecting duct and are stored
temporarily in the urinary bladder until released from the body. See Urinary
system.

Kingdom system of classification, five The largest and most inclusive
classification category is the kingdom. For many decades, living things were
considered to be either plants or animals and thus were grouped into one of the
two established kingdoms. In 1969, the ecologist Robert Whittaker proposed an
updated system of classification in which living things are grouped into one of
five kingdoms, based on the extent of their complexity and the methods by which
their nutritional needs are met.

Krebs cycle See Respiration.

Kwashiorkor A serious protein deficiency disease is kwashiorkor. This disease,
which threatens the lives of many children in Africa, causes misshapen heads,
barrel chests, bloated stomachs, spindly legs and arms, decreased mental
abilities, and poor vision.

Leaf The most important function of green leaves is to carry out
photosynthesis, the food-making process in which inorganic raw materials are
changed into organic nutrients. A leaf consists of two parts: a stalk or
petiole and the blade. The petiole attaches the blade to the stem. The blade is
the place where photosynthesis takes place. Leaves vary greatly in shape.

Leaf cross section Study of a leaf cross section under the microscope reveals
three types of tissue: upper and lower epidermis, mesophyll, and the vascular
bundles. The mesophyll consists of palisade cells and spongy cells. The
epidermis is a single layer of cells at the upper and lower surfaces of the
leaf. The cells have thick walls made of cutin and lack chloroplasts. Their
main function is to protect the underlying or overlying tissues from drying,
bacterial invasion, and mechanical injury. On the underside of the leaf, the
lower epidermis has pores known as stomates, the size openings of which are
regulated by a pair of guard cells. The stomates serve as passageways for
oxygen and carbon dioxide.

Legionnaires' disease An acute, pneumonia-like respiratory infection caused by
an air-borne bacterium and associated with water in air conditioning towers.
Symptoms are a feeling of malaise, chest pain, muscle aches, shortness of
breath, and a dry cough. Other symptoms include a high fever, chills, abdominal
pain, and sometimes abnormalities of the kidney and liver. If treated promptly
with the antibiotic erythromycin, the infected person recovers. Without prompt
treatment, the disease is fatal in 15 percent of cases.

Lichens Lichens are pioneer organisms that can inhabit bare rock and other
uninviting substrates. They live on the barks of trees and even on stone walls.
A lichen is a combination of two organisms -- an alga and a fungus -- that live
together in a mutualistic relationship. The alga carries on photosynthesis,
while the fungus absorbs water and mineral matter for its partner. The fungus
also anchors the lichen to the substrate.

Life functions Living things are highly organized systems. They are
self-regulating, self-reproducing, and capable of adapting to changes in the
environment. To satisfy all of the conditions necessary for life, all living
systems must be able to perform certain biochemical and biophysical activities
which collectively are known as life functions. Nutrition is the sum total of
those activities through which a living organism obtains nutrients (food
molecules) from the environment. Life functions include nutrition as well as
the processes of ingestion, digestion, and assimilation; respiration which
encompasses breathing and cellular respiration; reproduction of cells;
synthesis involving the biochemical processes of cells; regulation encompassing
processes of control by hormones, enzymes, coenzymes, and utilized by the
nervous system and the endocrine system; reproduction, both sexual and asexual.

Light reactions See Photosynthesis.

Linked genes Genes occurring on the same chromosome are inherited together.
They are said to form linkage groups. Crossing over breaks linkage groups.

Lipids The lipids are a group of organic compounds that include the fats and
fat-like substances. A lipid molecule contains the elements carbon, hydrogen,
and oxygen similar to a carbohydrate. Unlike the carbohydrates, however, in
lipid molecules the ratio of hydrogen to oxygen is much greater than 2:1. Most
lipids are made up of two basic units: alcohol (usually glycerol) and a class
of compounds called fatty acids.

Lock and key A simple analogy is used to explain the specificity of enzymes.
Specificity refers to the characteristic of enzymes that permits a particular
enzyme to form a complex with a specific substrate molecule only. The "lock and
key" analogy explains enzyme specificity: the substrate is viewed as a padlock
and the enzyme as the key able to unlock it. When unlocked (in the analogy) or
acted upon by the key, the padlock comes completely apart. The key remains
unchanged and ready to work again on another padlock of the same type.

Lungs The human body has two lungs. Each of these is enclosed in a double
membranous sac known as the pleural sac. Not only is this sac airtight, but it
also contains a lubricating fluid. The pleural sac and the lubricating fluid
prevent friction that might be caused by rubbing of the lungs against the chest
wall.

Lyme disease A flu-type disease transmitted from ticks to humans. The cause of
the disease is a species of spiral bacteria injected in the human bloodstream
through a bite by the infected deer tick, Ixodes dammini. Early treatment with
antibiotics will prevent the development of arthritic symptoms. See also Health
and Medicine.

Lymph The body cells are bathed with tissue fluid called lymph. Lymph comes
from the blood plasma, diffusing out of the capillaries into the tissue spaces
in the body. Lymph differs from plasma in that it has 50 percent fewer proteins
and does not contain red blood cells. Lymph has the important function of
bringing nutrients and oxygen to cells and removing from them the waste
products of respiration.

Lymphocytes White blood cells that are produced in the lymph nodes and
function in immune reactions of the blood.

Malpighian tubules These are long slender tubules, attached at one end to the
digestive tract controlling excretion in insects and certain other arthropods.
Nitrogen-containing wastes in the body fluid are changed into uric acid, which
is then moved through the Malpighian tubule to the end of the digestive tract
where it is ultimately excreted as dry crystals.

Mammals The characteristics that set mammals apart from other animals and made
them adaptable to a wide range of habitats are as follows:

1. Mammals have mammary glands (from which the name mammal) that supply the
young with milk directly after birth.

2. At some time during the life cycle, all mammals have hair.

3. Mammals are warm-blooded. Constant body temperature is due, in part, to the
four-chambered heart, a device that prevents the mixing of oxygenated and
deoxygenated blood.

4. Most species of mammals have sweat glands that provide a secondary means of
excreting water and salts.

5. Mammalian teeth have evolved into three different types: incisors for
tearing, canines for biting, molars and premolars for grinding.

6. All but a few species have seven vertebrae in the neck. These neck bones are
known as cervical vertabrae.

7. A muscular diaphragm separates the thoracic cavity (containing the lungs and
the heart) from the abdominal cavity (housing part of the digestive system, the
reproductive organs, and the excretory system).

Marsupials The marsupials are primitive mammals that do not have a placenta.
The young are about 5 centimeters long at birth and are in an extremely
immature condition. At birth they crawl into the mother's pouch or marsupium.
The rounded mouth is attached to a nipple and the mother expresses milk down
the throat of the helpless fetus. As development occurs, the young marsupial is
then able to obtain milk by sucking. There are 29 living genera of marsuspials,
28 of which live in Australia. The opossum Didelphys is indigenous to North,
South, and Central America and Caenolestes inhabits regions of Central America
only. Besides the opossum, other marsupials are the kangaroo, koala, Tasmanian
wolf, wombat, wallaby, and native cat.

Mastigophora The Mastigophora are protozoa that have one or more flagella.
Some species are free-living and inhabit fresh or salt water. Other
Mastigophora species live in a symbiotic relationship with organisms of other
species. For example, several species live in the intestines of termites,
cockroaches, and woodroaches, where they digest cellulose for these insects.
The genus Trypanosoma includes parasites that cause debilitating diseases in
human beings. Trypanosoma gambiense is the zooflagellate that causes African
sleeping sickness. Humans are infected with the trypanosome by the bite of an
infected tsetse fly.

Meiosis Or reduction division, is cell division that occurs in the primary sex
cells leading to the formation of viable egg and sperm cells. Meiosis reduces
the number of chromosomes to one half in each gamete so that upon fertilization
(the fusing of sperm and egg nuclei) the species chromosome number is kept
constant.

Menstruation The process in which a nonfertilized egg is discharged from the
body. The vascularized lining of the uterus, known as the endometrium,
disintegrates in response to decreased levels of estrogen and progesterone in
the blood. Menstrual bleeding lasts four to seven days in humans.

Messenger RNA The mRNA, carrying a code for a specific protein, moves from the
nucleus to the cytoplasm. The mRNA attaches itself to several ribosomes, each
having its own ribosomal RNA. Specific transfer RNA (tRNA) molecules bring to
the ribosomes their own kind of activated amino acids. Transfer RNA molecules
that fit the active sites of mRNA's on the ribosomes temporarily attach to
them. As a result, amino acids are lined up in the proper sequence. The RNA
code is a triplet code with one triplet, or codon, made up of three base coding
for a specific amino acid. See also Protein synthesis.

Metabolism An inclusive term concerning all of the biochemical activities
carried on by cells, tissues, organs, and systems necessary for the sustaining
of life. Metabolic activities in which large molecules are built from smaller
ones or in which nutrients are changed into protoplasm are called anabolic
activities, or anabolism. Destructive metabolism in which large molecules are
degraded for energy or changed into their smaller building blocks is called
catabolic activity, or catabolism.

Metamorphosis The change from tadpole to adult frog is known as metamorphosis,
a process controlled by the thyroid gland. The adult amphibian loses the gills,
lateral line senses, tail, unpaired fins, and muscles controlling them -- the
fish characteristics -- and develops structures adapted for life on land. An
adult amphibian breathes by means of lungs and has a three-chambered heart that
is more efficient at pumping blood between the lungs, heart, and rest of the
body. In most species the adult has limbs for movement, but no tail.

Microscope, electron Magnification more than 200,000 times. Using electrons
instead of light and magnets in place of lenses, the electron microscope has
revolutionized the study of the cell. The scanning electron microscope has
improved upon the resolution of fine detail made possible by electron
microscopy. A fine probe directs and focuses electron beams over the material
being studied, affording quick scanning and giving finer detail than is
possible with the standard electron microscope.

Microscope, light The best light microscope is capable of magnifying objects
2,000 times. The phase contrast microscope makes transparent specimens visible,
while the darkfield or the ultramicroscope gives vivid clarity to fragile and
transparent organisms such as the spirochetes that cause syphilis. The
ultraviolet microscope is used for photographing living bacteria and naturally
fluorescent substances.

Mitochondrion A membranous cellular organelle. It consists of a smooth outer
membrane and a folded inner membrane. The folds are called cristae. Like the
cell membrane, the mitochondrion membrane is composed of proteins and
phospholipids. The mitochondria are necessary for aerobic respiration to take
place in cells.

Mitosis Mitosis (also known as karyokinesis) concerns the cell nucleus and its
chomosomes. Before the onset of mitosis, the cell is in a stage known as
interphase. During interphase, the chromosomes are exceptionally long and very
thin, appearing as fine granules through the light microscope. It is during
this stage that DNA molecules in the nucleus replicate. The result of
replication is that each chromosome now has an exact copy of itself. When
interphase comes to an end, the cell has enough nuclear material for two cells.
The orderly process that divides the chromosomes equally between the two
daughter cells is known as mitosis. There are four stages of mitosis: prophase,
metaphase, anaphase, and telophase. The significant events that mark each of
these stages and interphase are shown below:

Mollusks Soft-bodied, nonsegmented, and usualIy enclosed within a calcium
carbonate shell. They are most abundant in marine waters, although some species
inhabit fresh water and others live on land. All mollusks have a mantle, a
flattened piece of tissue that covers the body and that may secrete the
calcareous shell. The body of the mollusk is described as being a head-foot, a
muscular mass having different shapes and functions in the various classes. The
mollusks include the chitons, snails, clams, scallops, squids, and octopuses.
This is one of the largest animal phyla and includes about 1,000 species.

Monera The monerans are prokaryotic cells. They lack a nuclear membrane,
mitochrondria, endoplasmic reticulum, Golgi apparatus, and lysosomes. The
prokaryotes do not have any membrane-bound organelles in the cytoplasm. The
monera are unicellular organisms usually invisible to the naked eye. Most
monerans live as independent cells, although some may occur in filaments
(chains of cells) or colonies of cells held together by a gelatinous coat. This
kingdom includes bacteria, blue-green bacteria, also known as blue-green algae.

Monotremes The monotremes are primitive egglaying mammals. The eggs, large and
full of yolk, house the developing monotreme embryos. Examples of the
monotremes are the "duckbill" or platypus (Ornithorhynchus) indigenous to
Australia and Tasmania; the spiny anteater (Echidna), also an inhabitant of
Australia; and a long snouted anteater (Proechidna) indigenous to New Guinea.
Modified sweat glands of the anteater secrete a milk substitute that the young
lick up from tufts of hair on the mother's belly.

Motor neuron See Nerve cell (neuron).

Muscle contraction Muscles contract due to a sliding filament mechanism. When
the thick and thin filaments slide past each other, the Z lines of the
sarcomeres are pulled closer together: in effect, contracting. Z lines form the
boundary between sarcomeres. When the sarcomeres contract, the myofibrils
contract, which causes the contraction of muscle fibers. Myofibrils are the
basic units of muscle fibers. The figure illustrates the sliding filament
mechanism of muscle contraction.

Muscular system Muscles represent 40 percent of the total weight of the human
body. Muscle tissue is characterized by contractility and electrical
excitability, two distinctive properties that enable it to effect movement of
the body and its parts. There are three types of muscle tissue: smooth,
striated, and cardiac.

Muscle, cardiac Cardiac muscle is present only in the heart, where the cells
form long rows of fibers. Unlike other muscle tissue, cardiac muscle contracts
independently of nerve supply since reflex activity and electrical stimuli are
contained within the cardiac muscle cells themselves.

Muscle, smooth Smooth muscle is present in the walls of the internal organs,
including the digestive tract, reproductive organs, bladder, arteries, and
veins. Because smooth muscle is contained in organs that do not respond to the
will of a person, these muscles are called involuntary muscles.

Muscle, striated Striated muscle is variously referred to as striped muscle,
voluntary muscle, or skeletal muscle -- terms describing its structure and
function. Located in the legs, arms, back, and torso, striated muscles attach
to and move the skeleton; since they are moved by the will of the person, they
are often termed voluntary muscles.

Mutations Genes can change and changes in genes are known as mutations.
Mutations are usually recessive and they are usually harmful. Mutations usually
occur at random and spontaneously. However, mutations may be induced by
radiation or by chemical contamination. There are several types of mutations. A
loss of a piece of a chromosome is known as a deletion. The genes on the broken
off piece of chromosome are lost. Sometimes a broken piece of chromosome sticks
on to another chromosome, thus adding too many genes; this type of mutation is
known as duplication. Sometimes a piece of chromosome becomes rearranged in the
chromosome where it belongs, thus changing the sequence of the genes on that
chromosome; this is known as an inversion, and it prevents gene for gene
matching when chromosomes line up during meiosis. Point mutations are changes
in individual genes.

Natural selection Charles Darwin (1809-1882) was an English naturalist who
together with his cousin, Alfred Russel Wallace (1823-1915), developed a theory
of evolution that laid the groundwork for modern biological thinking. Darwin's
theory of natural selection can be summed up thusly: large numbers of new
plants and animals are produced by nature. Many of these do not survive because
nature "weeds out" weak and feeble organisms by killing off those that cannot
adapt to changing environmental conditions. Only the strongest and most
efficient survive and produce progeny. Specific tenets of the Darwin-Wallace
theory of evolution follow:

Overproduction: Every organism produces more gametes. If every gamete produced
by a given species united in fertilization and developed into offspring, the
world would become so overcrowded that there would be no room for successive
generations. This does not happen because a balance is maintained in the
reproduction of all species. Therefore natural populations remain fairly
stable.

Competition: There is competition for life among organisms: competition for
food, room, and space. Therefore there is a struggle for existence in which
some organisms die and the more hardy survive.

Survival of the Fittest: Some organisms are better able to compete for survival
than others. The differences that exist between organisms of the same species
making one more fit to survive than another can be explained in terms of
variations. Variations exist in every species and in every trait in members of
the species. Therefore some organisms can compete more successfully for the
available food or space in which to grow or can elude their enemies better.
These variations are said to add survival value to an organism.

Nematodes See Worm-Parasites.

Nephridia Coiled tubules that serve as excretory organs in the segmented
worms. The nephridia filter out waste materials.

Nephron The unit of structure and function of the kidney is the nephron. There
are about one million of these microscopic units in each kidney. They actively
remove waste products from the blood and return water, glucose, sodium ions,
and chloride ions to the blood. The nephron is made up of several structures.
The first of these is a knot of capillaries called the glomerulus. The
glomerulus fits into a second portion -- the Bowman's capsule, a cup-shaped
cellular structure that leads into the third part, the kidney tubule. There are
four main parts of each kidney tubule: the proximal convoluted tubule, the loop
of Henle, the distal convoluted tubule, and the collecting duct.

Nerve cell (neuron) The basic unit of function of the nervous system is the
neuron, or nerve cell. The parts of the nerve cell are the cyton, or cell body,
the dendrites, and the axon. The dendrites receive signals from sense organs or
from other nerve cells and transmit them to the cyton. The cell body passes
signals to the axon, which then conducts the signals away from the dendrites
and cell body. The axon terminating in end brushes (known also as terminal
branches) is popularly called a nerve fiber. The nervous system has three types
of neurons. Sensory or afferent neurons receive impulses from the sense organs
and transmit them to the brain or spinal cord. Associative or interneurons are
located within the brain or spinal cord. These transmit signals from sensory
neurons and pass them along to motor neurons. Motor or efferent neurons conduct
signals away from the brain or spinal cord to muscles or glands, so-called
effector organs.

Nerve impulse Communication in the nervous system is made possible by signals
or impulses carried in a one-way direction along nerve cells. These impulses
are electrical and chemical in nature. When a neuron is not carrying an
impulse, it is at resting potential. When a nerve cell is stimulated to carry
an impulse, its electrical charge changes and it has an action potential.
Action potentials (nerve impulses) from any one nerve cell are always the same.
All impulses are of the same size, there being no graded responses. This is
known as the "all or none response," meaning that a nerve cell will transmit an
impulse totally or not at all.

Nerve net The simple nervous system in Hydra consisting of pointed sensory
cells scattered throughout the endoderm and ectoderm, specialized to receive
impulses.

Nervous system, autonomic A network of nerves known as the autonomic nervous
system controls the body's involuntary activities and the smooth muscles of the
internal organs, glands, and heart muscle. It is composed of motor (efferent)
neurons leaving the brain and spinal cord and also of peripheral efferent
neurons. The autonomic nervous system is divided into the sympathetic system
and the parasympathetic system. These subsystems are antagonists. When one set
of nerves activates the smooth muscles of the body, the other set inhibits the
action. For example: the parasympathetic nerves dilate the blood vessels and
slow the heartbeat; the sympathetic nerves constrict the blood vessels and
quicken heartbeat.

Nervous system, human The human being has a complex nervous system that is
composed of the principal functioning units of the central nervous system and
the autonomic nervous system. The central nervous system is composed of the
brain, spinal cord, and the nerves that radiate from the spinal cord. The brain
is divided into three main parts: the forebrain, midbrain, and hindbrain. The
forebrain controls all intelligent activities, voluntary muscle movements,
speech, and interpretation of sensory data. The midbrain relays sensory
information. The hindbrain controls all vital involuntary activities such as
breathing, heartbeat, digestion, and the like. The central nervous system is
the control center for all regulating activities in the body. See Brain.

Nitrogen cycle The cycle of events through which nitrogen in the air becomes
useful to plants and subsequently to heterotrophic organisms.

Nodules Bumps on the roots of leguminous plants (beans, clover, alfalfa) that
house nitrifying bacteria. The nitrifying bacteria convert ammonia, released
into the soil by breakdown of proteins to nitrites and then to nitrates.

Nucleic acids DNA is an important part of the chromosome structure of all
cells. DNA is a nucleic acid as is RNA. The unit of structure and function in
the nucleic acid is called a nucleotide. A nucleotide is composed of a
phosphate group, a five-carbon sugar, and a protein base. If the five-carbon
sugar is ribose, the nucleic acid is ribonucleic acid (RNA). If the five-carbon
sugar is deoxyribose, then the nucleic acid is deoxyribose nucleic acid (DNA).
The protein bases in nucleic acids are ring compounds. Those bases with single
rings are pyrimidines. Bases with double rings are purines. The pyrimidines in
nucleic acid are thymine, cytosine, and uracil. The purines are adenine and
guanine. The four bases that make up the DNA molecule are adenine (A), guanine
(G), thymine (T), and cytosine (C). The four bases that make up the RNA
molecule are adenine (A), guanine (G), cytosine (C), and uracil (U).

Nutrition Is the totality of methods by which an organism satisfies the
energy, fuel, and regulatory needs of its body cells. Those substances that
contribute to the nutritional needs of cells are the nutrients. Animals take
these nutrients into the body by the ingestion of food. Food, therefore, refers
to edible materials that supply the body nutrients. Nutrients needed in large
amounts are classified as macronutrients: carbohydrates, proteins, and fats.
Micronutrients -- vitamins and minerals -- are needed in smaller amounts.
Vitamins are organic compounds; minerals are inorganic. Malnutrition results
from the improper intake of nutrients.

Nutritional relationships When two different species of organisms live
together, the relationship is called symbiosis. If the relationship is of
mutual benefit to both species, it is called mutualism. When one species
benefits and the other does not but is not harmed by the association, the
condition is known as commensalism. When one species lives at the expense of
another, doing harm to its host, the relationship is parasitism.
Disease-producing organisms are parasites.

Obligate aerobes Most species of the eubacteria are aerobic, using molecular
oxygen in the process of breaking down carbohydrates to carbon dioxide and
water. Obligate aerobes are those organisms that can live only in an
environment that provides free or atmospheric oxygen. An example of an obligate
aerobe is Bacillus subtilis.

Obligate anaerobes Some bacteria are obligate anaerobes and derive their
energy by fermentation. These organisms cannot live in an environment of free
oxygen. Many obligate anaerobes are disease producers; included in this group
are Clostridium tetani, the causative organism of tetanus, and Clostridium
botulinum, the bacterium that induces food poisoning. Still other bacteria are
facultative anaerobes. These are basically aerobic bacteria, but they can live
and grow in an environment that lacks free oxygen.

Oogenesis The maturation of the egg cell in which the primary oocyte (egg
cell) goes through reduction division (meiosis). The outcome of these divisions
is the production of one viable egg cell and two or more polar bodies.

Open circulatory system Characteristic of mollusks and the arthropods such as
the grasshopper in which the blood is not confined in blood vessels and has
contact with body tissues.

Organelles Organized structures that are parts of cells, such as ribosomes,
nuclei, mitochondria, chloroplasts, cilia, contractile vacuoles, and
endoplasmic reticulum.

Organs Are groups of tissues that work together to carry out a special
function. Examples of organs are the heart, lungs, liver, and stomach.

Osmosis Is the diffusion of water through a membrane. Osmosis is a form of
passive transport.

Ovary Is the female gonad where egg cells (ova) are produced. At times the
ovary produces hormones and thus functions as an endocrine gland.

Oviparous Egg-laying, as in birds.

Ovoviviparous A condition in which eggs develop but are retained and hatched
inside the mother's reproductive tract, as in cartilaginous fish (sharks) and a
few snakes.

Ovulation The release of an egg cell (ovum) from a follicle in the ovary.
Within the follicle a mass known as the corpus luteum forms in mammals and
secretes the hormone progesterone. If the egg is not fertilized, the corpus
luteum disintegrates and an egg is released from another follicle during the
next menstrual cycle.

Ovules Immature seeds contained within the ovary of the flower.

Paramecium Representative of the ciliata. The cytoplasm in ciliates is
differentiated into rigid outer ectoplasm and a more fluid inner endoplasm. A
pellicle lies just inside of the cell membrane. Some species respond to adverse
environmental stimuli by discharging elongated threads called trichocysts that
serve as defense mechanisms or a means of anchoring the protist to floating
pond material while feeding. Characteristic of the ciliata is the presence of
two kinds of nuclei. The macronucleus controls metabolic activities, while the
smaller micronucleus directs cell division. The figure shows the structure of
the paramecium, a typical representative of the ciliata.

Parasites An organism that lives on or inside of the body of a plant or animal
of another species and does harm to the host. Parasites offer physical
discomfort to the host and tend to kill slowly, meanwhile having had time to
reproduce themselves for several generations. Ectoparasites live on the host's
body; body lice, dog fleas, ticks. Endoparasites live within the host's body
and exhibit several adaptations for life in an intestine or in muscle or blood.

Parthenogenesis Although an egg will not develop unless it has been fertilized
by sperm, some lower animals reproduce by parthenogenesis -- in which eggs
develop without sperm. This is true of aphids, or plant lice, water fleas, and
others. In bees the drone, or male, develops from an unfertilized egg.
Artificial parthenogenesis has been accomplished by scientists experimenting
with the eggs of sea urchins and frogs. Jacques Loeb stimulated frog eggs to go
through cleavage and eventually to form frogs without fertilization by sperms;
he used various stimuli, such as pricking the membrane with a needle, and
treatment with salt solutions or acids. Dr. Gregory Pincus was successful in
producing "fatherless rabbits" by removing the ova from female rabbits,
treating the ova with salt solutions and implanting them in other female
rabbits. The baby rabbits that developed were females, and were subsequently
mated to produce normal offspring.

Parturition The birth process is known as parturition. In humans the period of
gestation (period of embryo development) is about 9 months or 40 weeks. At the
end of that time, the uterus begins to contract in a process called labor to
expel the baby. The onset of uterine contractions is probably caused by the
release of oxytocin into the bloodstream by the posterior pituitary. The human
newborn passes through the neck of the uterus (cervix) head first and then
through the vagina to the outside.

Perennials Plants that live for many years, such as trees, shrubs, and
grasses. Fire, disease, and mismanagement by humans causes the death of
perennials.

pH The number of hydrogen ions in a solution is the basis of pH. The pH scale
ranges from 0-14. A pH of 7 is neutral. Below the pH of 7, the number of H+
ions increases and the solution becomes more acidic. The lower the pH number,
the stronger the acid. A pH above 7 indicates that there are more OH- ions than
H+ ions and the solution becomes more alkaline. Blood has a pH of 7.3. Special
indicators are used to show the acidity or alkalinity of a solution.

Phagocytosis A process by which a cell first surrounds and engulfs a particle
and then ingests it. Solid particles are ingested by cells through a process
known as phagocytosis. White blood cells ingesting bacteria serve as an example
of phagocytosis.

Phenotype The traits of an organism that can be seen.

Phloem Water-carrying (vascular) tissue in the plant leaf and stem specialized
for conducting water with dissolved nutrient molecules downward to the root.

Photoperiodism The physiological response made by plants to changes in day
length is known as photoperiodism. Some plants (short-day) flower only if they
are exposed to light for less than a certain amount of time each day, other
plants (long-day) must have a certain minimum length of photoperiod.
Researchers explain the ability of plants to measure time by the action of
phytochrome, a light absorbing pigment that is associated with the cell
membrane and with some of the cell's internal membranes.

Photosynthesis Photosynthesis takes place inside of chloroplasts, structures
within the cells of the leaf. Chloroplasts have fine structures within --
flattened membranous sacs named thylakoids. On the membranes of the thylakoids,
chlorophyll and the accessory pigments are organized into functional groups
known as photosystems. Each of these photosystems contains about 300 pigment
molecules that are involved directly or indirectly in the process of
photosynthesis. Photosynthesis involves four sets of biochemical events:
photochemical reactions, electron transport, chemiosmosis, and carbon fixation.
The photochemical reactions and electron transport activities take place on the
membranes of the thylakoids. The oval membranes of a thylakoid surround a
vacuole or reservoir in which hydrogen ions are stored until needed in the
Calvin cycle, or carbon fixation. Each thylakoid rests in the stroma or ground
substance of the chloroplast. The stroma is the place where carbon fixation
occurs.

Pinocytosis Cell drinking. The engulfing of molcules in solution through the
cell membrane requiring the use of the cell's energy.

Plants, green Green plants are grouped in the kingdom Plantae. Members of the
kingdom Plantae contain the green pigment chlorophyll. Not only does
chlorophyll color plant leaves and some stems green, it, more importantly,
traps light energy which is used in the process of photosynthesis. As an
outcome of photosynthesis nutrient molecules are made, serving as food for both
plants and animals. Most species of the kingdom Plantae are nonmotile, anchored
to one place, and unable to move about, but a few of the lower plants are
motile for at least part of the life cycle. However, the evolutionary trend
exhibited in green plants is toward stationary organisms that carry out their
life functions on land in locations where they remain for life. Lower plant
species equipped to swim live in salt and fresh water. Higher plants are
terrestrial (land-dwelling)

Pollination The transfer of pollen from the anther to the stigma. Most flowers
are insect- or wind-pollinated. Insect-pollinated flowers have petals and
nectar that attract insects. The pollen tends to be sticky and clings more
easily to insect bodies. Wind-pollinated flowers lack petals; or if present,
they are relatively inconspicuous. They seldom produce nectar. Pollen is light
and dry and sometimes has membranous appendages, all these characteristics
making it easier to become wind-borne. Windblown pollen is produced in large
quantities enhancing the chances for pollination.

Polymorphism The honeybee exhibits a specialized social structure called
polymorphism, a condition in which individuals of the same species are
specialized for different functions. In a honeybee colony, three classes of
individuals arise: fertile males called drones; fertile females, or queens; and
sterile females, or workers. The workers have a special concave surface on the
second pair of walking legs called a pollen basket used to carry pollen. The
queen bee receives sperm from the drone once during her lifetime. The sperm are
stored in a special organ called the spermatotheca in which they may live for
years. Fertilized eggs give rise to females, most of which remain workers. A
special female may be selected by the colony and fed a diet of "royal jelly"
that causes her to grow larger than the others and become fertile. This fertile
female will become a queen, and either take over the existing colony or start a
colony of her own. Drones develop from unfertilized eggs by the process of
parthenogenesis.

Population genetics See Hardy-Weinberg principle.

Predation This is the process of one species feeding upon another. The
predator is the consumer that seeks out the prey to be consumed. Predators have
special adaptations that enable them to hunt and catch their food supply: speed
and agility, stingers, fangs, poisons, claws, and camouflage. Plant species
have certain adaptations that protect them against predation such as thorns,
microscopic spines, and the production of poisons such as nicotine, morphine,
strychnine, mescaline, and aromatic compounds such as cinnamon and cloves.
Animal defenses against predation include protective coloration, spines of the
porcupine, the odor of the skunk, and the ability to hide through camouflage.

Proteins Complex molecules built from amino acids. About 20 amino acids are
essential to living systems. From these a large number of different kinds of
proteins are formed. The great variety of protein molecules is possible because
of the many ways in which amino acid molecules can be arranged. Changing the
sequence of just one amino acid in a chain will change the protein molecule.
Much of the work of the cell is concerned with the synthesizing of protein
molecules. Some of these proteins such as hormones, enzymes, and hemoglobin are
used in complex biochemical activities. Other proteins contribute to the
structure of cells such as those that make up the plasma membrane and other
cellular membranes.

Protein synthesis The code for proteins is present in messenger RNA molecules,
which are complementary to DNA molecules. For example: let us suppose that a
portion of a DNA molecule carries a code such as AAC GGC AAA TTT. Its mRNA
complement would be as follows: UUG CCG UUU AAA. The mRNA, carrying this code,
now moves from the nucleus to the cytoplasm. The mRNA attaches itself to
several ribosomes, each having its own ribosomal RNA. Specific transfer RNA
(tRNA) molecules bring to the ribosomes their own kind of activated amino
acids. Transfer RNA (tRNA) molecules, each with a particular amino acid
attached, attach temporarily to the codons on the mRNA and their amino acids
are linked together in the correct sequence, one amino acid at a time. Note
that the RNA code is a triplet code with one triplet, or codon, made up of
three bases coding for a specific amino acid. See also Messenger RNA.

Protists All of the species assigned to the kingdom Protista are eukaryotic,
for example, Amoeba and Paramecia. Most protists carry out their lives within a
single cell as free-living organisms. However, some protist species are
organized into colonies where each cell carries out its own life functions and
where, also, there may be some simple division of labor among the cells in the
grouping. An impressive variety of species are classified as protists, and they
probably descended from diverse evolutionary lines. The protists themselves
represent evolutionary modification and are probably the ancestors of the
modern fungi, plants, and animals.

Protozoa Protozoa, meaning "first animals," are one-celled heterotrophs.
Species of protozoa number in the thousands. They live in fresh water, salt
water, dry sand, and moist soil. Some species live as parasites on or inside of
the bodies of other organisms. Reproduction in the protozoans is usually
described as being asexual by means of mitosis, but recent research has
revealed that many protozoa augment asexual reproduction with a sexual cycle.
Usually, the sexual cycle occurs during periods of adverse environmental
conditions, and the cell arising from the fusion of gametes (zygote) can resist
unfavorable conditions. The thick wall and the decreased metabolic rate of the
cyst permits survival during periods of cold, drought, or famine. The protozoa
are divided into four phyla, based primarily on the methods of locomotion.

Punctuated equilibrium There is a time frame for evolution. The concept of
gradualism supports the idea that evolutionary change is slow, gradual, and
continuous. The concept of punctuated equilibrium sets forth the idea that
species have long periods of stability, lasting for four or five million years,
and then change as the result of some geological or other environmental change.

Punnett square A Punnett square is a diagrammatic device used to predict the
genotypic and phenotypic ratios that will result when certain gametes fuse.
Remember that as a result of meiosis each gamete has only one half the number
of chromosomes that are in the somatic cells.

Problem: In fruit flies, long wing (L) is dominant over vestigial wing (l).
What is the result of a cross between two flies that are heterozygous (Ll) for
wing length?

Solution:
Parents: Male x Female
Ll Ll
Gametes: L l L l
Punnett square: L l
L LL Ll
l Ll ll
F2: LL = 1 homozygous dominant long winged fly
Ll = 2 heterozygous dominant long winged flies
ll = 1 homozygous recessive short winged fly

Race All humans belong to the species Homo sapiens. This means that the
genetic material of all people is so similar that all humans can interbreed and
produce fertile offspring. The human species is really a group of interbreeding
populations. Populations that have adapted to certain environments become
genetically different based on the frequency with which certain genes appear.
Skin color, hair texture, body build, and facial bone structure are a few of
the characteristics that identify human population groups known as races.
Although we can make broad generalizations about the identifying
characteristics of racial groups, not every member of each group fits these
specifications. A set of physical characteristics can be drawn up that will fit
individuals of several different races. Therefore, it is difficult for
biologists and anthropologists to agree on the number of human races.

Radioactive dating Scientists have determined that certain elements
disintegrate by giving off radiations spontaneously and at a regular rate. Such
elements are said to be radioactive. In the process of emitting radiations, the
radioactive substance changes to something else. For example: uranium-238
changes to lead. The half-life of U-238, the rate at which one half of the
uranium in a rock sample will change to lead, is 4.5 billion years. Uranium's
rate of decay is not affected by any chemical or physical conditions. Therefore
measuring the uranium-lead ratio in a sample of rock is a very reliable method
for estimating the age of the rock. Dating of the oldest rocks found on earth
indicates that they are about 3 billion years old. To allow time for the
original formation of the rocks, geologists add another 2 billion years to this
figure, thus arriving at the 4.5-5 billion-year estimate of the age of the
earth.

Recessive trait A characteristic that will appear if there are two like
(recessive) genes for the trait. A trait that is masked by a dominant gene. For
example: in sheep, white wool is dominant over black wool. A sheep with black
wool must have two recessive genes for the trait.

Recombinant DNA Recombinant DNA is DNA combined from two different organisms
to produce characteristics not found in nature. The technique for doing this
involves the use of a class of special enzymes called "restriction enzymes."
These enzymes have the ability of splitting a DNA strand. The fragments of DNA
have sticky ends, and when they touch a strip of DNA from another organism,
they stick to it. In this way new genes can be introduced into an organism.
Once done, the cell can synthesize the protein coded for by the newly acquired
genes. Some proteins that have been produced in this way are interferon,
insulin, and human growth hormone.

Red blood cells (Erythrocytes) The human body contains about 25 trillion
erythrocytes, each one lasting about 120 days. New red cells are produced by
the bone marrow at the rate of one million per second. Erythrocytes contain
hundreds of molecules of the iron-protein compound hemoglobin. In the lungs,
oxygen binds loosely to hemoglobin forming the compound oxyhemoglobin. As
erythrocytes pass body cells with low oxygen content, oxygen is released from
hemoglobin and diffuses into tissue cells. Carbon dioxide combines with another
portion of the hemoglobin molecule and is transported to the lungs where it is
exhaled.

Reflex If you accidentally touch a flame, your finger is automatically pulled
back. A sensory neuron carries the impulse from the receptor, at the end of
your finger, to the spinal cord, here an associative neuron receives it and
sends it over a motor neuron to a muscle in your arm, which contracts, and
pulls your finger away. There is no thought involved in the reaction. It is
centered in the spinal cord. A moment later, you are aware of it, because
impulses are sent up the spinal cord to the brain. Fortunately you do not have
to think about such stimuli. The response is inborn and is a factor in your
survival. Such a reflex protects you.

Regulation Encompasses all processes that control and coordinate the many
activities of a living thing. Chemical activities inside of cells are
controlled by enzymes, coenzymes, vitamins, minerals, and hormones. The nervous
and endocrine systems of higher animals integrate and coordinate body
activities. Growth and development of plants is regulated by auxins and other
growth-control substances. Gene activity is regulated by special molecules that
turn the gene on and off.

Replication The process through which a DNA molecule makes an exact duplicate
of itself.

Reproduction The process by which new individuals are produced by parent
organisms. Basic to the understanding of reproduction is the concept that
organisms produce the same kind of individuals as themselves. There are two
major kinds of reproduction: asexual and sexual.

Reproductive system, female The female reproductive system serves three
important functions: the production of egg cells, the disintegration of
nonfertilized egg cells, and the protection of the developing embryo. The
reproductive system has specialized organs to carry out these functions. Two
oval-shaped ovaries lie one on each side of the midline of the body in the
lower region of the abdomen. On a monthly alternating basis each ovary produces
a mature egg. Eggs are located in spaces in the ovary called follicles. As an
egg matures, it bursts out of the ovarian follicle and is released into the
appropriate branch of the fallopian tube, a tube that leads from the region of
the ovary to the uterus. If the egg is fertilized, it becomes implanted in the
uterus where it goes through a series of cell divisions known as cleavage.

Reproductive system, male In the male reproductive system some organs are
located outside of the body and others are positioned internally. The scrotum
contains the testes, glands that produce sperm, and the male hormone
testosterone. Also positioned outside of the body is the penis, the organ that
delivers the sperm into the body of the female. Each testis contains thousands
of seminiferous tubules. Within these tubules, the sperm cells are
manufactured. Each epididymis tubule functions as a storage place for sperm and
also serves as a pathway that carries the sperm to a duct called a vas
deferens. In its travels to the vas deferens, the sperm pass the seminal
vesicles where they obtain nutrients. From the vas deferens, the sperm are
conducted to the urethra, a single tube that extends from the bladder through
the penis. Sperm cells leave the body through the penis. See Sperm.

Reptiles Reptiles have a dry leathery skin covered with epidermal scales. A
somewhat flattened skull contains a brain having a cerebrum much larger than
that of the fish or amphibians. The eyes have secreting glands that keep the
surface moist. Reptiles are air-breathers and have rather well-developed lungs.
The heart is composed of two atria and a ventricle; in some species the
ventricle is almost divided into two compartments, an evolutionary signpost
pointing to the four-chambered heart. The body temperature of reptiles is not
constant, changing with the external environment. In popular speech, such
animals are called cold-blooded; in technical language, poikilotherms or
ectothermic.

Respiration Consists of breathing and cellular respiration. Breathing refers
to the pumping of air into and out of the lungs of air-breathing animals or the
movement of water over the gills of fish. During breathing, oxygen diffuses
into the air sacs in the lungs and carbon dioxide moves out of the lungs
through the nose and mouth. Cellular respiration is a combination of
biochemical processes that release energy from glucose and store it in ATP
(adenosine triphosphate) molecules.

Respiratory system The respiratory system includes the structures through
which oxygen comes into the body to reach the bloodstream, and through which
carbon dioxide and water vapor leave. Summary of route of air through
respiratory system: (1) nostrils, (2) nasal cavity and sinuses, (3) pharynx,
(4) larynx, (5) trachea, (6) bronchi, (7) bronchial tubes in the lungs, (8) air
sacs (alveoli).

Retrovirus A virus in which the genetic information is coded in ribonucleic
acid (RNA) instead of in deoxyribonucleic acid (DNA). HIV, the virus of AIDS,
is a retrovirus that produces reverse transcriptase, an enzyme that enables the
retrovirus to replicate, and to make more of itself.

Ribonucleic acid (RNA) A nucleic acid consisting of a large number of
nucleotides, each of which contains ribose sugar and four bases, including
uracil, and a phosphate radical. See also Protein synthesis.

Ribose A sugar of chemical formula C5H10O5: one of the building blocks of
nucleic acid.

Ribosome A small organelle that is the site of protein synthesis.

Rickettsiae The rickettsiae are very small Gram negative intracellular
parasites. They were first described in 1909 by Harold Taylor Ricketts, who
found them in the blood of patients suffering from Rocky Mountain spotted
fever. The rickettsiae are nonmotile, nonspore forming, nonencapsulated
organisms. In length, these organisms range from 0.3-1 micrometers. They live
in the cells of ticks and mites and are transmitted to humans through insect
bites. The rickettsiae are responsible for several febrile (fever-producing)
diseases in humans, such as typhus fever, trench fever, and Q fever.

Roots Roots anchor plants to the soil and absorb water and dissolved minerals
from the ground. The absorbed materials enter the root by way of root hairs
which are one-cell extensions of the epidermis. From the root hairs, dissolved
materials pass through the cortex, endodermis, and pericycle into xylem cells.
The xylem cells conduct the dissolved materials upward. The root cortex serves
in the storage of food and water.

Saprophytes (saprobes) These organisms obtain their food by absorbing
nutrients from dead organic matter. Examples are some species of eubacteria and
species of fungi. The fungus has specialized structures that secrete digestive
enzymes into the food substrate on which they live, such as dead logs. The
digestive enzymes liquefy the small portions of the log, releasing organic
molecules that are absorbed by the fungus. Saphrophytic bacteria secrete
digestive enzymes onto the food substrate for digestion. The released organic
molecules then are absorbed by the bacteria.

Scientific method Scientific problem solving depends upon accuracy of
observation and precision of method. Inherent in scientific thinking is
orderliness of approach, which invites the forming of conclusions from
hypotheses, theories, principles, generalizations, concepts, and laws.
Scientific problem solving follows a pattern of behaviors that are collectively
known as the scientific method.

Semi-permeable membrane The cell membrane controls the passage of materials
into and out of the cell. It is often referred to as a living gatekeeper. The
cell membrane is semi-permeable and highly selective: not every ion or molecule
can cross its boundary. The movement of materials across the cell boundary and
into or out of the cell is given the general term of transport. It is
controlled by the globular proteins, phospholipids, and pores of the membrane
and by the electrochemical nature of protoplasm, the living substance of the
cell.

Sense organs The human body has five major senses -- sight, hearing, taste,
smell, and touch -- that provide information about the external environment and
transmit the stimuli to sensory nerves and ultimately to the brain for
processing.

Sex determination In human beings, there are 22 pairs of autosomes,
chromosomes that affect all characteristics not involved in sex determination.
One pair of chromosomes determines the sex of an individual. In normal females,
the sex chromosomes are designated as XX. In normal males, the sex chromosomes
are XY. Certain disorders are sex-linked, usually passed from mother to son by
a defective gene on the X chromosome; red-green color blindness is one such
sex-linked trait that is found more frequently in males and hardly at all in
females. Hemophilia is another sex-linked trait that affects males with greater
frequency than females.

Sharks Cartilaginous fish in which the skeleton is made of cartilage instead
of bone. The body of the shark is covered with placoid scales which arise from
the ectoderm, also forming teeth in the jaws and on the roof of the mouth.
Sharks have no swim bladder and the gill slits are uncovered. The fertilization
is internal; the embryos are nourished from food from the egg.

Sinoatrial node Each heartbeat is started by self activating electrical
activity of the heart's pacemaker known as the sinoatrial node (S-A node),
positioned in the wall of the right atrium. From the S-A node, the impulse
spreads throughout the atrium to the antrioventricular node (A-V node), a
specialized bundle of cardiac muscle located on the atrium near the ventricles.
The impulse spreads from the A-V node to all parts of the ventricles causing
simultaneous contractions in the ventricles.

Skeletal system The human skeleton, like that of all vertebrates, is a living
endoskeleton that grows with the body. At birth, the human baby has a body that
is made up of 270 bones. Due to the fusion of separate bones, the mature
skeleton is composed of 206 bones. The human skeleton is a magnificent feat of
engineering. The primary purpose of the skeleton is to carry the weight of the
body and to support and protect the internal organs. The skeleton must be
strong and able to absorb reasonable amounts of shock without fracturing. At
the same time, the body framework must be flexible and light enough in weight
to permit movement. Skeletal bones move in response to muscles that work like
levers, allowing a variety of movements such as walking, running, hopping,
sitting, bending, lifting, and stooping.

Slime molds Slime molds live on the forest floor where they grow in damp soil,
on or around rotting logs and on decaying vegetation. They appear to be
shapeless globs of slime of varying colors: white, yellow, or red. The life
cycle includes either an amoeboid slime mass called a plasmodium or a
plant-like sporangia stage that is supported by stalks called fruiting bodies.
A multicellular mass is called a plasmodium. A multinucleated mass is called a
pseudoplasmodium. The diagram above shows stages in the life cycle of true
slime mold.

Sodium-potassium pump At times molecules are forced out of cells by
exocytosis, a means by which they are carried to the cell surface by vacuoles
or vesicles. The sodium-potassium pump is a means by which excess sodium ions
are forcibly extruded from nerve cells while potassium ions are pulled into the
cell.

Species The basic unit of classification is the species. A species is a group
of similar organisms that can mate and produce fertile offspring. The red wolf,
African elephant, red oak, house fly, hair cap moss -- each belong to a
separate and distinct species. For example, the red wolf belongs to the red
wolf species in which the male mates with the female and produces fertile red
wolf offspring. Upon maturity these red wolf offspring will reproduce just as
their parents did. Species is a reproductive unit, not one defined by
geography. Once brought together, a Mexican male Chihuahua can mate with a
female Chihuahua born in France because they are species compatible. In rare
instances, members of closely related species -- horses and donkeys, for
example -- can mate and produce offspring. But the products of interspecies
matings are not fertile and therefore cannot reproduce. When a male donkey
mates with a female horse (mare), a mule is produced. The mule is an infertile
hybrid. The mating of a male horse (stallion) and a female donkey results in a
hinny, also an infertile hybrid.

Sperm The male gamete is structured for motility. It leaves the male
reproductive system and is able to swim through the female reproductive system
where it may fertilize (penetrate) an egg cell. Sperm cells are haploid, having
half the number of chromosomes of a fertilized egg.

Spirochetes Most bacteria belonging to this group are anaerobic; many are
disease producers. These bacteria are long, thin, and curved, moving with a
wriggling, corkscrew-like motion, made possible by an axial filament. In some
ways, the spirochetes resemble protozoa, but they are nonnucleated. They do not
form spores or branches. They reproduce by transverse fission. The spirochete
Treponema pallidum causes syphilis.

Spongy cells See Leaf cross section.

Sporozoa The Sporozoa are parasitic spore-formers. The adult forms are
incapable of locomotion, although immature organisms may move by means of
pseudopodia. Some species of sporozoa go through a complicated life cycle
requiring different hosts during different life stages. For example, the
species Plasmodium vivax -- the agent that causes malaria -- requires two
hosts: the Anopheles mosquito and a human.

Starfish The starfish is an excellent representative of the echinoderms. It
has all of the distinguishing characteristics: pentaradial symmetry, spiny
skin, tube feet controlled by a water vascular system, no head, excretory or
respiratory system. Protruding from the wall of the coelom and extending out
between the calcareous plates into the sea water, are the papulae, sac-like
structures that function as respiratory and excretory organs. The mouth is
located in the center of the disc on the underside of the body. The nervous
system is composed of a nerve ring located in the disc from which a ventral and
radial nerve branch into each arm. Starfish sexes are separate. Starfish have
remarkable powers of regeneration. If an arms breaks off, the arm grows back.
Should a piece of the central disc be attached to the amputated arm, a new
individual will grow from the dismembered part. Starfish prey on oysters.

Stem See Woody stems.

Stoma or stomate Small pore in the plant epidermis that permits gas exchange.
Each stoma is bounded by a pair of guard cells whose osmotic status regulates
the size of the opening.

Substrate The molecules on which an enzyme exerts catalytic action. The
surface that supports any organism. See Enzyme-substrate complex.

Synthesis Involves those biochemical processes in cells by which small
molecules are built into larger ones. As a result of synthesis, amino acids,
the building blocks of proteins, are changed into enzymes, hormones, and
protoplasm.

Tadpole See Fertilization, frogs.

Tapeworm The life cycle of the tapeworm includes two hosts. A mature
proglottid contains a sac filled with hundreds of fertilized eggs. When the
proglottid walls rupture, the ground becomes infected with fertilized eggs. If
these eggs are ingested by a pig, the protective walls surrounding each egg are
digested, releasing developing embryos of the tapeworm into the digestive
system of the pig. These embryos bore into the pig's capillaries and are
carried by the blood to the muscles where the scolex forms a cyst. The worm now
remains encysted in the muscles (meat) of the pig. If the butchered pig (now
called pork) is improperly cooked and eaten by a human, the encysted worm
becomes activated. Its head begins to bud proglottids and the cycle of
infection repeats.

Taste buds The taste buds, which receive stimuli for the four taste sensations
of sweet, sour, bitter, and salty, are located in different parts of the
tongue. The senses of taste are related to that of smell. When we have a bad
head cold, or hold our nose, taste is affected.

Testes Male gonads that produce sperm, the male sex cells. Each gonad consists
of tightly coiled seminiferous tubules where sperm production begins.

Thallophyta Most algae are not differentiated into roots, stems, and leaves.
The simple body of the plant -- either a single cell or a flat sheet of simple
cells -- is known as a thallus. Thallophytes do not have specialized tissues to
carry water, anchor the plants, or to grow new cells. As a rule, the sex cells
of the thallophytes are produced in rather simple sex organs that are not
protected by a surrounding wall of cells. The zygotes of the thallophytes do
not develop into embryos that are contained in a female reproductive organ.
Most species of thallophytes live in fresh water, although there are a few
saltwater forms. Some species live in damp soil or on the bark of trees.

Tissues Cells in the body of the multicellular organism are arranged in
structural and functional groups called tissues. A tissue is a group of similar
cells that work together to perform a particular function. Tissues that are
grouped together and work for a common cause form organs. Groups of organs that
contribute to a particular set of functions are called systems. The ability of
cells to carry out special functions in addition to the usual work of cells
exemplifies specialization. When different jobs are accomplished by the various
tissues in an organ, we call this division of labor.

Transpiration Loss of water vapor through plant leaves is termed
transpiration, a process that is responsible for the rise of sap in trees.
Guttation is the loss of liquid water through the leaves of plants with short
stems. Guttation is caused by the effect of root pressure on water flow and has
hardly any physiological advantage to the plant.

Transport Involves the absorption of materials by living things, including the
movement and distribution of materials within the body of the organism. There
are several transport methods, including diffusion, active transport, and
circulation. Diffusion is the flow of molecules from an area where these
molecules are in great concentration to an area where there are fewer of them.
Active transport is the movement of molecules powered by energy. Circulation is
the movement of fluid and its dissolved materials throughout the body of an
organism or within the cytoplasm of a single cell.

Turtles and tortoises The chelonians are the turtles and tortoises. The
skeleton is modified to form a box-like covering, the upper curved portion of
which is called the carapace, the lower part, the plastron. The head, tail, and
legs are the only movable parts of the animal. The jaws are horny and
toothless. Chelonians live on land, in freshwater, and in the sea. More turtles
live on the American continent than anywhere else.

Twins There are two types of twins: identical and fraternal. Identical twins
result from the fertilization of one egg and have the same genetic makeup. They
are of the same sex and are almost identical in appearance. They develop in a
common chorionic sac and share a common placenta. However, the umbilical cords
are separate. Fraternal twins develop from two separate fertilized eggs. They
do not share a common genetic makeup and are no more alike than siblings born
at separate times. The sexes may be different. Each fraternal twin has its own
chorionic membrane and its own placenta.

Umbilical cord The implanted embryo is attached to the uterus by means of the
umbilical cord, a structure that contains blood vessels that function in
carrying nutrients and oxygen to the embryo and transporting waste away from
it. The umbilical cord connects with the placenta, a vascularized organ made up
of tissues of the mother and the embryo. The blood of the embryo that
circulates in the capillaries of the placenta is separated from the blood of
the mother by layers of cells thin enough to allow diffusion between the two
circulatory systems. There is no mixing of the blood of the mother with the
blood of the embryo. (The term fetus is used in mammalian embryos after the
organs have formed.)

Urinary system The human urinary system is located dorsally in the abdomen.
This system consists of two kidneys, tubes known as ureters extending from each
kidney to a urinary bladder and a single urethra, a tube that leads out of the
bladder.

Uterus A muscular sac inside the body of the female mammal specialized for
holding the developing young.

Vacuole A space in the cytoplasm enclosed by a membrane and filled with
liquid. Some vacuoles are digestive vesicles; some are storage areas.
Contractile vacuole is an organelle that pumps excess water out of freshwater
protozoans such as the Paramecium.

Vascular plants The vascular plants are truly land-dwelling plants. They have
developed adaptations that permit them to live on land independent of bodies of
water. The word "vascular" means that these plants have a water-carrying
system. Water is conducted upward from the roots by xylem tubules. Fluid
compounds are conducted downward from the leaves to lower plant organs by the
phloem tubules.

Vegetative propagation In some cases, flowering plants can reproduce more of
their kind without the use of flowers, but from stems, leaves, or roots. Since
these are the vegetative parts of the plant normally used for nutrition, this
method of reproduction is known as vegetative propagation. It is a form of
asexual reproduction, since it involves only one parent.

Values of vegetative propagation:

1. The plants are of the same type as the parents. They do not vary, as might
usually be the case in sexual reproduction, where the characteristics of two
parents are inherited.

2. Plants are reproduced much more quickly and in larger numbers, than if they
were grown from seeds.

3. Seedless fruits such as oranges and grapes can be maintained and propagated.

Venereal diseases (VD) Largely spread through sexual intercourse between
people who are infected with the germs that cause the diseases. The infection
of two such diseases, syphilis and gonorrhea, generally takes place through the
mucous membranes of the reproductive organs or the mouth. A pregnant woman who
has one of these diseases may give birth to a deformed or dead baby. Blindness
may also affect the newborn. As a precaution, it is common practice in
hospitals to treat the eyes of newborn babies with silver nitrate.

Vertebrates Animals that have a true backbone composed of segmented parts
called vertebrae belong to the chordate subphylum Vertebrata. The vertebrae may
be made of cartilage or bone: if made of the latter, cartilage cushions prevent
the bones from rubbing together. The backbone is built around the notochord and
usually obliterates it. Vertebrates vary in size from large to small, but all
have a living endoskeleton usually made of bone. All vertebrate species have
marked development of the head where a brain is enclosed in a cranium. Blood is
pumped through a closed circulatory system by means of a ventral heart, having
at least two chambers: an atrium and a ventricle. The hepatic portal system
carries blood laden with food from the intestines to the liver before it
reaches the body cells. Vertebrate red blood cells contain the iron-bearing
pigment hemoglobin that is specialized to carry oxygen. Such a system of closed
blood vessels prevents blood from entering the body cavity. Most vertebrates
(except humans) have a post-anal tail that is a continuation of the vertebral
column. Although there are never more than two sets of paired appendages, some
adult vertebrates show only one such set or none at all, the appendages having
been lost over evolutionary time. Evidence of lost appendages may be seen in
embryonic forms or may be demonstrated by vestigial structures. The coccyx bone
in humans is a remnant (vestigial structure) of a post-anal tail. Other
characteristics of vertebrates include a mouth that is closed by a movable
lower jaw and a thyroid gland derived from the ventral wall of the pharynx. In
the invertebrate chordates, the endostyle is an evolutionary signpost pointing
to the development of the thyroid gland.

Viruses Viruses are not living. They are not cells and they do not exhibit the
characteristics of life as do cells. Viruses can reproduce only within living
cells. The virus particle is known as a virion. It merely consists of a protein
coat and a nucleic acid core. The protein coat, called a capsid, may be shaped
like a rod, be polyhedral, or have a tail with extending fibers. In some
viruses, including those that cause influenza, a cytoplasmic membrane surrounds
the protein coat. This surrounding envelope may come from the plasma membrane
of the host cell or may be synthesized by the host's cytoplasm. However,
virologists have found that the envelope contains proteins that are
virus-specific. Viral nucleic acid may be a single molecule consisting of as
few as five genes or may have as many as several hundred. Viral nucleic acid
may be single or it may be double-stranded; it may be circular or linear. Some
viral nucleic acid is made of DNA (deoxyribonucleic acid); others have only an
RNA (ribonucleic acid) core. Viruses never contain both DNA and RNA. See
Bacteriophage.

Vitamins Are organic compounds. They are classified as water soluble or fat
soluble. In general, the water soluble vitamins are coenzymes necessary to the
proper sequence of biochemical events that occur during cellular respiration.
It is interesting to note that the primates (Homo sapiens included) and guinea
pigs are the only vertebrate animals that cannot synthesize their own vitamin C
from carbohydrates. Therefore, the daily requirements of ascorbic acid must be
met through food intake. The functions of the fat soluble vitamins are not
clearly understood. For detailed information on vitamins, See Health and
Medicine.

Water, conservation of Water is rendered useless for drinking, bathing,
irrigation, and as a habitat for fish when polluted by the chemical wastes from
industry and by human sewage. Sewage treatment plants clean up sewage before it
is dumped into waterways. Special treatment must be given to chemical wastes to
detoxify them before disposal. Water is a renewable resource. However, people
on the earth are using more water than ever before in industry, refrigeration,
agriculture, and the like. Humans are dependent on rainfall to maintain an
adequate water table (level of groundwater) and to replenish water stores in
reservoirs. The wasting of water through careless use can have serious
consequences for human life.

Water cycle There are three ways in which water vapor enters the atmosphere.
Water evaporates from land surfaces and from the surfaces of all bodies of
water. Water vapor enters the air as a waste product of respiration of animals
and plants. For example: every time you exhale, water vapor is released into
the air. Great amounts of water are lost from plants through the openings in
the leaves; this water loss due to evaporation is called transpiration. Water
vapor in the air is carried to high altitudes where it is cooled and forms
clouds by condensation. Eventually, clouds fall to the earth in the form of
precipitation: rain, snow, or sleet. Most of the precipitation returns to the
oceans, lakes, or stream and less than 1 percent of it falls on land. Of the
water that does fall on land, about 25 percent of it will evaporate from the
various land surfaces before it can be absorbed by plants or used by animals.
Water that does not evaporate enters the soil and becomes available to plant
roots and soil organisms. Soil water that is not absorbed by plants seeps down
into the ground until it reaches an impervious layer of rock. The water moves
along this rock as groundwater until it reaches an outlet into a larger body of
water such as a lake or an ocean. The water cycle repeats.

White blood cells (Leucocytes) There are five types of white blood cells,
functioning to protect the body against invading foreign proteins. The
amoeboid-like neutrophils and monocytes behave as phagocytes, engulfing
bacteria and other foreign proteins. Eosinophils detoxify histamine-like
secretions. Lymphocytes participate in immune responses and basophils produce
anticoagulants.

Woody stems Stems have three major functions. First, they conduct water upward
from the roots to the leaves and conduct dissolved food materials downward from
the leaves to the roots. Second, stems produce and support leaves and flowers.
Third, they provide the mechanisms for the storage of food. Woody stems are
composed of primary and secondary tissues. Primary tissues are those that
develop from the meristems (embryonic tissue) of the buds on twigs during the
first year of growth. After the first year, growth in the woody stem takes
place in the secondary tissues. These are tissues that arise from the cambium.

Worm-Parasites
Diseases Caused by Diseases Caused by
Flatworms Roundworms
Tapeworm Infection Hookworm
Sheep Liver Fluke Trichinosis
Infection Ascaris Infection
Chinese Liver Fluke Pinworm Infection
Infection Filariasis

Xylem Water-carrying (vascular) tissue in plants that conducts water from the
roots upward.

Zoospores Free-swimming cells with flagella that are produced by asexual cell
division of the green algae Chlamydomonas. The zoospores mature, develop a
large chloroplast, and become vegetative Chlamydomonas cells.

Zygote Fertilized egg formed by the union of egg and sperm. The zygote has the
2N chromosome number. Following zygote formation, the cell goes into a series
of divisions known as cleavage.