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Old Wednesday, August 08, 2007
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Default Glossary of Biology

A

Abiogenesis: Early theory that held that some organisms originated from nonliving material.

Abnormal hemoglobin: Hemoglobin molecule with a different shape due to an altered amino acid sequence (ultimately caused by an altered DNA base sequence), such as in the inherited disease sickle-cell anemia.

Abscisic acid: A plant hormone that promotes dormancy in perennial plants and causes rapid closure of leaf stomata when a leaf begins to wilt.

Absolute time: One of the two types of geologic time (relative time being the other), with a definite age date established mostly by the decay of radioactive elements, although ages may also be obtained by counting tree rings, decay of a specific type of atom, or annual sedimentary layers (such as varves in lakes or layers in a glacier). The term is in some disfavor because it suggests an exactness that may not be possible to obtain.

Absorption: The process by which the products of digestion are transferred into the body's internal environment, enabling them to reach the cells.

Absorptive feeders: Animals such as tapeworms that ingest food through the body wall.

Acetylcholine: A chemical released at neuromuscular junctions that binds to receptors on the surface of the plasma membrane of muscle cells, causing an electrical impulse to be transmitted. The impulse ultimately leads to muscle contraction.

Acetyl CoA: An intermediate compound formed during the breakdown of glucose by adding a two-carbon fragment to a carrier molecule (Coenzyme A or CoA).

Acid: A substance that increases the number of hydrogen ions in a solution.

Acid rain: The precipitation of sulfuric acid and other acids as rain. The acids form when sulfur dioxide and nitrogen oxides released during the combustion of fossil fuels combine with water and oxygen in the atmosphere.

Acoelomates: Animals that do not have a coelom or body cavity; e.g., sponges and flatworms.

Acquired immunodeficiency syndrome (AIDS): A collection of disorders that develop as a result of infection by the human immunodeficiency virus (HIV), which attacks helper T cells, crippling the immune system and greatly reducing the body's ability to fight infection; results in premature death brought about by various diseases that overwhelm the compromised immune system.



Actin: The protein from which microfilaments are composed; forms the contractile filaments of sarcomeres in muscle cells.

Action potential: A reversal of the electrical potential in the plasma membrane of a neuron that occurs when a nerve cell is stimulated; caused by rapid changes in membrane permeability to sodium and potassium.

Active transport: Transport of molecules against a concentration gradient (from regions of low concentration to regions of high concentration) with the aid of proteins in the cell membrane and energy from ATP.



Adaptation: Tendency of an organism to suit its environment; one of the major points of Charles Darwin's theory of evolution by natural selection: organisms adapt to their environment. Those organisms best adapted will have a greater chance of surviving and passing their genes on to the next generation.

Adaptive radiation: The development of a variety of species from a single ancestral form; occurs when a new habitat becomes available to a population. Evolutionary pattern of divergence of a great many taxa from a common ancestral species as a result of novel adaptations or a recent mass extinction. Examples: mammals during the Cenozoic Era after the extinction of dinosaurs at the close of the Mesozoic Era flowering plants during the Cretaceous Period diversified because of their reproductive advantages over gymnosperm and non-seed plants that dominated the floras of the world at that time.

Adenine: One of the four nitrogen-containing bases occurring in nucleotides, the building blocks of the organic macromolecule group known as nucleic acids (DNA and RNA). Adenine is also the base in the energy carrying molecule ATP (adenosine triphosphate) which is the energy coin of the cell.



Adenosine diphosphate (ADP): Lower energy form of ATP, having two (instead of the three in ATP) phosphhate groups attached to the adenine base and ribose sugar.

Adenosine triphosphate (ATP): A common form in which energy is stored in living systems; consists of a nucleotide (with ribose sugar) with three phosphate groups. The energy coin of the cell.

Adhesion: The ability of molecules of one substance to adhere to a different substance.

Adrenocorticotropic hormone (ACTH): A hormone produced by the anterior pituitary that stimulates the adrenal cortex to release several hormones including cortisol.

Adventitious roots: Roots that develop from the stem following the death of the primary root. Branches from the adventitious roots form a fibrous root system in which all roots are about the same size; occur in monocots.

Age structure: The relative proportion of individuals in each age group in a population.

Aggregates: Fairly random associations of animals with little or no internal organization; form in response to a single stimulus and disperse when the stimulus is removed; one of the three broad classes of social organization.

Albinism: Genetic condition caused by the body's inability to manufacture pigments; an autosomal recessive trait.

Aldosterone: A hormone secreted by the adrenal glands that controls the reabsorption of sodium in the renal tubule of the nephron.

Alleles: Alternate forms of a gene.

Allergens: Antigens that provoke an allergic reaction.

Alpha decay: Type of radioactive decay in which a radioisotope emits a large but slow-moving particle consisting of two protons and two neutrons.

Alternation of generations: A life cycle in which a multicellular diploid stage is followed by a haploid stage and so on; found in land plants and many algae and fungi.

Altitudinal gradient: As altitude increases, a gradient of cooler, drier conditions occurs.

Alveoli: Tiny, thin-walled, inflatable sacs in the lungs where oxygen and carbon dioxide are exchanged.

Amensalism: A symbiotic relationship in which members of one population inhibit the growth of another population without being affected.

Amino acids: The subunits (monomers) from which proteins (polymers) are assembled. Each amino acid consists of an amino functional group, and a carboxyl acid group, and differs from other amino acids by the composition of an R group.



Amino acid sequence: Also known as the primary structure of a protein/polypeptide; the sequence of amino acids in a protein/polypeptide controlled by the sequence of DNA bases.

Amniocentesis: A method of prenatal testing in which amniotic fluid is withdrawn from the uterus through a needle. The fluid and the fetal cells it contains are analyzed to detect biochemical or chromosomal disorders.

Amniote egg: An egg with compartmentalized sacs (a liquid-filled sac in which the embryo develops, a food sac, and a waste sac) that allowed vertebrates to reproduce on land.

Amoebocytes: Amoeboid cells in sponges that occur in the matrix between the epidermal and collar cells. They transport nutrients.

Amphibians: Class of terrestrial vertebrates which lay their eggs (and also mate) in water but live on land as adults following a juvenile stage where they live in water and breathe through gills. Amphibians were the first group of land vertebrates; today they are mostly restricted to moist habitats.

Anabolic reactions: Reactions in cells in which new chemical bonds are formed and new molecules are made; generally require energy, involve reduction, and lead to an increase in atomic order.

Anaerobic: Refers to organisms that are not dependent on oxygen for respiration.

Analogous structures: Body parts that serve the same function in different organisms, but differ in structure and embryological development; e. g., the wings of insects and birds.

Anaphase: Phase of mitosis in which the chromosomes begin to separate.

Anaphylaxis: A severe allergic reaction in which histamine is released into the circulatory system; occurs upon subsequent exposure to a particular antigen; also called anaphylactic shock.

Androecium: Collective term applied to all of the male (stamen) parts of the flower.

Aneuploidy: Variation in chromosome number involving one or a small number of chromosomes; commonly involves the gain or loss of a single chromosome.

Angina: Chest pain, especially during physical exertion or emotional stress, that is caused by gradual blockage of the coronary arteries.

Angiosperms: Flowering plants. First appearing at least 110 million years ago from an unknown gymnosperm ancestor, flowering planbts have risen to dominance in most of the world's floras. The male gametophyte is 2-3 cells contained within a pollen grain; the female gametophyte is usually eight cells contained within an ovule which is retaind on the sporophyte phase of the plant's life cycle.

Animalia: Animal Kingdom. Multicellular eukaryotic group characterized by heterotrophic nutritional mode, usually organ and tissue development, and motility sometime during the organism's life history.

Annuals: Plants that grow and reproduce sexually during one year.

Antagonistic muscles: A pair of muscles that work to produce opposite effects&emdash;one contracts as the other relaxes: for example, the bicep and tricep muscles on opposite sides of your upper arm.

Anther: The top of a stamen's filament; divided into pollen sacs in which the pollen grains form.



Antibiotics: Substances produced by some microorganisms, plants, and vertebrates that kill or inhibit the growth of bacteria.

Antibiotic resistance: Tendency of certain bacteria to develop a resistance to commonly over-used antibiotics.

Antibodies: Proteins produced by immune system cells that bind to foreign molecules and microorganisms and inactivate them.

Antibody-mediated immunity: Immune reaction that protects primarily against invading viruses and bacteria through antibodies produced by plasma cells; also known as humoral immunity.

Anticodon: A sequence of three nucleotides on the transfer RNA molecule that recognizes and pairs with a specific codon on a messenger RNA molecule; helps control the sequence of amino acids in a growing polypeptide chain.



Antidiuretic hormone (ADH): and released by the A hormone produced by the hypothalamuspituitary gland that increases the permeability of the renal tubule of the nephron and thereby increases water reabsorption; also known as vasopressin.

Antigenic determinant: The site on an antigen to which an antibody binds, forming an antigen-antibody complex.

Antigens: Molecules carried or produced by microorganisms that initiate antibody production; mostly proteins or proteins combined with polysaccharides.

Antinutrients: Chemicals produced by plants as a defense mechanism; inhibit the action of digestive enzymes in insects that attack and attempt to eat the plants.

Anus: The posterior opening of the digestive tract.

Aorta: The artery that carries blood from the left ventricle for distribution throughout the tissues of the body. The largest diameter and thickest walled artery in the body.

Apical meristem: A meristem (embryonic tissue) at the tip of a shoot or root that is responsible for increasing the plant's length.

Apnea: A disorder in which breathing stops for periods longer than 10 seconds during sleep; can be caused by failure of the automatic respiratory center to respond to elevated blood levels of carbon dioxide.

Apocrine glands: Sweat glands that are located primarily in the armpits and groin area; larger than the more widely distributed eccrine glands.

Appendicular skeleton: The bones of the appendages (wings, legs, and arms or fins) and of the pelvic and pectoral girdles that join the appendages to the rest of the skeleton; one of the two components of the skeleton of vertebrates.

Appendix: Blind sac at the end of the large intestine that usually ruptures during final exams; a vestigial organ in humans.

Archaea: Proposed, but not widely accepted, sixth taxonomic kingdom that would include the archaebacteria.

Archaebacteria: Ancient (over 3.5 billion years old) group of prokaryotes; some biologists want to place this group into a separate Kingdom, the Archaea. Most currently place it within the Kingdom Monera.

Archaeocyathids: An extinct group of animals that were part of Cambrian-aged reef environments, but which were extinct by the close of the Cambrian Period.

Archean/Proterozoic Era: The period of time beginning 4.6 billion years ago with the formation of the Earth and ending 570 million years ago.

Aridity: The condition of receiving sparse rainfall; associated with cooler climates because cool air can hold less water vapor than warm air. Many deserts occur in relatively warm climates, however, because of local or global influences that block rainfall.

Arrector pili: A muscle running from a hair follicle to the dermis. Contraction of the muscle causes the hair to rise perpendicular to the skin surface, forming "goose pimples."

Arteries: Thick-walled vessels that carry blood away from the heart.

Arterioles: The smallest arteries; usually branch into a capillary bed.

Artificial selection: The process in which breeders choose the variants to be used to produce succeeding generations.

Ascomycetes: Division of fungi that contains the yeasts and morels; ascomycetes produce an ascus (or sac) in which ascospores are produced.

Ascus: Structure produced by sac fungi in which sexual ascospores develop.

Asexual reproduction: A method of reproduction in which genetically identical offspring are produced from a single parent; occurs by many mechanisms, including fission, budding, and fragmentation.

Assortment: A way in which meiosis produces new combinations of genetic information. Paternal and maternal chromosomes line up randomly during synapsis, so each daughter cell is likely to receive an assortment of maternal and paternal chromosomes rather than a complete set from either.

Aster: Short fibers produced by cells during mitosis and meiosis. These radiate from the centriole (if it is present).

Asteroid impacts: Hypothesis that links certain mass extinction events with the impact of a comet or asteroid, most notably the mass extinction 65 million years that caused the disappearance of dinosaurs and many other reptilian groups. Asteroid impacts early in earth history also contributed to the formation of the atmosphere and oceans.

Asthma: A respiratory disorder caused by allergies that constrict the bronchioles by inducing spasms in the muscles surrounding the lungs, by causing the bronchioles to swell, or by clogging the bronchioles with mucus.

Asymmetrical: In animals, a term referring to organisms that lack a general body plan or axis of symmetry that divides the body into mirror-image halves.

Atmosphere: The envelope of gases that surrounds the Earth; consists largely of nitrogen (78%) and oxygen (21%).

Atom: The smallest indivisible particle of matter that can have an independent existence.

Atomic number: The number of protons in the nucleus of an atom.

Atomic weight: The sum of the weights of an atom's protons and neutrons, the atomic weight differs between isotopes of the same element.

Atrioventricular (AV) node: Tissue in the right ventricle of the heart that receives the impulse from the atria and transmits it through the ventricles by way of the bundles of His and the Purkinje fibers.

Atrioventricular (AV) valve: The valve between each auricle and ventricle of the heart.

Auricle: The chamber of the heart that receives blood from the body returned to the heart by the veins. Also referred to as atrium.

Autonomic system: The portion of the peripheral nervous system that stimulates smooth muscle, cardiac muscle, and glands; consists of the parasympathetic and sympathetic systems.

Autosomes: The chromosomes other than the sex chromosomes. Each member of an autosome pair (in diploid organisms) is of similar length and in the genes it carries.

Autotrophic: Refers to organisms that synthesize their nutrients and obtain their energy from inorganic raw materials.

Autotrophs: Organisms that synthesize their own nutrients; include some bacteria that are able to synthesize organic molecules from simpler inorganic compounds.

Auxins: A group of hormones involved in controlling plant growth and other functions; once thought responsible for phototropism by causing the cells on the shaded side of a plant to elongate, thereby causing the plant to bend toward the light.

Axial skeleton: The skull, vertebral column, and rib cage; one of the two components of the skeleton in vertebrates.

Axillary buds: Buds borne in the axil (where the leaf meets the stem) of a stem.

Axons: Long fibers that carry signals away from the cell body of a neuron.
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Old Wednesday, August 08, 2007
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B

bacteriophages Viruses that attack and kill bacterial cells; composed only of DNA and protein.

bark The outer layer of the stems of woody plants; composed of an outer layer of dead cells (cork) and an inner layer of phloem.



Barr body Inactivated X-chromosome in mammalian females. Although inactivated, the Barr body is replicated prior to cell division and thus is passed on to all descendant cells of the embryonic cell that had one of its X-chromosomes inactivated.



barriers to gene flow Factors, such as geographic, mechanical, and behavioral isolating mechanisms that restrict gene flow between populations, leading to populations with differing allele frequencies.

basal body A structure at the base of a cilium or flagellum; consists of nine triplet microtubules arranged in a circle with no central microtubule.

base A substance that lowers the hydrogen ion concentration in a solution.

basidia Specialized club-shaped structures on the underside of club fungi (Basidiomycetes) within which spores form (sing.: basidium).

basidiomycetes The club fungi, a major group of fungi that all produce a structure (basidium) on which basidiospores are produced. Includes mushrooms and toadstools.



basidiospores The spores formed on the basidia of club fungi (Basidiomycetes).

B cells Type of lymphocyte responsible for antibody-mediated immunity; mature in the bone marrow and circulate in the circulatory and lymph systems where they transform into antibody-producing plasma cells when exposed to antigens.

benthic zone One of the two basic subdivisions of the marine biome; includes the sea floor and bottom-dwelling organisms.

beta decay Type of radioactive decay in which a radioisotope emits a small, negatively-charged and fast-moving particle from its nucleus. The beta particle is similar in size, charge, and speed to an electron and is formed when a neutron in the radioisotope's nucleus converts to a proton.

bicarbonate ions A weak base present in saliva that helps to neutralize acids in food.

big bang theory A model for the evolution of the universe that holds that all matter and energy in the universe were concentrated in one point, which suddenly exploded. Subsequently, matter condensed to form atoms, elements, and eventually galaxies and stars.

bilateral symmetry In animals, refers to those that have a single axis of symmetry.

biliary system The bile-producing system consisting of the liver, gallbladder, and associated ducts.

binary fission The method by which bacteria reproduce. The circular DNA molecule is replicated; then the cell splits into two identical cells, each containing an exact copy of the original cell's DNA.

binding sites Areas on the ribosome within which tRNA-amino acid complexes fit during protein synthesis.

binomial system of nomenclature A system of taxonomy developed by Linnaeus in the early eighteenth century. Each species of plant and animal receives a two-term name; the first term is the genus, and the second is the species.

biochemical cycle The flow of an element through the living tissue and physical environment of an ecosystem; e. g., the carbon, hydrogen, oxygen, nitrogen, sulfur, and phosphorus cycles.

biochemical reactions Specific chemical processes that occur in living things.

biochemistry Chemical processes associated with living things.

biodiversity Biological diversity; can be measured in terms of genetic, species, or ecosystem diversity.

biogeography The study of the distribution of plants and animals across the Earth.

bioluminescent Refers to organisms that emit light under certain conditions.

biomass The total weight of living tissue in a community.

biome A large-scale grouping that includes many communities of a similar nature.

biosphere All ecosystems on Earth as well as the Earth's crust, waters, and atmosphere on and in which organisms exist; also, the sum of all living matter on Earth.

birds Taxonomic class of terrestrial vertebrates that are characterized by endothermy and feathers; descended from some group of reptiles (or possibly dinosaurs).

birth rate The ratio between births and individuals in a specified population at a particular time.

bladder and expels it through the A hollow, distensible organ with muscular walls that stores urineurethra.

blastocoel The fluid-filled cavity at the center of a blastula.

blastocyst The developmental stage of the fertilized ovum by the time it is ready to implant; formed from the morula and consists of an inner cell mass, an internal cavity, and an outer layer of cells (the trophoblast).

blastula A ball of cells surrounding a fluid-filled cavity (the blastocoel) that is produced by the repeated cleavage of a zygote.



blending Term applied to 19th century belief that parental traits "blended" in their offspring; disproven by Mendel's work.

blood group or type One of the classes into which blood can be separated on the basis of the presence or absence of certain antigens; notably, the ABO types and the Rh blood group.

B memory cells Long-lived B cells that are produced after an initial exposure to an antigen and play an important role in secondary immunity. They remain in the body and facilitate a more rapid responce if the antigen is encountered again.

body fossil The actual remains (however permineralized, compressed or otherwise post-mortem altered) of an organism; includes bones, shells, and teeth.

bolus A mass of chewed food mixed with salivary secretions that is propelled into the espohagus during the swallowing phase of digestion.

bony fish A term applied collectively to all groups of fish with bony (as opposed to cartilaginous) skeletons.

bottlenecks Drastic short-term reductions in population size caused by natural disasters, disease, or predators; can lead to random changes in the population's gene pool.

brachiopods A phylum of hinge-shelled animals that have left an excellent fossil record; brachiopods live on or in the ocean floor.



brachydactly Human genetic disorder that causes production of an extra digit; an autosomal dominant trait.Sometimes referred to as polydactly.
brain The most anterior, most highly developed portion of the central nervous system.

brain stem and consists of the The portion of the brain that is continuous with the spinal cordmedulla oblongata and pons of the hindbrain and the midbrain.

bronchi Tubes that carry air from the trachea to the lungs (sing.: bronchus).

bronchioles Small tubes in the lungs that are formed by the branching of the bronchi; terminate in the alveoli.

bronchitis A respiratory disorder characterized by excess mucus production and swelling of the bronchioles; caused by long-term exposure to irritants such as cigarette smoke and air pollutants.

brown algae Multicellular protistans placed in the Division Phaeophyta, includes kelp.

brush border The collection of microvilli forming a border on the intestinal side of the epithelial cells of the small intestine.

bryophytes The nonvascular plants, characterized by life cycles dominated by the gametophyte phase. This group includes the mosses, liverworts, and hornworts, which lack lignified conducting tissues.

budding 1. Asexual production of new organisms; usually found in yeast; 2. the process by which HIV and similar viruses leave the cell (other than by lysing).

bud sports Buds that produce fruit that is different from the rest of the fruit on the tree; vegetatively propagated by grafting cuttings onto another plant.

buffers Chemicals that maintain pH values within narrow limits by absorbing or releasing hydrogen ions.

bulbourethral glands Glands that secrete a mucus-like substance that is added to sperm and provides lubrication during intercourse.

bursae Small sacs lined with synovial membrane and filled with synovial fluid; act as cushions to reduce friction between tendons and bones.
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C

calcitonin A hormone produced by the thyroid that plays a role in regulating calcium levels.

calcium carbonate
Chemical that also occurs in limestone and marble.

Calvin cycle
(aka Calvin-Benson Cycle or Carbon Fixation) Series of biochemical, enzyme-mediated reactions during which atmospheric carbon dioxide is reduced and incorporated into organic molecules, eventually some of this forms sugars. In eukaryotes, this occurs in the stroma of the chloroplast.

cambium
A lateral meristem in plants. Types of cambiums include vascular, cork, and intercalary.

Cambrian
Geologic period that begins the Paleozoic Era 570 million years ago. Marked in its beginning by a proliferation of animals with hard, preservable parts, such as brachiopods, trilobites, and archaeocyathids.

campodactyly
A dominant trait in which a muscle is improperly attached to bones in the little finger, causing the finger to be permanently bent.

capillaries
Small, thin-walled blood vessels that allow oxygen to diffuse from the blood into the cells and carbon dioxide to diffuse from the cells into the blood.

capillary bed
A branching network of capillaries supplied by arterioles and drained by venules.

capsid
The protein "shell" of a free virus particle.

capsule
1. Structure produced around certain bacteria; 2. Structure produced by the bryophytesporophyte that contains spores produced by meiosis.

carbohydrates
Organic molecules composed of carbon, hydrogen, and oxygen that serve as energy sources and structural materials for cells of all organisms.

cardiac cycle
One heartbeat; consists of atrial contraction and relaxation, ventricular contraction and relaxation, and a short pause.

cardiac muscle
The type of muscle that is found in the walls of the heart. Cardiac muscle is striated but branched, unlike the straight-shaped striated skeletal muscle cells.

cardiovascular system
The human circulatory system consisting of the heart and the vessels that transport blood to and from the heart.

carnivores
Term applied to a heterotroph, usually an animal, that eats other animals. Carnivores function as secondary, tertiary, or top consumers in food chains and food webs.

carotenoids
Major group of accessory pigments in plants; includes beta carotene.

carpals
The bones that make up the wrist joint.

carpels
The female reproductive structures of a flower; consisting of the ovary, style, and stigma.

carrageenan
Chemical extracted from red algae that is added to commercial ice creams as an emulsifying agent.

carrying capacity
The maximum population size that can be regularly sustained by an environment; the point where the population size levels off in the logistic growth model.

Casparian strip
In plants, an impermeable waxy layer between the cells of the endodermis that stops water and solutes from entering the xylem, except by passing through the cytoplasm of adjacent cells.



cast
Type of fossil preservation where the original material of the fossil has decayed and been replaced later by another material, much the way a plaster cast is made in a mold.

catabolic reactions
Reactions in cells in which existing chemical bonds are broken and molecules are broken down; generally produce energy, involve oxidation, and lead to a decrease in atomic order.

catastrophism
Once-popular belief that events in earth history had occurred in the past a sudden events and by processes unlike those operating today. Periods of catastrophic change were followed by long periods of little change. A subgroup, the Diluvialists, contended that Noah's Flood was the last of many floods which had occurred throughout earth history.

cell body
In a neuron, the part that contains the nucleus and most of the cytoplasm and the organelles.



cell cycle
The sequence of events from one division of a cell to the next; consists of mitosis (or division) and interphase.



cell-mediated immunity
Immune reaction directed against body cells that have been infected by viruses and bacteria; controlled by T cells.

cell plate
In plants, a membrane-bound space produced during cytokinesis by the vesicles of the Golgi apparatus. The cell plate fuses with the plasma membrane, dividing the cell into two compartments.

cells
The smallest structural units of living matter capable of functioning independently.



cell theory
One of the four (or five) unifying concepts in biology. The cell theory states that all living things are composed of at least one cell and that the cell is the fundamental unit of function in all organisms. Corollaries: the chemical composition of all cells is fundamentally alike; all cells arise from preexisting cells through cell division.

cellular respiration
The transfer of energy from various molecules to produce ATP; occurs in the mitochondria of eukaryotes, the cytoplasm of prokaryotes. In the process, oxygen is consumed and carbon dioxide is generated.

cellulose
A polysaccharide that is composed of unbranched chains of glucose; the major structural carbohydrate of plants, insoluble in water, and indigestible in the human intestine.

cell wall
Structure produced by some cells outside their cell membrane; variously composed of chitin, peptidoglycan, or cellulose.

Cenozoic Era
The period of geologic time beginning after the end of the Mesozoic Era 65 million years ago and encompassing the present. Commonly referred to as the age of mammals.

central nervous system (CNS)
The division of the nervous system that includes the brain and spinal cord.

centriole
Paired cellular organelle which functions in the organization of the mitotic spindle during cell division in eukaryotes.

centromere
A specialized region on each chromatid to which kinetochores and sister chromatids attach.

cephalization
The concentration of sensory tissues in the anterior part of the body (head).

cerebellum
That part of the brain concerned with fine motor coordination and body movement, posture, and balance; is part of the hindbrain and is attached to the rear portion of the brain stem.

cerebral cortex
The outer layer of gray matter in the cerebrum; consists mainly of neuronal cell bodies and dendrites in humans; associated with higher functions, including language and abstract thought.

cerebrum
The part of the forebrain that includes the cerebral cortex; the largest part of the human brain.

cervix
The lower neck of the uterus that opens into the vagina.

channels
Transport proteins that act as gates to control the movement of sodium and potassium ions across the plasma membrane of a nerve cell.

chemical equilibrium
The condition when the forward and reverse reaction rates are equal and the concentrations of the products remain constant.

chemiosmosis
The process by which ATP is produced in the inner membrane of a mitochondrion. The electron transport system transfers protons from the inner compartment to the outer; as the protons flow back to the inner compartment, the energy of their movement is used to add phosphate to ADP, forming ATP.



chemotrophs
Organisms (usually bacteria) that derive energy from inorganic reactions; also known as chemosynthetic.

chiasma
The site where the exchange of chromosome segments between homologous chromosomes takes place (crossing-over) (pl.: chiasmata).

chitin
A polysaccharide contained in fungi; also forms part of the hard outer covering of insects.

chlamydia
A sexually transmitted disease caused by a parasitic bacterium that lives inside cells of the reproductive tract.

chlorofluorocarbons (CFCs)
Chemical substances used in refrigerators, air conditioners, and solvents that drift to the upper stratosphere and dissociate. Chlorine released by CFCs reacts with ozone, eroding the ozone layer.

chlorophyll
The pigment in green plants that absorbs solar energy.

chlorophyll a
The green photosynthetic pigment common to all photosynthetic organisms.

chlorophyll b
An accessory chlorophyll found in green algae and plants.

chlorophyll c
An accessory chlorophyll found in some protistans.

Chlorophyta
The taxonomic division that contains what are commonly called the green algae.

chloroplasts
Disk-like organelles with a double membrane found in eukaryotic plant cells; contain thylakoids and are the site of photosynthesis. ATP is generated during photosynthesis by chemiosmosis.



cholecystokinin
A hormone secreted in the duodenum that causes the gallbladder to release bile and the pancreas to secrete lipase..

chorion
The two-layered structure formed from the trophoblast after implantation; secretes human chorionic gonadotropin.

chorionic villi sampling (CVS)
A method of prenatal testing in which fetal cells from the fetal side of the placenta (chorionic villi) are extracted and analyzed for chromosomal and biochemical defects.

chromatid
Generally refers to a strand of a replicated chromosome; consists of DNA and protein.

chromatin
A complex of DNA and protein in eukaryotic cells that is dispersed throughout the nucleus during interphase and condensed into chromosomes during meiosis and mitosis.

chromosomes
Structures in the nucleus of a eukaryotic cell that consist of DNA molecules that contain the genes.

chromosome theory of inheritance
Holds that chromosomes are the cellular components that physically contain genes; proposed in 1903 by Walter Sutton and Theodore Boveri.

Chrysophytes Protistan
division that is referred to as the golden brown algae; includes the diatoms.

cilia
Hair-like organelles extending from the membrane of many eukaryotic cells; often function in locomotion (sing.: cilium).

circadian rhythms
Biorhythms that occur on a daily cycle.

circulatory system
One of eleven major body organ systems in animals; transports oxygen, carbon dioxide, nutrients, and waste products between cells and the respiratory system and carries chemical signals from the endocrine system; consists of the blood, heart, and blood vessels.

circulatory system, closed
A system that uses a continuous series of vessels of different sizes to deliver blood to body cells and return it to the heart; found in echinoderms and vertebrates.



circulatory system, open
A system in which the circulating fluid is not enclosed in vessels at all times; found in insects, crayfish, some mollusks, and other invertebrates.

classes Taxonomic
subcategories of phyla.

clavicle
The collar bone.

cleavage furrow
A constriction of the cell membrane at the equator of the cell that marks the beginning of cytokinesis in animal cells. The cell divides as the furrow deepens.

climax community
The stage in community succession where the community has become relatively stable through successful adjustment to its environment.

clitoris
A short shaft with a sensitive tip located where the labia minora meet; consists of erectile tissue and is important in female sexual arousal.

clone
An exact copy of a DNA segment; produced by recombinant DNA technology.

closed community
A community in which populations have similar range boundaries and density peaks; forms a discrete unit with sharp boundaries.

codominance
A type of inheritance in which heterozygotes fully express both alleles.

codon
A sequence of three nucleotides in messenger RNA that codes for a single amino acid.

coelom
In animals, a body cavity between the body wall and the digestive system that forms during preadult development.

coelomates
Animals that have a coelom or body cavity lined with mesoderm.

coenzymes
Chemicals required by a number of enzymes for proper functioning; also known as enzyme cofactors.

cohesion
The force that holds molecules of the same substance together.

cohesion-adhesion theory
Describes the properties of water that help move it through a plant. Cohesion is the ability of water molecules to stick together (held by hydrogen bonds), forming a column of water extending from the roots to the leaves; adhesion is the ability of water molecules to stick to the cellulose in plant cell walls, counteracting the force of gravity and helping to lift the column of water.

collenchyma
One of the three major cell types in plants; are elongated and have thicker walls than parenchyma cells and are usually arranged in strands; provide support and are generally in a region that is growing.



colonial
1. Level of organization intermediate between unicellular and multicellular - organisms are composed of multiple cells but fail to exhibit specialization of those cells. Examples: Volvox, a colonial alga. 2. Term applied to organisms that occur in a fixed location, with one generation growing atop previous generations, as in coral reefs.

commensalism
A symbiotic relationship in which one species benefits and the other is not affected.

community
All species or populations living in the same area.

community age
One of the factors that helps cause the latitudinal diversity gradient. Tropical communities have had more time to evolve because they have been less disrupted by advancing ice sheets and other relatively recent climatic changes.

community simplification
The reduction of overall species diversity in a community; generally caused by human activity.

community succession
The sequential replacement of species in a community by immigration of new species and by local extinction of old ones.

compact bone
The outer dense layer that forms the shaft of the long bones; made up of concentric layers of mineral deposits surrounding a central opening.

companion cells
Specialized cells in the phloem that load sugars into the sieve elements and help maintain a functional plasma membrane in the sieve elements.



competition
One of the biological interactions that can limit population growth; occurs when two species vie with each other for the same resource.

competitive exclusion
Competition between species that is so intense that one species completely eliminates the second species from the area.

competitive release
Occurs when one of two competing species is removed from an area, thereby releasing the remaining species from one of the factors that limited its population size.

complementary nucleotides
The bonding preferences of nucleotides, Adenine with Thymine, and Cytosine with Guanine. Also referred to as complementary base pairing.

complement system
A chemical defense system that kills microorganisms directly, supplements the inflammatory response, and works with, or complements, the immune system.



complete dominance
The type of inheritance in which both heterozygotes and dominant homozygotes have the same phenotype.

complete flower
Condition in which all flower parts are present. Example: lily.

compound
A substance formed by two or more elements combined in a fixed ratio.

compound leaf
A leaf in which the blade forms small leaflets. Compound leaves that have several small leaflets originating from a central axis are termed pinnately compound; example: rose. Compound leaves that have their leaflets originating from a common point are termed palmately compound; example: palm.

compression
Type of fossilization in which the fossil is flattened (compressed)m by the weight of overlying sediment.

conditioned response
The response to a stimulus that occurs when an animal has learned to associate the stimulus with a certain positive or negative effect.

cones
Light receptors in primates' eyes that operate in bright light; provide color vision and visual acuity.

conifers
Group of gymnosperms that reproduce by cones and have needle-like leaves (in general); includes the pines.

connective tissue
Animal tissue composed of cells embedded in a matrix (gel, elastic fibers, liquid, or inorganic minerals). Includes loose, dense, and fibrous connective tissues that provide strength (bone, cartilage), storage (bone, adipose), and flexibility (tendons, ligaments).

consumers
The higher levels in a food pyramid; consist of primary consumers, which feed on the producers, and secondary consumers, which feed on the primary consumers.

continuous variation
Occurs when the phenotypes of traits controlled by a single gene cannot be sorted into two distinct phenotypic classes, but rather fall into a series of overlapping classes.

contractile vacuole
Organelle in many eukaryotes that acts as a bilge pump in the active transport of excess water from the cell.

contrast
In relation to microscopes, the ability to distinguish different densities of structures.

convergent evolution
The development of similar structures in distantly related organisms as a result of adapting to similar environments and/or strategies of life. Example: wings of birds and insects, the body shape of dolphins, sharks, and the extinct marine reptiles known as ichthyosaurs.

convergent plate boundary
The boundary between two plates that are moving toward one another.

coprolites
Fossilized feces.

cork
The outer layer of the bark in woody plants; composed of dead cells.

cork cambium
A layer of lateral meristematic tissue between the cork and the phloem in the bark of woody plants.

coronary arteries Arteries
that supply the heart's muscle fibers with nutrients and oxygen.

corpus callosum
Tightly bundled nerve fibers that connect the right and left hemispheres of the cerebrum.

corpus luteum
A structure formed from the ovulated follicle in the ovary; secretes progesterone and estrogen.

cortex
1) The outer part of an organ, e.g., the adrenal cortex, which produces several steroidhormones; 2) in plants, the region of the stem or root between the epidermis and the vascular bundle(s).



cortisol
The primary glucocorticoidhormone; released by the adrenal cortex.

cotyledon
A leaf-like structure that is present in the seeds of flowering plants; appears during seed germination and sometimes is referred to as a seed leaf.



countercurrent flow
An arrangement by which fish obtain oxygen from the water that flows through their gills. Water flows across the respiratory surface of the gill in one direction while blood flows in the other direction through the blood vessels on the other side of the surface.

courtship behavior
Behavioral sequences that precede mating.

covalent bond
A chemical bond created by the sharing of electrons between atoms.

The Cranial Nerves The cranial nerves are composed of twelve pairs of nerves that emanate from the nervous tissue of the brain. In order reach their targets they must ultimately exit/enter the cranium through openings in the skull. Hence, their name is derived from their association with the cranium. The function of the cranial nerves is for the most part similar to the spinal nerves, the nerves that are associated with the spinal cord.



cranium
The braincase; composed of several bones fitted together at immovable joints.

Cretaceous Period
The geologic period between the Jurassic Period (140 milliojn years ago) and the Tertiary Period (beginning 65 million years ago). The Cretaceous was marked by a mass extinction that closed the period along with the reign of the nonavian dinosaurs.

cristae
Structures formed by the folding of the inner membrane of a mitochondrion (sing.: crista).

crossing-over
During the first meioticprophase, the process in which part of a chromatid is physically exchanged with another chromatid to form chromosomes with new allele combinations.

crossopterygians
A type of lobe-finned fish with lungs that were ancestral to amphibians.

crustaceans
A large taxonomic class of arthropods that includes lobsters, shrimps, and crabs.

cuticle
A film composed of wax and cutin that occurs on the external surface of plant stems and leaves and helps to prevent water loss.

cyanobacteria
Blue-green bacteria; unicellular or filamentous chains of cells that carry out photosynthesis.

cycadeoids
A group of gymnosperm seed plants not closely rated to, but superficially similar to, the cycads. Cycads and cycadeoids were dominant floristic elements of early and middle Mesozoic landscapes. This groupo is also known as the Bennettitaleans.

cycads
Group of gymnosperm seed plants that have large fern-like leaves and reproduce by cones but not flowers.

cycle
A recurring sequence of events; e. g., the secretion of certain hormones at regular intervals.

cyclin
A protein found in the dividing cells of many organisms that acts as a control during cell division.

cystic fibrosis
An autosomalrecessive genetic disorder that causes the production of mucus that clogs the airways of the lungs and the ducts of the pancreas and other secretory glands.

cytokinesis
The division of the cytoplasm during cell division.

cytokinins
A group of hormones that promote cell division and inhibit aging of green tissues in plants.

cytology
The branch of biology dealing with cell structure.

cytoplasm
The viscous semiliquid inside the plasma membrane of a cell; contains various macromolecules and organelles in solution and suspension.

cytosine
One of the pyrimidine nitrogenous bases occurring in both DNA and RNA.

cytoskeleton
A three-dimensional network of microtubules and filaments that provides internal support for the cells, anchors internal cell structures, and functions in cell movement and division.

cytoxic T cells T cells
that destroy body cells infected by viruses or bacteria; also attack bacteria, fungi, parasites, and cancer cells and will kill cells of transplanted organs if they are recognized as foreign; also known as killer T cells.
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D

dark reactions The photosynthetic process in which food (sugar) molecules are formed from carbon dioxide from the atmosphere with the use of ATP; can occur in the dark as long as ATP is present.

death rate The ratio between deaths and individuals in a specified population at a particular time.

decay series Most radioisotopes do not decay into a stable daughter element in one single decay, but rather through a series of radioactive intermediaries.

deciduous Term applied to trees that lose the leaves and have a dormancy period at least once per year.

deletion The loss of a chromosome segment without altering the number of chromosomes.

dendrites Short, highly branched fibers that carry signals toward the cell body of a neuron.



dendrochronology The process of determining the age of a tree or wood used in structures by counting the number of annual growth rings.

deoxyribonucleic acid (DNA) A nucleic acid composed of two polynucleotide strands wound around a central axis to form a double helix; the repository of genetic information. Nucleic acid that functions as the physical carrier of inheritance for 99% of all species. The molecule is double-stranded and composed of two strands in an antiparallel and complementary arrangement. The basic unit, the nucleotide, consists of a molecule of deoxyribose sugar, a phosphate group, and one of four nitrogenous bases.



deoxyribose Five-carbon sugar found in nucleotides of DNA.



depth diversity gradient The increase in species richness with increasing water depth until about 2000 meters below the surface, where species richness begins to decline.

dermal system Plant organ system that provides the covering for the plant.

dermis One of the two layers of skin; a connective tissue layer under the epidermis containing elastic and collagen fibers, capillary networks, and nerve endings.

desert biome Characterized by dry conditions and plants and animals that have adapted to those conditions; found in areas where local or global influences block rainfall.

desmosome A circular region of membrane cemented to an adjacent membrane by a molecular glue made of polysaccharides; found in tissues that undergo stretching.

deuterostomes Animals in which the first opening that appears in the embryo becomes the anus while the mouth appears at the other end of the digestive system. Main groups include chordates and echinoderms.

Devonian Period of geologic time from 410 - 360 million years before the present. Life on land diversified, with the amphibians appearing late in this period. Plants underwent major changes, including the development of forests and seeds. In the water, fish diversified into all modern groups as well as numerous now-extinct forms.

diabetes mellitus, Types I and II A disorder associated with defects in insulin action. Type I diabetes is characterized by inadequate insulin secretion; Type II diabetes is characterized by impaired insulin secretion in response to elevated blood glucose levels or by loss of sensitivity to insulin by target cells.

diaphragm A dome-shaped muscle that separates the thoracic and abdominal cavities.

diastole The filling of the ventricle of the heart with blood.

diatomaceous earth Fossilized deposits of diatoms; used for abrasives, polishes and as a filtering agent.

dicots One of the two main types of flowering plants; characterized by having two cotyledons, floral organs arranged in cycles of four or five, and leaves with reticulate veins; include trees (except conifers) and most ornamental and crop plants.



dictyosomes produced on the Organelles in plant cells composed of a series of flattened membrane sacs that sort, chemically modify, and package proteinsrough endoplasmic reticulum. Also known as the Golgi Apparatus.

diencephalon Part of the forebrain; consists of the thalamus and hypothalamus.

diffusion The spontaneous movement of particles from an area of higher concentration to an area of lower concentration.

digestion The process of breaking down food into its molecular and chemical components so that these nutrient molecules can cross plasma membranes.

digestive system One of eleven major body organ systems in animals; converts food from the external environment into nutrient molecules that can be used and stored by the body and eliminates solid wastes; involves five functions: movement, secretion, digestion, absorption, and elimination.

dihybrid cross In genetics, a cross that involves two sets of characteristics.



dinoflagellates Single-celled to colonial protistans characterized by two flagella, one girdling the cell and the other trailing the cell. Some dinoflagellates exist in coral, in a symbiotic relationship. These dinoflagellates are termed the zooxanthellae. Other dinoflagellates occur in such high numbers that the water is colored red, a phenomenon known as a red tide.



dinosaurs Any of the Mesozoic diapsids (once considered to be reptiles) belonging to the groups designated as ornithischians and saurischians.

dioecious Term applied to plants having separate male and female plants.

diploid Cells that contain homologous chromosomes. The number of chromosomes in the cells is the diploid number and is equal to 2n (n is the number of homologous pairs).

directional selection A process of natural selection that tends to favor phenotypes at one extreme of the phenotypic range.

disaccharides 1.Sugars made up of two monosaccharides held together by a covalent bond; e.g., sucrose and lactose. 2. Type of sugar (saccharide) composed of two sugar molecules bonded together with an ester (covalent) bond examples include sucrose, maltose, and lactose.

discontinuous variation Occurs when the phenotypes of traits controlled by a single gene can be sorted into two distinct phenotypic classes.

disruptive selection A process of natural selection that favors individuals at both extremes of a phenotypic range.

distal tubule The section of the renal tubule where tubular secretion occurs.

divergent evolution The divergence of a single interbreeding population or species into two or more descendant species.

divergent plate boundary The boundary between two tectonic plates that are moving apart.

diversity The different types of organisms that occur in a community.

DNA hybridization The formation of hybrid DNA molecules that contain a strand of DNA from two different species. The number of complementary sequences in common in the two strands is an indication of the degree of relatedness of the species.

DNA ligase In recombinant DNA technology, an enzyme that seals together two DNA fragments from different sources to form a recombinant DNA molecule.

DNA polymerase In DNA replication, the enzyme that links the complementary nucleotides together to form the newly synthesized strand.

dominance The property of one of a pair of alleles that suppresses the expression of the other member of the pair in heterozygotes.

dominance hierarchy A social structure among a group of animals in which one is dominant and the others have subordinate nonbreeding positions.

dominant Refers to an allele of a gene that is always expressed in heterozygotes.

double fertilization carries two sperm cells to the female A characteristic of angiosperms in which a pollen tubegametophyte in the ovule. One sperm cell fuses with the egg cell and gives rise to a diploid embryo The other sperm cell fuses with the two polar cells to form a triploid cell that develops into the endosperm.

duodenum The upper part of the small intestine.

duplication An extra copy of a chromosome segment without altering the number of chromosomes.

dystrophin Protein making up only 0.002% of all protein in skeletal muscle but which appears vital for proper functioning of the muscle. Sufferers of muscular dystrophy appear to lack dystrophin.
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E

eccrine glands Sweat glands that are linked to the sympathetic nervous system and are widely distributed over the body surface.

ecological niche The role an organism occupies and the function it performs in an ecosystem; closely associated with feeding.

ecological time A timescale that focuses on community events that occur on the order of tens to hundreds of years.

ecology The study of how organisms interact with each other and their physical environment.

ecosystem The community living in an area and its physical environment.

ecotones Well-deŜned boundaries typical of closed communities.

ecotype A subdivision of a species; a stage in the formation of a species such that reproductive isolation has occurred.

ectoderm The outer layer of cells in embryonic development; gives rise to the skin, brain, and nervous system. Also, the outermost tissue layer in ŝatworms.

ectotherms Animals with a variable body temperature that is determined by the environment. Examples: fish, frogs, and reptiles.

effector In a closed system, the element that initiates an action in response to a signal from a sensor. In human systems, a muscle or gland often serves as an effector.

ejaculatory duct from each In males, a short duct that connects the vas deferenstestis to the urethra.

electron A subatomic particle with a negative charge. Electrons circle the atom's nucleus in regions of space known as orbitals.

electron acceptor A molecule that forms part of the electron transport system that transfers electrons ejected by chlorophyll during photosynthesis. Part of the energy carried by the electrons is transferred to ATP, part is transferred to NADPH, and part is lost in the transfer system.

electron transport 1) A series of coupled oxidation/reduction reactions where electrons are passed like hot potatoes from one membrane-bound protein/enzyme to another before being finally attached to a terminal electron acceptor (usually oxygen or NADPH). ATP is formed by this process. 2) coupled series of oxidation/reduction reactions during which ATP is generated by energy transfer as electrons move from high reducing state to lower reducing state.

electrostatic attraction The attraction between atoms of opposite charge that holds the atoms together in ionic bonds.

element A substance composed of atoms with the same atomic number; cannot be broken down in ordinary chemical reactions.

elongation During protein synthesis, the growth of the polypeptide chain through the addition of amino acids; the second step in translation.

embryo Term applied to the zygote after the beginning of mitosis that produces a multicellular structure.

embryo sac contained within a Alternate term applied to the angiosperm female gametophytemegaspore.

emphysema Lung disease characterized by shortness of breath, often associated with smoking.

endergonic Chemical reactions that require energy input to begin.
endochondral ossification The process by which human bones form from cartilage.

endocrine system One of eleven major body organ systems in animals; a system of glands that works with the nervous system in controlling the activity of internal organs, especially the kidneys, and in coordinating the long-range response to external stimuli.

endocytosis The incorporation of materials from outside the cell by the formation of vesicles in the plasma membrane. The vesicles surround the material so the cell can engulf it.

endoderm The inner layer of cells in embryonic development that gives rise to organs and tissues associated with digestion and respiration. Also, the inner tissue layer in ŝatworms.

endodermis A layer of cells surrounding the vascular cylinder of plants.



endometrium The inner lining of the uterus.

endoplasmic reticulum (ER) A network of membranous tubules in the cytoplasm of a cell; involved in the production of phospholipids, proteins, and other functions. Rough ER is studded with ribosomes; smooth ER is not.



endoskeleton An internal supporting skeleton with muscles on the outside; in vertebrates, consists of the skull, spinal column, ribs, and appendages.

endosperm A food storage tissue that provides nutrients to the developing embryo in angiosperms; formed from the triploid cell produced when a sperm cell fertilizes the central cell. Some endosperm is solid (as in corn), some is liquid (as in coconut).

endosymbiosis Theory that attempts to explain the origin of the DNA-containing mitochondria and chloroplasts in early eukaryotes by the engulfing of various types of bacteria that were not digested but became permanent additions to the ancestral "eukaryote".

endothermic A reaction that gives off energy. The product is in a lower energy state than the reactants.

endotherms Animals that have the ability to maintain a constant body temperature over a wide range of environmental conditions.

endothermy The internal control of body temperature; the ability to generate and maintain internal body heat.

energy The ability to bring about changes or to do work.

energy flow The movement of energy through a community via feeding relationships.

energy of activation The minimum amount of energy required for a given reaction to occur; varies from reaction to reaction.

entropy The degree of disorder in a system. As energy is transferred from one form to another, some is lost as heat; as the energy decreases, the disorder in the system&emdash;and thus the entropy&emdash;increases.
enzymes Protein molecules that act as catalysts in biochemical reactions.

eon The longest umit of geological time.

epidermis 1. The outermost layer of skin consisting of several layers of epithelial cells&emdash;notably, keratinocytes&emdash;and, in the inner layer of the epidermis, basal cells and melanocytes. 2. The outer layer of cells in the plant body, often covered by a waxy cuticle.

epididymis A long, convoluted duct on the testis where sperm are stored.

epiglottis A ŝap of tissue that closes off the trachea during swallowing.

epinephrine A hormone produced by the adrenal medulla and secreted under stress; contributes to the "Ŝght or ŝight" response.

epistasis The masking of the effects of one gene by the action of another, example: widow's peak masked by the baldness gene.

epithelial tissue Cells in animals that are closely packed in either single or multiple layers, and which cover both internal and external surfaces of the animal body. Also referred to as epithelium.



epoch Subdivision of a geological period.

eras One of the major divisions of the geologic time scale.

erythrocytes Red blood cells; doubly concave, enucleated cells that transport oxygen in the blood.

esophagus The muscular tube extending between and connecting the pharynx to the stomach.

estrogen A female sex hormone that performs many important functions in reproduction.

ethylene A gaseous plant hormone that stimulates fruit ripening and the dropping of leaves..

Eubacteria The subunit of the Monera that includes the true bacteria such as E. coli. One of the three major groups of prokaryotes in the Kingdom Monera. The eubacteria have cell walls containing peptidoglycan.

Euglenoids Term applied to a division of protozoans that have one long flagellum, no cell wall, and which may have chloroplasts.

eukaryote A type of cell found in many organisms including single-celled protists and multicellular fungi, plants, and animals; characterized by a membrane-bounded nucleus and other membraneous organelles; an organism composed of such cells. The first eukaryotes are encountered in rocks approximately 1.2-1.5 billion years old.

euphotic zone The upper part of the marine biome where light penetrates and photosynthesis occurs; usually extends to about 200 meters below the water surface.

eutrophication "Runaway" growth of aquatic plants that occurs when agricultural fertilizers containing phosphorus and nitrogen run off into lakes and ponds; also ultimately increases the plant death rate with the result that the bacterial decomposition of the dead plants uses up oxygen, causing Ŝsh and other organisms to suffocate.

evaporation The part of the hydrologic cycle in which liquid water is converted to vapor and enters the atmosphere.

evolution 1) The change in life over time by adaptation, variation, over-reproduction, and differential survival/reproduction, a process referred to by Charles Darwin and Alfred Wallace as natural selection. 2) Descent with modification.

evolutionary tree A diagram showing the evolutionary history of organisms based on differences in amino acid sequences. Organisms with fewer differences are placed closer together while those with more differences are further apart.

excretion The process of removing the waste products of cellular metabolism from the body.

excretory system One of eleven major body systems in animals; regulates the volume and molecular and ionic constitution of internal body ŝuids and eliminates metabolic waste products from the internal environment.

exine Outer covering of pollen grains, often containing sporopollenin, an acid-resistant polysaccharide that allows pollen grains to become fossils.

exocytosis The process in which a membrane-enclosed vesicle Ŝrst fuses with the plasma membrane and then opens and releases its contents to the outside.

exon The DNA bases that code for an amino acid sequence. Exons are separated by introns that code for no amino acid sequences.

exoskeleton A hard, jointed, external covering that encloses the muscles and organs of an organism; typical of many arthropods including insects.

exothermic A reaction where the product is at a higher energy level than the reactants.

exponential rate An extremely rapid increase, e.g., in the rate of population growth.

expression In relation to genes, the phenotypic manifestation of a trait. Expression may be age-dependent (e.g., Huntington disease) or affected by environmental factors (e.g., dark fur on Siamese cats).

extinction The elimination of all individuals in a group, both by natural (dinosaurs, trilobites) and human-induced (dodo, passenger pigeon, liberals) means.

extracellular digestion A form of digestion found in annelids, crustaceans, and chordates including vertebrates; takes place within the lumen of the digestive system, and the resulting nutrient molecules are transferred into the blood or body ŝuid.

extracellular route Path taken by water through the root in which water moves through the spaces between cell walls of the cortex parenchyma.

eyespot 1.A pigmented photoreceptor in euglenoids. The eyespot senses light and orients the cell for maximum rates of photosynthesis. 2. Term applied to a photosenstive area in starfish.
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F

families 1. In taxonomy, term applied to subcategories within orders. 2. Term applied to a group of similar things, such as languages, chromosomes, etc.

fats 1. Triglycerides that are solid at room temperature. 2. A legendary pool player from Minnesota?

fauna Term referring collectively to all animals in an area. The zoological counterpart of flora.

feces Semisolid material containing undigested foods, bacteria, bilirubin, and water that is produced in the large intestine and eliminated from the body. Frequently noted as "hitting the fan".

femur The upper leg bone.

fermentation The synthesis of ATP in the absence of oxygen through glycolysis.



fertilization The fusion of two gametes (sperm and ovum) to produce a zygote that develops into a new individual with a genetic heritage derived from both parents. Strictly speaking, fertilization can be divided into the fusion of the cells (plasmogamy) and the fusion of nuclei (karyogamy).

fibroblast A term applied to a cell of connective tissue that is separated from similar cells by some degree of matrix material; fibroblasts secrete elastin and collagen protein fibers.



fibrous root A root system found in monocots in which branches develop from the adventitious roots, forming a system in which all roots are about the same size and length.

filaments Slender, thread-like stalks that make up the stamens of a ŝower; topped by the anthers.

filter feeders Organisms such as sponges that feed by removing food from water that Ŝlters through their body.

filtration The removal of water and solutes from the blood; occurs in the glomerulus of the nephron.

first law of thermodynamics (conservation) Energy is neither created nor destroyed, it changes from one form to another.

fitness A measure of an individual's ability to survive and reproduce; the chance that an individual will leave more offspring in the next generation than other individuals.

flagella long, whip-like locomotion organelles found in both prokaryotic and eukaryotic cells; sing.: flagellum. Eukaryotic flagella have an internal arrangement of microtubules in a 9 + 2 array.

flame cell A specialized cell at the blind end of a nephridium that Ŝlters body ŝuids.

flora Term collectively applied to all of the plants in an area. The botanical counterpart of fauna.

flowers The reproductive structures in angiosperm sporophytes where gametophytes are generated.

fluid feeders Animals such as aphids, ticks, and mosquitoes that pierce the body of a host plant or animal and obtain food from ingesting its ŝuids.

fluid-mosaic Widely accepted model of the plasma membrane in which proteins (the mosaic) are embedded in lipids (the ŝuid).

follicles (ovary) Structures in the ovary consisting of a developing egg surrounded by a layer of follicle cells.

follicles (thyroid) Spherical structures that make up the thyroid gland; contain a gel-like colloid surrounded by a single layer of cells, which secrete thyroglobulin into the colloid.

follicle-stimulating hormone (FSH) A hormone secreted by the anterior pituitary that promotes gamete formation in both males and females.

fontanels Membranous areas in the human cranial bones that do not form bony structures until the child is 14 to 18 months old; know as "soft spots."

food chain The simplest representation of energy ŝow in a community. At the base is energy stored in plants, which are eaten by small organisms, which in turn are eaten by progressively larger organisms; the food chain is an oversimpliŜcation in that most animals do not eat only one type of organism.

food pyramid A way of depicting energy ŝow in an ecosystem; shows producers (mostly plants or other phototrophs) on the Ŝrst level and consumers on the higher levels.

food web A complex network of feeding interrelations among species in a natural ecosystem; more accurate and more complex depiction of energy ŝow than a food chain.

foraminifera Single-celled protists that secrete a shell or test. Accumulations of the shells of dead foraminifera and other microscopic sea creatures form chalk deposits.



forebrain The part of the brain that consists of the diencephalon and cerebrum.

fossil 1. The remains or traces of prehistoric life preserved in rocks of the Earth's crust. 2. Any evidence of past life.

fossil fuels Fuels that are formed in the Earth from plant or animal remains; e.g., coal, petroleum, and natural gas.

fossil record The observed remains of once-living organisms taken as a whole.

founder effect The difference in gene pools between an original population and a new population founded by one or a few individuals randomly separated from the original population, as when an island population is founded by one or a few individuals; often accentuates genetic drift.

fovea The area of the eye in which the cones are concentrated.

freshwater biome The aquatic biome consisting of water containing fewer salts than the waters in the marine biome; divided into two zones: running waters (rivers, streams) and standing waters (lakes, ponds).

frontal lobe The lobe of the cerebral cortex that is responsible for motor activity, speech, and thought processes.

fruit A ripened ovary wall produced from a flower.

fucoxanthin Brown accessory pigment found in and characteristic of the brown algae.

Fungi Nonmobile, heterotrophic, mostly multicellular eukaryotes, including yeasts and mushrooms.
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G

Gaia A hypothetical superorganism composed of the Earth's four spheres: the biosphere, hydrosphere, lithosphere, and atmosphere..

gametes Haploid reproductive cells (ovum and sperm).

gametophyte The haploid stage of a plant exhibiting alternation of generations, generates gametes by the process of mitosis.

ganglia Clusters of neurons that receive and process signals; found in ŝatworms and earthworms.

gap junctions Junctions between the plasma membranes of animal cells that allow communication between the cytoplasm of adjacent cells.

gastric pits The folds and grooves into which the stomach lining is arranged.

gastrin A hormone produced by the pyloric gland area of the stomach that stimulates the secretion of gastric acids.

gastroesophageal sphincter A ring of muscle at the junction of the esophagus and the stomach that remains closed except during swallowing to prevent the stomach contents from entering the esophagus.

gene pool The sum of all the genetic information carried by members of a population. Note: there is no diving in the deep end of the gene pool!

genera Taxonomic subcategories within families (sing.: genus), composed of one or more species.

genes SpeciŜc segments of DNA that control cell structure and function; the functional units of inheritance. Sequence of DNA bases usually code for a polypeptide sequence of amino acids.

gene therapy The insertion of normal or genetically altered genes into cells through the use of recombinant DNA technology; usually done to replace defective genes as part of the treatment of genetic disorders.

genetic code The linear series of nucleotides, read as triplets, that speciŜes the sequence of amino acids in proteins. Each triplet speciŜes an amino acid, and the same codons are used for the same amino acids in almost all life-forms, an indication of the universal nature of the code.

genetic divergence The separation of a population's gene pool from the gene pools of other populations due to mutation, genetic drift, and selection. Continued divergence can lead to speciation.

genetic drift Random changes in the frequency of alleles from generation to generation; especially in small populations, can lead to the elimination of a particular allele by chance alone.

genetic maps Diagrams showing the order of and distance between genes; constructed using crossover information.

genetics The study of the structure and function of genes and the transmission of genes from parents to offspring.

genital herpes A sexually transmitted disease caused by the herpes virus; results in sores on the mucus membranes of the mouth or genitals.

genome 1. The set of genes carried by an individual. 2. The set of genes shared by members of a reproductive unit such as a population or species.

genotype The genetic (alleleic) makeup of an organism with regard to an observed trait.

geographic isolation Separation of populations of a species by geographic means (distance, mountains, rivers, oceans, etc.) that lead to reproductive isolation of those populations.

geographic range The total area occupied by a population.

geological time The span of time that has passed since the formation of the Earth and its physical structures; also, a timescale that focuses on events on the order of thousands of years or more.

geotropism Plants' response to gravity: roots grow downward, showing positive geotropism, while shoots grow upward in a negative response.



germ cells Collective term for cells in the reproductive organs of multicellular organisms that divide by meiosis to produce gametes.

gestation Period of time between fertilization and birth of an animal. Commonly called pregnancy.

gibberellins A group of hormones that stimulate cell division and elongation in plants. Gibberellic acid (GA), the first of this class to be discovered, causes bolting (extreme elongation) of stems. GA is also applied to certain plants to promote larger fruits.

gill slits Opening or clefts between the gill arches in Ŝsh. Water taken in by the mouth passes through the gill slits and bathes the gills. Also, rudimentary grooves in the neck region of embryos of air-breathing vertebrates such as humans; a characteristic of chordates.

ginkgos Group of seed plants today restricted to a single genus (Ginkgo biloba); ginkgos were more diverse during the Mesozoic Era.

glial cells Nonconducting cells that serve as support cells in the nervous system and help to protect neurons.

glomerulus A tangle of capillaries that makes up part of the nephron; the site of Ŝltration.

glucagon A hormone released by the pancreas that stimulates the breakdown of glycogen and the release of glucose, thereby increasing blood levels of glucose. Glucagon and insulin work together to maintain blood sugar levels.

glucocorticoids A group of steroid hormones produced by the adrenal cortex that are important in regulating the metabolism of carbohydrates, fats, and proteins.

glucose A six-carbon single sugar; the most common energy source.



glycogen Polysaccharide consisting of numerous monosaccharide glucoses linked together. The animal equivalent of starch.

glycolipids Polysaccharides formed of sugars linked to lipids, a part of the cell membrane.

glycolysis The universal cellular metabolic process in the cell's cytoplasm where 6-carbon glucose is split into two 3-carbon pyruvate molecules, and some ATP and NADH are produced.

glycoproteins Polysaccharides formed of sugars linked to proteins. On the outer surface of a membrane, they act as receptors for molecular signals originating outside the cell.

gnetales Group of seed plants restricted to three genera today (Gnetum, Ephedra, and Welwitschia); the possible outgroup for flowering plants.

golden brown algae Common name applied to the protistan division Chrysophyta.

Golgi complex Organelles in animal cells composed of a series of ŝattened sacs that sort, chemically modify, and package proteins produced on the rough endoplasmic reticulum.



gonadotropin-releasing hormone (GnRH) A hormone produced by the hypothalamus that controls the secretion of luteinizing hormone.

gonadotropins Hormones produced by the anterior pituitary that affect the testis and ovary; include follicle-stimulating hormone and luteinizing hormone.

gonads The male and female sex organs.

Gondwana Name applied to the ancient (Paleozoic-early Mesozoic) southern hemisphere supercontinent that rifted apart to form present-day Antarctica, India, Africa, Australia, and South America. The southern part of Pangaea.

gonorrhea A sexually transmitted disease that is caused by a bacterium that inŝames and damages epithelial cells of the reproductive system.

grana A series of stacked thylakoid disks containing chlorophyll; found in the inner membrane of chloroplasts.

grasslands biome Occurs in temperate and tropical regions with reduced rainfall or prolonged dry seasons; characterized by deep, rich soil, an absence of trees, and large herds of grazing animals.

green algae Common name for algae placed in the division Chlorophyta.

greenhouse effect The heating that occurs when gases such as carbon dioxide trap heat escaping from the Earth and radiate it back to the surface; so-called because the gases are transparent to sunlight but not to heat and thus act like the glass in a greenhouse.

ground system Plant tissue system, composed mainly of parenchyma cells with some collenchyma and sclerenchyma cells, that occupies the space between the epidermis and the vascular system; is involved in photosynthesis, water and food storage, and support; one of the four main tissue systems in plants.

growth hormone (GH) A peptide hormone produced by the anterior pituitary that is essential for growth.

growth rings Features of woody stems produced by plants growing in areas with seasonal (as opposed to year-long) growth. The growth ring marks the position of the vascular cambium at the cessation of the previous year's growth.

guard cells Specialized epidermal cells that flank stomates and whose opening and closing regulates gas exchange and water loss.

guanine One of the nitrogenous bases in nucleic acids, guanine is one of the two purine bases.



gymnosperms Flowerless, seed-bearing land plants; the Ŝrst seed plants; living groups include the pines, ginkgos, and cycads. Naked seeds.

gynoecium Collective term for all of the carpels (or pistils) in a flower. Some flowers have many pistils that are partially or wholly fused.
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H

habitat disruption A disturbance of the physical environment in which a population lives.

hair bulb The base of a hair; contains cells that divide mitotically to produce columns of hair cells.

hair root The portion of a hair that extends from the skin's surface to the hair bulb.

hair shaft The portion of a hair that extends above the skin's surface.

half-life The time required for one-half of an original unstable radioactive element to be converted to a more stable daughter element.

halophiles A group of archaebacteria that are able to tolerate high salt concentrations.

haploid Cells that contain only one member of each homologous pair of chromosomes (haploid number = n). At fertilization, two haploid gametes fuse to form a single cell with a diploid number of chromosomes.

hardwoods Term applied to dicot trees, as opposed to softwoods, a term applied to gymnosperms.

Haversian canal The central opening of compact bone; contains nerves and blood vessels.



heart The multicellular, chambered, muscular structure that pumps blood through the circulatory system by alternately contracting and relaxing.

heartwood Inner rings of xylem that have become clogged with metabolic by-products and no longer transport water; visible as the inner darker areas in the cross section of a tree trunk.

helper T cells A type of lymphocyte that stimulates the production of antibodies by activating B cells when an antigen is present.

hemizygous Having one or more genes that have no allele counterparts. Usually applied to genes on the male's X chromosome (in humans).

hemoglobin A red pigment in red blood cells that can bind with oxygen and is largely responsible for the blood's oxygen-carrying capacity. Hemoglobin is composed of four polypeptide chains, two alpha (a) and two beta (b) chains.

hemophilia A human sex-linked recessive genetic disorder that results in the absence of certain blood-clotting factors, usually Factor VII. Hemophiliacs suffer from an inability to clot their blood.

hepatitis B A potentially serious viral disease that affects the liver; can be transmitted through sexual contact or through contact with infected blood.

herbaceous Term applied to a nonwoody stem/plant with minimal secondary growth.

herbivores Term pertaining to a heterotroph, usually an animal, that eats plants or algae. Herbivores function in food chains and food webs as primary consumers.

heterogametic sex The sex with two different chromosomes, such as males in humans and Drosophila.

heterotrophic Refers to organisms, such as animals, that depend on preformed organic molecules from the environment (or another organism) as a source of nutrients/energy.

heterotrophs Organisms that obtain their nutrition by breaking down organic molecules in foods; include animals and fungi.

heterozygous Having two different alleles (one dominant, one recessive) of a gene pair.

histamine A chemical released during the inŝammatory response that increases capillary blood ŝow in the affected area, causing heat and redness.

histone proteins Proteins associated with DNA in eukaryote chromosomes.

homeobox genes Pattern genes that establish the body plan and position of organs in response to gradients of regulatory molecules.

homeostasis The ability to maintain a relatively constant internal environment.

hominid Primate group that includes humans and all fossil forms leading to man only.

hominoid Primate group that includes common ancestors of humans and apes.

homologous structures Body parts in different organisms that have similar bones and similar arrangements of muscles, blood vessels, and nerves and undergo similar embryological development, but do not necessarily serve the same function; e.g., the ŝipper of a whale and the forelimb of a horse.

homologues A pair of chromosomes in which one member of the pair is obtained from the organism's maternal parent and the other from the paternal parent; found in diploid cells. Also commonly referred to as homologous chromosomes.

homozygous Having identical alleles for a given gene.

hormones Chemical substances that are produced in the endocrine glands and travel in the blood to target organs where they elicit a response.

human chorionic gonadotropin (hCG) A peptide hormone secreted by the chorion that prolongs the life of the corpus luteum and prevents the breakdown of the uterine lining.

Human Genome Project base sequence of every gene in the human Federally funded project to determine the DNAgenome.

human immunodeficiency virus (HIV) The retrovirus that attacks T-cells in the human immune system, destroying the body's defenses and allowing the development of AIDS.

Huntington disease A progressive and fatal disorder of the nervous system that develops between the ages of 30 and 50 years; caused by an expansion of a trinucleotide repeat and inherited as a dominant trait.

hydrogen bond A weak bond between two atoms (one of which is hydrogen) with partial but opposite electrical charges.

hydrophilic Water-loving. Term applied to polar molecules that can form a hydrogen bond with water.

hydrophobic Water-fearing.Term applied to nonpolar molecules that cannot bond with water.

hydrophytic leaves The leaves of plants that grow in water or under conditions of abundant moisture.

hydrosphere The part of the physical environment that consists of all the liquid and solid water at or near the Earth's surface.

hydrostatic skeleton Fluid-Ŝlled closed chambers that give support and shape to the body in organisms such as jellyŜsh and earthworms. No to be confused with the water-vascular system of echinoderms.

hypertension High blood pressure; blood pressure consistently above 140/90.

hypertonic A solution having a high concentration of solute.

hyphae The multinucleate or multicellular Ŝlaments that make up the mycelium (body) of a fungus (sing.: hypha).

hypothesis An idea that can be experimentally tested; an idea with the lowest level of confidence.

hypothalamus functions aimed at maintaining A region in the brain beneath the thalamus; consists of many aggregations of nerve cells and controls a variety of autonomichomeostasis.



hypotonic A solution having a low concentration of solute.
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ice age Interval of geologic time between 2 million and 10,000 years ago during which the northern hemisphere experienced several episodes of continental glacial advance and retreat along with a climatic cooling. The icing over of Antarctica was also completed during this time.

ileum The third and last section of the small intestine.

immovable joint A joint in which the bones interlock and are held together by Ŝbers or bony processes that prevent the joint from moving; e.g., the bones of the cranium.

immune system One of the eleven major body organ systems in vertebrates; defends the internal environment against invading microorganisms and viruses and provides defense against the growth of cancer cells.

immunoglobulins The Ŝve classes of protein to which antibodies belong (IgD, IgM, IgG, IgA, IgE).



implantation The process in which the blastocyst embeds in the endometrium.

incomplete dominance has a A type of inheritance in which the heterozygotephenotype intermediate to those of the homozygous parents.



incomplete flower Condition in which one or more "typical" flower parts are absent. Example: grass flowers such as corn tassels which are male.

incus One of the three bones comprising the middle ear of mammals.

inflammation A reaction to the invasion of microorganisms through the skin or through the epithelial layers of the respiratory, digestive, or urinary system; characterized by four signs: redness, swelling, heat, and pain.

inflammatory response The body's reaction to invading infectious microorganisms; includes an increase in blood ŝow to the affected area, the release of chemicals that draw white blood cells, an increased ŝow of plasma, and the arrival of monocytes to clean up the debris.

ingestive feeders Animals that ingest food through a mouth.

inheritance of acquired characteristics Lamarck's view that features acquired during an organism's lifetime would be passed on to succeeding generations, leading to inheritable change in species over time.

initiation molecule, a The Ŝrst step in translation; occurs when a messenger RNAribosomal subunit, and a transfer RNA molecule carrying the Ŝrst amino acid bind together to form a complex; begins at the start codon on mRNA.

initiation codon (AUG) Three-base sequence on the messenger RNA that codes for the amino acid methionine; the start command for protein synthesis.

insertion A type of mutation in which a new DNA base is inserted into an existing sequence of DNA bases. This shifts the reference frame in protein synthesis, resulting (sometimes) in altered amino acid sequences.

insulin A hormone secreted by the pancreas that stimulates the uptake of glucose by body cells. Insulin works antagonistically with glucagon to control blood sugar levels.

integration The process of combining incoming information; one of the functions of the nervous system.

integument Something that covers or encloses, e.g., the skin.

integumentary system The skin and its derivatives (hair, nails, feathers, horns, antlers, and glands), which in multicellular animals protect against invading foreign microorganisms and prevent the loss or exchange of internal ŝuids.

interferons Proteins released by cells in response to viral infection; activate the synthesis and secretion of antiviral proteins.

internal environment In multicellular organisms, the aqueous environment that is outside the cells but inside the body.

interneurons Neurons that process signals from one or more sensory neurons and relay signals to motor neurons. Aka connector neurons.

internodes The stem regions between nodes in plants.

interphase The period between cell divisions when growth and replacement occur in preparation for the next division; consists of gap 1 (G1), synthesis (S), and gap 2 (G2).

interstitial Being situated within a particular organ or tissue.

interstitial fluid Fluid surrounding the cells in body tissues; provides a path through which nutrients, gases, and wastes can travel between the capillaries and the cells.

intracellular digestion A form of digestion in which food is taken into cells by phagocytosis; found in sponges and most protozoa and coelenterates.

intracellular parasites Viruses that enter a host cell and take over the host's cellular machinery to produce new viral particles.

intracellular route Path taken by water through the cells of the root between the epidermis and the xylem, moving through plasmodesmata.

intron In eukaryotes, bases of a gene transcribed but later excised from the mRNA prior to exporting from the nucleus and subsequent translation of the message into a polypeptide.



inversion A reversal in the order of genes on a chromosome segment.
ion An atom that has lost or gained electrons from its outer shell and therefore has a positive or negative charge, respectively; symbolized by a superscript plus or minus sign and sometimes a number, e.g., H+, Na+1, Cl-2.

ionic bond A chemical bond in which atoms of opposite charge are held together by electrostatic attraction.

isotonic Term applied to two solutions with equal solute concentrations.

isotopes Atoms with the same atomic number but different numbers of neutrons; indicated by adding the mass number to the element's name, e.g., carbon 12 or 12C.
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jejunum The second portion of the small intestine. Also, a popular month for weddings!

Jurassic Period Middle period of the Mesozoic Era, between 185-135 million years ago. Characterized by the (possible) origin of angiosperms and the continued split of the worldwide supercontinent of Pangaea.

karyotype The chromosomal characteristics of a cell; also, a representation of the chromosomes aligned in pairs.

keratin A Ŝbrous protein that Ŝlls mature keratinocytes near the skin's surface.

keratinocytes The basic cell type of the epidermis; produced by basal cells in the inner layer of the epidermis.

kidney stones Crystallized deposits of excess wastes such as uric acid, calcium, and magnesium that may form in the kidney.

kilocalorie The energy needed to heat 1000 grams of water from 14.5 to 15.5 degrees C.

kinetochores Structures at the centromeres of the chromosomes to which the Ŝbers of the mitotic spindle connect.



kingdoms Five broad taxonomic categories (Monera, Protista, Plantae, Fungi, Animalia) into which organisms are grouped, based on common characteristics.

Klinefelter syndrome In humans, a genetically determined condition in which the individual has two X and one Y chromosome. Affected individuals are male and typically tall and infertile.

Kreb's cycle Biochemical cycle in cellular aerobic metabolism where acetyl CoA is combined with oxaloacetate to form citric acid; the resulting citric acid is converted into a number of other chemicals, eventually reforming oxaloacetate; NADH, some ATP, and FADH2 are produced and carbon dioxide is released.

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