Abdomen (ab´ duh mun) [L. belly] • In arthropods, the posterior segments of the body; in mammals, the part of the body containing the intestines and most other internal organs, posterior to the thorax.
Abdomen (ab´ duh mun) [L. belly] • In arthropods, the posterior segments of the body; in mammals, the part of the body containing the intestines and most other internal organs, posterior to the thorax.
Abiotic (a bye ah tick) [Gk. a: not + bios: life] • Nonliving.
Abscisic acid (ab sighs´ ik) [L. abscissio: breaking off] • A plant growth substance having growth-inhibiting action. Causes stomata to close.
Abscission (ab sizh´ un) [L. abscissio: breaking off] • The process by which leaves, petals, and fruits separate from a plant.
Absolute temperature scale • Also known as the Kelvin scale. A temperature scale in which zero is the state of no molecular motion, or "absolute zero" (-273° on the Celsius scale).
Absorption • (1) Of light: complete retention, without reflection or transmission. (2) Of liquids: soaking up (taking in through pores or cracks).
Absorption spectrum • A graph of light absorption versus wavelength of light; shows how much light is absorbed at each wavelength.
Abyssal zone (uh biss´ ul) [Gr. abyssos: bottomless] • The deep ocean, below the point that light can penetrate.
Accessory pigments • Pigments that absorb light and transfer energy to chlorophylls for photosynthesis.
Acetylcholine • A neurotransmitter substance that carries information across vertebrate neuromuscular junctions and some other synapses.
Acetylcholinesterase • An enzyme that breaks down acetylcholine.
Acetyl CoA (acetyl coenzyme A) • Compound that reacts with oxaloacetate to produce citrate at the beginning of the citric acid cycle; a key metabolic intermediate in the formation of many compounds.
Acid [L. acidus: sharp, sour] • A substance that can release a proton in solution. (Contrast with base.)
Acid precipitation • Precipitation that has a lower pH than normal as a result of acid-forming precursors introduced into the atmosphere by human activities.
Acidic • Having a pH of less than 7.0 (a hydrogen ion concentration greater than 10-7 molar).
Acoelomate • Lacking a coelom.
Acquired Immune Deficiency Syndrome • See AIDS.
Acrosome (a´ krow soam) [Gr. akros: highest + soma: body] • The structure at the forward tip of an animal sperm which is the first to fuse with the egg membrane and enter the egg cell.
ACTH (adrenocorticotropin) • A pituitary hormone that stimulates the adrenal cortex.
Actin [Gr. aktis: a ray] • One of the two major proteins of muscle; it makes up the thin filaments. Forms the microfilaments found in most eukaryotic cells.
Action potential • An impulse in a neuron taking the form of a wave of depolarization or hyperpolarization imposed on a polarized cell surface.
Action spectrum A graph of a biological process versus light wavelength; shows which wavelengths are involved in the process.
Activating enzymes • Also called aminoacyl-tRNA synthetases; these enzymes catalyze the addition of amino acids to their appropriate tRNAs.
Activation energy (Ea) • The energy barrier that blocks the tendency for a set of chemical substances to react.
Active site • The region on the surface of an enzyme where the substrate binds, and where catalysis occurs.
Active transport • The energy-dependent transport of a substance across a biological membrane against a concentration gradient-that is, from a region of low concentration (of that substance) to a region of high concentration. (See primary active transport, secondary active transport; contrast with facilitated diffusion.)
Adaptation (a dap tay´ shun) • In evolutionary biology, a particular structure, physiological process, or behavior that makes an organism better able to survive and reproduce. Also, the evolutionary process that leads to the development or persistence of such a trait.
Adenine(a´ den een) • A nitrogen-containing base found in nucleic acids, ATP, NAD, etc.
Adenosine triphosphate • See ATP.
Adenylate cyclase • Enzyme catalyzing the formation of cyclic AMP (cAMP) from ATP.
Adrenal (a dree´ nal) [L. ad-: toward + renes: kidneys] • An endocrine gland located near the kidneys of vertebrates, consisting of two glandular parts, the cortex and medulla.
Adrenaline • See epinephrine.
Adrenocorticotropin • See ACTH.
Adsorption • Binding of a gas or a solute to the surface of a solid.
Aerobic (air oh´ bic) [Gr. aer: air + bios: life] • In
the presence of oxygen; requiring oxygen.
Afferent (af´ ur unt) [L. ad: to + ferre: to bear] • Carrying to, as in a neuron that carries impulses to the central nervous system, or a blood vessel that carries blood to a structure. (Contrast with efferent.)
AIDS (acquired immune deficiency syndrome) • ondition caused by a virus (HIV) in which the body's helper T lymphocytes are reduced, leaving the victim subject to opportunistic diseases.
Aldehyde (al´ duh hide) • A compound with a -CHO functional group. Many sugars are aldehydes. (Contrast with ketone.)
Aldosterone (al dahs´ ter own) • A steroid hormone produced in the adrenal cortex of mammals. Promotes secretion of potassium and reabsorption of sodium in the kidney.
Alga (al´ gah) (plural: algae) [L.: seaweed] • Any one of a wide diversity of mostly photosynthetic protests.
Allantois (al lan to is) • A sac-like extraembryonic membrane that contains nitrogen waste from embryo.
Allele (a leel´) [Gr. allos: other] • he alternate forms of a genetic character found at a given locus on a chromosome.
Allele frequency • The relative proportion of a particular allele in a specific population.
Allergy [Ger. allergie: altered reaction] • An overreaction to amounts of an antigen that do not affect most people; often involves IgE antibodies.
Allometric growth • A pattern of growth in which some parts of the body of an organism grow faster than others, resulting in a change in body proportions as the organism grows.
Allopatric speciation (al´ lo pat´ rick) [Gr. allos: other + patria: fatherland] • Also called geographical speciation, this is the formation of two species from one when reproductive isolation occurs because of the interposition of (or crossing of) a physical geographic barrier such as a river. (Contrast with parapatric speciation, sympatric speciation.)
Allostery (al´ lo steer´ y) [Gr. allos: other + stereos: structure] • Regulation of the activity of a protein by the binding of an effector molecule at a site other than the active site.
Alpha helix • Type of protein secondary structure; a right-handed spiral.
Alternation of generations • The succession of multicellular haploid and diploid phases in some sexually reproducing organisms, notably plants.
Alturism • Behavior that harms the individual who performs it but benefits other individuals.
Alveolus (al ve´ o lus) (plural: alveoli) [L. alveus: cavity] • A small, baglike cavity, especially the blind sacs of the lung.
Ambient That which surrounds; the immediate environment.
Amensalism (a men´ sul ism) • Interaction in which one animal is harmed and the other is unaffected. (Contrast with commensalism, mutualism.)
Amine • An organic compound with an amino group (see Amino acid).
Amino acid • An organic compound containing both NH2 and COOH groups. Proteins are polymers of amino acids.
Ammonotelic (am moan´ o teel´ ic) [Gr. telos: end] • Describes an organism in which the final product of breakdown of nitrogen-containing compounds (primarily proteins) is ammonia. (Contrast with ureotelic, uricotelic.)
Amniocentesis • A medical procedure in which cells from the fetus are obtained from the amniotic fluid. The genetic material of the cells is then examined.
Amnion(am nee on) • The fluid-filled sac in which the embryos of reptiles, birds, and mammals develop.
Amniote • Any of the vertebrate animals whose embryos are enclosed in an amnion: reptiles, birds, and mammals.
Amniote egg • A shelled egg surrounding four extraembryonic membranes and embryo-nourishing yolk. This adaptation allowed animals to colonize the terrestrial environment.
Amphipathic (am´ fi path´ ic) [Gr. amphi: both + pathos: emotion] • Of a molecule, having both hydrophilic and hydrophobic regions.
Amylase (am´ ill ase) • Any of a group of enzymes that digest starch.
Anabolism (an ab´ uh liz´ em) [Gr. ana: up, throughout + ballein: to throw] • Synthetic reactions of metabolism, in which complex molecules are formed from simpler ones. (Contrast with catabolism.)
Anaerobic (an ur row´ bic) [Gr. an: not + aer: air + bios: life] • Occurring without the use of molecular oxygen, O2.
Anagenesis • Evolutionary change in a single lineage over time.
Analogy (a nal´ o jee) [Gr. analogia: resembling] • A resemblance in function, and often appearance as well, between two structures that is due to convergent evolution rather than to common ancestry. (Contrast with homology.)
Anaphase (an´ a phase) [Gr. ana: upward progress] • The stage in nuclear division at which the first separation of sister chromatids (or, in the first meiotic division, of paired homologues) occurs.
Anaphylactic shock • A precipitous drop in blood pressure caused by loss of fluid from capillaries because of an increase in their permeability stimulated by an allergic reaction.
Ancestral trait • Trait shared by a group of organisms as a result of descent from a common ancestor.
Androgens (an´ dro jens) • The male sex steroids.
Aneuploidy (an´ you ploy dee) • A condition in which one or more chromosomes or pieces of chromosomes are either lacking or present in excess.
Angiosperm (an´ jee oh spurm) [Gr. angion: vessel + sperma: seed] • One of the flowering plants; literally, one whose seed is carried in a "vessel" (i.e., fruit).
Angiotensin (an´ jee oh ten´ sin) • A peptide hormone that raises blood pressure by causing peripheral vessels to constrict. Also maintains glomerular filtration by constricting efferent vessels and stimulates thirst and the release of aldosterone.
Animal [L. animus: breath, soul] • A member of the kingdom Animalia. In general, a multicellular eukaryote that obtains its food by ingestion.
Animal hemisphere • The metabolically active upper portion of some animal eggs, zygotes, and embryos; does not contain the dense nutrient yolk. (Contrast with vegetal hemisphere.)
Anion (an´ eye one) • A negatively charged ion. (Contrast with cation.)
Anisogamy (an´ eye sog´ a mee) [Gr. aniso: unequal + gamos: marriage] • The existence of two dissimilar gametes (egg and sperm).
Annual • Referring to a plant whose life cycle is completed in one growing season. (Contrast with biennial, perennial.)
Antenna system • In photosynthesis, a group of different molecules that cooperate to absorb light energy and transfer it to a reaction center.
Anterior pituitary • The portion of the vertebrate pituitary gland that derives from gut epithelium and produces tropic hormones.
Anther (an´ thur) [Gr. anthos: flower] • A pollen-bearing portion of the stamen of a flower.
Antheridium (an´ thur id´ ee um) (plural: antheridia) [Gr. antheros: blooming] • The multicellular structure that produces the sperm in bryophytes and ferns.
Antibody • One of the millions of proteins produced by the immune system that specifically binds to a foreign substance and initiates its removal from the body.
Anticodon • The three nucleotides in transfer RNA that pair with a complementary triplet (a codon) in messenger RNA.
Antidiuretic hormone • A hormone that controls water reabsorption in the mammalian kidney. Also called vasopressin.
Antigen (an´ ti jun) • Any substance that stimulates the production of an antibody or antibodies in the body of a vertebrate.
Antigenic determinant • A specific region of an antigen, which is recognized by and binds to a specific antibody.
Antiparallel • Pertaining to molecular orientation in which a molecule or parts of a molecule have opposing directions.
Antipodal cell • At one end of the megagametophyte, one of the three cells which eventually degenerate.
Antiport • A membrane transport process that carries one substance in one direction and another in the opposite direction. (Contrast with symport.)
Antisense nucleic acid • A single-stranded RNA or DNA complementary to and thus targeted against the mRNA transcribed from a harmful gene such as an oncogene.
Anus (a´ nus) • Opening through which digestive wastes are expelled, located at the posterior end of the gut.
Aorta (a or´ tuh) [Gr. aorte: aorta] • The main trunk of the arteries leading to the systemic (as opposed to the pulmonary) circulation.
Apex (a´ pecks) • The tip or highest point of a structure, as the apex of a growing stem or root.
Apical (a´ pi kul) • Pertaining to the apex, or tip, usually in reference to plants.
Apical dominance • Inhibition by the apical bud of the growth of axillary buds.
Apical meristem • The meristem at the tip of a shoot or root; responsible for the plant's primary growth.
Apomixis (ap oh mix´ is) [Gr. apo: away from + mixis: sexual intercourse] • The asexual production of seeds.
Apoplast (ap´ oh plast) • in plants, the continuous meshwork of cell walls and extracellular spaces through which material can pass without crossing a plasma membrane. (Contrast with symplast.)
Apoptosis(ay´ pu toh sis) • A series of genetically programmed events leading to cell death.
Aquaporin • A transport protein in plant and animal cells through which water passes in osmosis.
Aquatic (a kwa tic) [L. aqua: water] • Living in water. (Compare with marine, terrestrial.)
Aqueous(a kwee us) [L. aqua: water] • Pertaining to water or a watery solution.
Archegonium (ar´ ke go´ nee um) [Gr. archegonos: first of a kind] • The multicellular structure that produces eggs in bryophytes, ferns, and gymnosperms.
Archenteron (ark en´ ter on) [Gr. archos: beginning + enteron: bowel] • The earliest primordial animal digestive tract.
Arteriosclerosis • See atherosclerosis.
Artery • A muscular blood vessel carrying oxygenated blood away from the heart to other parts of the body. (Contrast with vein.)
Ascus (ass´ cuss) [Gr. askos: bladder] • In ascomycete fungi (sac fungi), the club-shaped sporangium within which spores (ascospores) are produced by meiosis.
Asexual • Without sex.
Assortative mating • A breeding system in which mates are selected on the basis of a particular trait or group of traits.
Atherosclerosis (ath´ er oh sklair oh´ sis) • A disease of the lining of the arteries characterized by fatty, cholesterol-rich deposits in the walls of the arteries. When fibroblasts infiltrate these deposits and calcium precipitates in them, the disease become arteriosclerosis, or "hardening of the arteries."
Atmosphere • The gaseous mass surrounding our planet. Also a unit of pressure, equal to the normal pressure of air at sea level.
Atom [Gr. atomos: indivisible] • The smallest unit of a chemical element. Consists of a nucleus and one or more electrons.
Atomic mass (also called atomic weight) • The average mass of an atom of an element; the average depends on the relative amounts of different isotopes of the element on Earth. Also called atomic weight.
Atomic number • The number of protons in the nucleus of an atom; also equals the number of electrons around the neutral atom. Determines the chemical properties of the atom.
ATP (adenosine triphosphate) • An energy-storage compound containing adenine, ribose, and three phosphate groups. When it is formed from ADP, useful energy is stored; when it is broken down (to ADP or AMP), energy is released to drive endergonic reactions.
ATP synthase • An integral membrane protein that couples the transport of proteins with the formation of ATP.
Atrium (a´ tree um) • An internal chamber. In the hearts of vertebrates, the thin-walled chamber(s) entered by blood on its way to the ventricle(s). Also, the outer ear.
Autoimmune disease • A disorder in which the immune system attacks the animal's own antigens.
Autonomic nervous system • The system that controls such involuntary functions as those of guts and glands.
Autosome • Any chromosome (in a eukaryote) other than a sex chromosome.
Autotroph (au´ tow trow´ fik) [Gr. autos: self + trophe: food] • An organism that is capable of living exclusively on inorganic materials, water, and some energy source such as sunlight or chemically reduced matter. (Contrast with heterotroph.)
Auxin (awk´ sin) [Gr. auxein: increase] • In plants, a substance (the most common being indoleacetic acid) that regulates growth and various aspects of development.
Auxotroph (awks´ o trofe) [Gr. auxanein: to grow + trophe: food] • A mutant form of an organism that requires a nutrient or nutrients not usually required by the wild type. (Contrast with prototroph.)
Axon [Gr.: axle] • The part of a neuron that conducts action potentials away from the cell body.
Axon hillock • The junction between an axon and its cell body, where action potentials are generated.
Axon terminals • The endings of an axon; they form synapses and release neurotransmitter.
Axoneme (ax´ oh neem) • The complex of microtubules and their crossbridges that forms the motile apparatus of a cilium.