A
(1) Adenine (or the corresponding nucleoside or nucleotide) in a nucleic
acid.
(2) Alanine (see amino acids).
A °
(° Angstr¨om unit) 10−10 m (= 10−1 nm).
2–5A
See interferons.
A-DNA
See DNA.
A layer
An S layer associated with virulence in strains of Aeromonas
salmonicida.
A-protein
In Tobacco Mosaic Virus: a mixture of small oligomers and monomers of
coat protein subunits which occur in equilibrium with the larger ‘disc’
aggregates under conditions of physiological pH and ionic strength; coat
protein occurs mainly as A-protein under conditions of high pH and low ionic
strength.
A site
(of a ribosome) See Protein Synthesis.
A-tubule
(A-subfibre) See flagellum
A-type inclusion body
See Poxviridae.
A-type particles
Intracellular, non-infectious, retrovirus-like particles. Many embryonic
and transformed mouse cells contain retrovirus-like ‘intracisternal A-type
particles’ (IAPs) which form by budding at the endoplasmic reticulum; these
particles have reverse transcriptase activity and an RNA genome coding for the
structural protein of the particles. The mouse genome contains ca. 1000 copies
(per haploid genome) of
DNA sequences homologous to IAP-associated RNA; these sequences appear
to be capable of transposition within the mouse genome – probably via an RNA
intermediate i.e., they may be Retrotransposons. Some A-type particles are
non-enveloped precursors of B-type particles.
A23187
An ionophore which transports divalent cations, particularly Ca2+; it
can effect the transmembrane exchange of 1Ca2+ (or 1Mg2+) for 2H+ without
causing perturbation in the gradients of other monovalent cations.
AAA ATPases
‘ATPases associated with diverse cellular activities’. AAA ATPases occur
e.g. in PEROXISOMES and as components of eukaryotic proteasomes.
AAA pathway
Aminoadipic Acid Pathway.
AAC
Aminoglycoside acetyltransferase
(see Aminoglycoside Antibiotics).
AAD
Aminoglycoside adenylyltransferase
(see Aminoglycoside Antibiotics).
AAS
Aminoalkylsilane (3-aminopropyltriethoxy-silane, APES; 3(triethoxysilyl)-propylamine,
TESPA): a reagent used for binding a tissue section to the surface of a glass
slide (e.g. for in situ hybridization); it reacts with silica glass and
provides aminoalkyl groups which bind to aldehyde or ketone groups in the
tissue section.
aat gene
In Escherichia coli: a gene whose product promotes the early
degradation of those proteins whose N-terminal amino acid is either arginine or
lysine. aat encodes an ‘amino acid transferase’ which catalyses the
addition of a leucine or phenylalanine
residue to the N-terminus of the protein; this destabilizes the protein,
facilitating its degradation.
ABA
Abscisic acid.
Abacavir
A nucleoside reverse transcriptase inhibitor.
Abbe condenser
a simple two- or three-lens substage condenser which is uncorrected for
spherical or chromatic aberrations.
ABC
(1) (immunol.) antigen-binding cell.
(2) see ABC transporter.
ABC excinuclease
see excision repair.
ABC exporter
An ABC transporter concerned with export/ secretion. These systems are
found in both prokaryotic and eukaryotic microorganisms and in higher animals,
including
man.
ABC immunoperoxidase method
An immunoperoxidase method involving the use of a preformed
avidin–biotin–peroxidise complex (ABC) which has surplus biotin-binding
capacity. Initially, a (‘primary’) antiserum is raised against the required antigen;
if the primary antiserum is derived from e.g. a rat, a ‘secondary’ anti-rat
antiserum is prepared, and the anti-rat Ig antibodies are BIOTINylated. To
locate a specific antigen, the section is treated with primary antiserum,
washed, and then treated with secondary antiserum; the subsequent addition of ABC
localizes peroxidase at the site of specific antigen
abdomen
In
vertebrates, the portion of the trunk containing visceral organs other than
heart and lungs; in arthropods, the posterior portion of the body, made up of
similar segments and containing the reproductive organs and part of the
digestive tract.
abiotic
[Gk. a, not,
without + bios, life]
Nonliving; specifically, the nonliving components of an ecosystem, such as temperature, humidity, the mineral content of the soil, etc.
Nonliving; specifically, the nonliving components of an ecosystem, such as temperature, humidity, the mineral content of the soil, etc.
abscisic
acid (ABA)
(ab-sis-ik)
[L. ab, away, off + scissio, dividing]
A plant hormone that generally acts to inhibit growth, promote dormancy, and help the plant tolerate stressful conditions.
A plant hormone that generally acts to inhibit growth, promote dormancy, and help the plant tolerate stressful conditions.
abscission
[L. ab, away,
off + scissio, dividing]
In plants, the dropping of leaves, flowers, fruits, or stems at the end of a growing season, as the result of formation of a two-layered zone of specialized cells (the abscission zone) and the action of a hormone (ethylene).
In plants, the dropping of leaves, flowers, fruits, or stems at the end of a growing season, as the result of formation of a two-layered zone of specialized cells (the abscission zone) and the action of a hormone (ethylene).
absorption
[L. absorbere, to
swallow down]
The movement of water and dissolved substances into a cell, tissue, or organism.
The movement of water and dissolved substances into a cell, tissue, or organism.
absorption
spectrum
The range
of a pigment's ability to absorb various wavelengths of light.
abyssal
zone
(uh-bis-ul)
The portion of the ocean floor where light does not penetrate and where temperatures are cold and pressures intense.
The portion of the ocean floor where light does not penetrate and where temperatures are cold and pressures intense.
acclimatization
(uh-kly-mih-ty-zay-shun)
Physiological adjustment to a change in an environmental factor.
Physiological adjustment to a change in an environmental factor.
accommodation
The
automatic adjustment of an eye to focus on near objects.
acetylcholine
(asset-ill-coal-een)
One of the most common neurotransmitters; functions by binding to receptors and altering the permeability of the postsynaptic membrane to specific ions, either depolarizing or hyperpolarizing the membrane.
One of the most common neurotransmitters; functions by binding to receptors and altering the permeability of the postsynaptic membrane to specific ions, either depolarizing or hyperpolarizing the membrane.
acetyl
CoA
The entry
compound for the Krebs cycle in cellular respiration;
formed from a fragment of pyruvate attached to a coenzyme.
acid
[L. acidus, sour]
A substance that increases the hydrogen ion concentration in a solution.
A substance that increases the hydrogen ion concentration in a solution.
acid
precipitation
Rain,
snow, or fog that is more acidic than pH 5.6.
acoelomate
(a-seel-oh-mate)
A solid-bodied animal lacking a cavity between the gut and outer body wall.
A solid-bodied animal lacking a cavity between the gut and outer body wall.
acrosome
(ak-ruh-some)
An organelle at the tip of a sperm cell that helps the sperm penetrate the egg.
An organelle at the tip of a sperm cell that helps the sperm penetrate the egg.
ACTH
Abbreviation
of adrenocorticotropic hormone.
actin
[Gk. aktis, a
ray]
A globular protein that links into chains, two of which twist helically about each other, forming microfilaments in muscle and other contractile elements in cells.
A globular protein that links into chains, two of which twist helically about each other, forming microfilaments in muscle and other contractile elements in cells.
action
potential
A rapid
change in the membrane potential of an excitable cell, caused by
stimulus-triggered, selective opening and closing of voltage-sensitive gates in
sodium and potassium ion channels.
activation
energy
The
energy that must be possessed by atoms or molecules in order to react.
active
site
The
specific portion of an enzyme that attaches to the substrate by means of weak
chemical bonds.
active
transport
The
movement of a substance across a biological membrane against its concentration
or electrochemical gradient, with the help of energy input and specific
transport proteins.
adaptation
[L. adaptare, to
fit]
(1) The evolution of features that make a group of organisms better suited to live and reproduce in their environment. (2) A peculiarity of structure, physiology, or behavior that aids the organism in its environment.
(1) The evolution of features that make a group of organisms better suited to live and reproduce in their environment. (2) A peculiarity of structure, physiology, or behavior that aids the organism in its environment.
adaptive
peak
An
equilibrium state in a population when the gene pool has allele frequencies
that maximize the average fitness of a population's members.
adaptive
radiation
The
emergence of numerous species from a common ancestor introduced into an
environment, presenting a diversity of new opportunities and problems.
adenosine
diphosphate (ADP)
A
nucleotide consisting of adenine, ribose, and two phosphate groups; formed by
the removal of one phosphate from an ATP molecule.
adenosine
monophosphate (AMP)
A
nucleotide consisting of adenine, ribose, and one phosphate group; can be
formed by the removal of two phosphates from an ATP molecule; in its cyclic
form, functions as a "second messenger" for a number of vertebrate
hormones and neurotransmitters.
adenosine
triphosphate (ATP)
An
adenine-containing nucleoside triphosphate that releases free energy when its
phosphate bonds are hydrolyzed. This energy is used to drive endergonic
reactions in cells.
adenylyl
cyclase
An enzyme
that converts ATP to cyclic AMP in response to a chemical signal.
ADH
Abbreviation
of antidiuretic hormone.
adhesion
[L. adhaerere, to
stick to]
The tendency of different kinds of molecules to stick together.
The tendency of different kinds of molecules to stick together.
ADP
Abbreviation
of adenosine diphosphate.
adrenal
gland
(uh-dree-nul)
[L. ad, near + renes, kidney]
An endocrine gland located adjacent to the kidney in mammals; composed of two glandular portions: an outer cortex, which responds to endocrine signals in reacting to stress and effecting salt and water balance, and a central medulla, which responds to nervous inputs resulting from stress.
An endocrine gland located adjacent to the kidney in mammals; composed of two glandular portions: an outer cortex, which responds to endocrine signals in reacting to stress and effecting salt and water balance, and a central medulla, which responds to nervous inputs resulting from stress.
adrenaline
A
hormone, produced by the medulla of the adrenal gland, that increases the
concentration of glucose in the blood, raises blood pressure and heartbeat
rate, and increases muscular power and resistance to fatigue; also a
neurotransmitter across synaptic junctions. Also called epinephrine.
adrenocorticotropic
hormone (ACTH)
A
hormone, produced by the anterior lobe of the pituitary gland, that stimulates
the production of cortisol by the adrenal cortex.
adventitious
[L. adventicius, not
properly belonging to]
Referring to a structure arising from an unusual place, such as roots growing from stems or leaves.
Referring to a structure arising from an unusual place, such as roots growing from stems or leaves.
aerobic
(air-oh-bik)
[Gk. aer, air + bios, life]
Containing oxygen; referring to an organism, environment, or cellular process that requires oxygen.
Containing oxygen; referring to an organism, environment, or cellular process that requires oxygen.
afferent
[L. ad, near
+ ferre, to carry]
Bringing inward to a central part, applied to nerves and blood vessels.
Bringing inward to a central part, applied to nerves and blood vessels.
agar
A
gelatinous material prepared from certain red algae that is used to solidify
nutrient media for growing microorganisms.
age
structure
The
relative number of individuals of each age in a population.
agnathan
(ag-naa-thun)
A member of a jawless class of vertebrates represented today by the lampreys and hagfishes.
A member of a jawless class of vertebrates represented today by the lampreys and hagfishes.
agonistic
behavior
(ag-on-is-tik)
A type of behavior involving a contest of some kind that determines which competitor gains access to some resource, such as food or mates.
A type of behavior involving a contest of some kind that determines which competitor gains access to some resource, such as food or mates.
AIDS
(acquired immunodeficiency syndrome)
The name
of the late stages of HIV infection; defined by a specified reduction of T
cells and the appearance of characteristic secondary infections.
aldehyde
(al-duh-hyde)
An organic molecule with a carbonyl group located at the end of the carbon skeleton.
An organic molecule with a carbonyl group located at the end of the carbon skeleton.
aldosterone
(al-dah-stair-own)
[Gk. aldaino, to nourish + stereo, solid]
An adrenal hormone that acts on the distal tubules of the kidney to stimulate the reabsorption of sodium (Na+) and the passive flow of water from the filtrate.
An adrenal hormone that acts on the distal tubules of the kidney to stimulate the reabsorption of sodium (Na+) and the passive flow of water from the filtrate.
aleurone
layer
[Gk. aleuron, flour]
The outermost cell layer of the endosperm of the grains (seeds) of wheat and other grasses; when acted upon by gibberellin, the aleurone layer releases enzymes that digest the stored food of the endosperm into small nutrient molecules that can be taken up by the embryo.
The outermost cell layer of the endosperm of the grains (seeds) of wheat and other grasses; when acted upon by gibberellin, the aleurone layer releases enzymes that digest the stored food of the endosperm into small nutrient molecules that can be taken up by the embryo.
alga pl. algae
(al-gah, al-jee)
A photosynthetic, plantlike protist.
A photosynthetic, plantlike protist.
alkaline
Pertaining
to substances that increase the relative number of hydroxide ions (OH–)
in a solution; having a pH greater than 7; basic; opposite of acidic.
all-or-none
event
An action
that occurs either completely or not at all, such as the generation of an
action potential by a neuron.
allantois
(al-an-toh-iss)
[Gk. allant, sausage]
One of four extraembryonic membranes; serves as a repository for the embryo's nitrogenous waste.
One of four extraembryonic membranes; serves as a repository for the embryo's nitrogenous waste.
allele
(al-eel)
[Gk. allelon, of one another]
An alternative form of a gene.
An alternative form of a gene.
allele
frequency
The
proportion of a particular allele in a population.
allergic
reaction
An
inflammatory response triggered by a weak antigen (an allergen) to which most
individuals do not react; involves the release of large amounts of histamine
from mast cells.
allometric
growth
(al-oh-met-rik)
The variation in the relative rates of growth of various parts of the body, which helps shape the organism.
The variation in the relative rates of growth of various parts of the body, which helps shape the organism.
allopatric
speciation
(al-oh-pat-rik)
[Gk. allos, other + patra, fatherland,
country]
A mode of speciation induced when the ancestral population becomes segregated by a geographical barrier.
A mode of speciation induced when the ancestral population becomes segregated by a geographical barrier.
allopolyploid
(al-oh-pol-ee-ploid)
A common type of polyploid species resulting from two different species interbreeding and combining their chromosomes.
A common type of polyploid species resulting from two different species interbreeding and combining their chromosomes.
allosteric
site
(al-oh-steer-ik)
A specific receptor site on an enzyme molecule remote from the active site. Molecules bind to the allosteric site and change the shape of the active site, making it either more or less receptive to the substrate.
A specific receptor site on an enzyme molecule remote from the active site. Molecules bind to the allosteric site and change the shape of the active site, making it either more or less receptive to the substrate.
allozymes
Slightly
different versions of the same enzyme, distinguishable via gel electrophoresis.
alpha
helix
A spiral
shape constituting one form of the secondary structure of proteins, arising
from a specific hydrogen-bonding structure.
alternation
of generations
A life
cycle in which there is both a multicellular diploid form, the sporophyte, and
a multicellular haploid form, the gametophyte; characteristic of plants.
alternative
splicing
In
alternative splicing, the same pre-mRNA molecule, which consists of introns and exons, is spliced in different ways to
produce mature mRNAs of different lengths and different functionality.
altruistic
behavior
(al-troo-is-tik)
The aiding of another individual at one's own risk or expense.
The aiding of another individual at one's own risk or expense.
alveolus pl. alveoli
(al-vee-oh-lus)
[L. dim. of alveus, cavity, hollow]
(1) One of the deadend, multilobed air sacs that constitute the gas exchange surface of the lungs. (2) One of the milk-secreting sacs of epithelial tissue in the mammary glands.
(1) One of the deadend, multilobed air sacs that constitute the gas exchange surface of the lungs. (2) One of the milk-secreting sacs of epithelial tissue in the mammary glands.
amino
acid
(am-ee-no)
[Gk. Ammon, referring to the Egyptian sun god, near whose
temple ammonium salts were first prepared from camel dung]
An organic molecule possessing both carboxyl and amino groups. Amino acids serve as the monomers of proteins.
An organic molecule possessing both carboxyl and amino groups. Amino acids serve as the monomers of proteins.
amino
group
A
functional group that consists of a nitrogen atom bonded to two hydrogen atoms;
can act as a base in solution, accepting a hydrogen ion and acquiring a charge
of +1.
aminoacyl—tRNA
synthetases
A family
of enzymes, at least one for each amino acid, that catalyze the attachment of
an amino acid to its specific tRNA molecule.
ammonification
The
process by which decomposers break down proteins and amino acids, releasing the
excess nitrogen in the form of ammonia (NH3) or ammonium ion (NH4+).
amniocentesis
(am-nee-oh-sen-tee-sis)
A technique for determining genetic abnormalities in a fetus by the presence of certain chemicals or defective fetal cells in the amniotic fluid, obtained by aspiration from a needle inserted into the uterus.
A technique for determining genetic abnormalities in a fetus by the presence of certain chemicals or defective fetal cells in the amniotic fluid, obtained by aspiration from a needle inserted into the uterus.
amnion
(am-nee-on)
[Gk. dim. of amnos, lamb]
The innermost of four extraembryonic membranes; encloses a fluid-filled sac in which the embryo is suspended.
The innermost of four extraembryonic membranes; encloses a fluid-filled sac in which the embryo is suspended.
amniote
A
vertebrate possessing an amnion surrounding the embryo; reptiles, birds, and
mammals are amniotes.
amniotic
egg
A
shelled, water-retaining egg that enables reptiles, birds, and egg-laying
mammals to complete their life cycles on dry land.
amoeboid
[Gk. amoibe, change]
Moving or feeding by means of pseudopodia (temporary cytoplasmic protrusions from the cell body).
Moving or feeding by means of pseudopodia (temporary cytoplasmic protrusions from the cell body).
AMP
Abbreviation
of adenosine monophosphate.
Amphibia
The
vertebrate class of amphibians, represented by frogs, salamanders, and
caecilians.
amphipathic
molecule
A
molecule that has both a hydrophilic region and a hydrophobic region.
anabolic
steroids
Synthetic
chemical variants of the male sex hormone testosterone; they produce increased
muscle mass but also suppress testosterone production, leading to shrinkage of
the testes, growth of the breasts, and premature baldness; long-term use
increases the risk of kidney and liver damage and of liver cancer.
anabolism
[Gk. ana, up
+ -bolism (as in metabolism)]
Within a cell or organism, the sum of all biosynthetic reactions (that is, chemical reactions in which larger molecules are formed from smaller ones).
Within a cell or organism, the sum of all biosynthetic reactions (that is, chemical reactions in which larger molecules are formed from smaller ones).
anaerobic
an-air-oh-bik)
[Gk. an, without + aer, air + bios, life]
Lacking oxygen; referring to an organism, environment, or cellular process that lacks oxygen and may be poisoned by it.
Lacking oxygen; referring to an organism, environment, or cellular process that lacks oxygen and may be poisoned by it.
anagenesis
(an-uh-jen-eh-sis)
A pattern of evolutionary change involving the transformation of an entire population, sometimes to a state different enough from the ancestral population to justify renaming it as a separate species; also called phyletic evolution.
A pattern of evolutionary change involving the transformation of an entire population, sometimes to a state different enough from the ancestral population to justify renaming it as a separate species; also called phyletic evolution.
analogy
The
similarity of structure between two species that are not closely related;
attributable to convergent evolution.
analogous
[Gk. analogos, proportionate]
Applied to structures similar in function but different in evolutionary origin, such as the wing of a bird and the wing of an insect.
Applied to structures similar in function but different in evolutionary origin, such as the wing of a bird and the wing of an insect.
anaphase
(anna-phase)
[Gk. ana, up + phasis, form]
The third stage of mitosis, beginning when the centromeres of duplicated chromosomes divide and sister chromotids separate from each other, and ending when a complete set of daughter chromosomes are located at each of the two poles of the cell.
The third stage of mitosis, beginning when the centromeres of duplicated chromosomes divide and sister chromotids separate from each other, and ending when a complete set of daughter chromosomes are located at each of the two poles of the cell.
androgen
(an-droh-jens)
[Gk. andros, man + genos, origin, descent]
The principal male steroid hormones, such as testosterone, which stimulate the development and maintenance of the male reproductive system and secondary sex characteristics.
The principal male steroid hormones, such as testosterone, which stimulate the development and maintenance of the male reproductive system and secondary sex characteristics.
aneuploidy
(an-yoo-ploy-dee)
A chromosomal aberration in which certain chromosomes are present in extra copies or are deficient in number.
A chromosomal aberration in which certain chromosomes are present in extra copies or are deficient in number.
angiosperm
(an-jee-o-sperms)
[Gk. angeion, vessel + sperma, seed]
A flowering plant, which forms seeds inside a protective chamber called an ovary.
A flowering plant, which forms seeds inside a protective chamber called an ovary.
anion
(an-eye-on)
A negatively charged ion.
A negatively charged ion.
annual
[L. annus, year]
A plant that completes its entire life cycle in a single year or growing season.
A plant that completes its entire life cycle in a single year or growing season.
antennae
Long,
paired sensory appendages on the head of many arthropods.
anterior
[L. ante, before,
toward, in front of]
Referring to the head end of a bilaterally symmetrical animal.
Referring to the head end of a bilaterally symmetrical animal.
anther
[Gk. anthos, flower]
The terminal pollen sac of a stamen, inside which pollen grains with male gametes form in the flower of an angiosperm.
The terminal pollen sac of a stamen, inside which pollen grains with male gametes form in the flower of an angiosperm.
antheridium pl. antheridia
In
plants, the male gametangium, a moist chamber in which gametes develop.
anthocyanin
Natural water-soluble
pigments of blue, purple or red which are dissolved in the cell-sap vacuole of
plant cells.
anthropoid
[Gk. anthropos, man,
human]
A higher primate; includes monkeys, apes, and humans.
A higher primate; includes monkeys, apes, and humans.
antibiotic
[Gk. anti, against
+ bios, life]
A chemical that kills bacteria or inhibits their growth.
A chemical that kills bacteria or inhibits their growth.
antibody
[Gk. anti, against]
An antigen-binding immunoglobulin, produced by B cells, that functions as the effector in an immune response.
An antigen-binding immunoglobulin, produced by B cells, that functions as the effector in an immune response.
anticodon
A
specialized base triplet on one end of a tRNA molecule that recognizes a
particular complementary codon on an mRNA molecule.
antidiuretic
hormone (ADH)
[Gk. anti, against
+ diurgos, thoroughly wet + hormaein, to
excite]
A hormone important in osmoregulation.
A hormone important in osmoregulation.
antigen
(an-teh-jen)
[Gk. anti, against + genos, origin, descent]
A foreign macromolecule that does not belong to the host organism and that elicits an immune response.
A foreign macromolecule that does not belong to the host organism and that elicits an immune response.
aorta
(a-ore-ta)
[Gk. aeirein, to lift, heave]
The major artery in blood-circulating systems; the aorta sends blood to the other body tissues.
The major artery in blood-circulating systems; the aorta sends blood to the other body tissues.
aphotic
zone
(ay-foe-tik)
[Gk. aeirein, to lift, heave]
The part of the ocean beneath the photic zone, where light does not penetrate sufficiently for photosynthesis to occur.
The part of the ocean beneath the photic zone, where light does not penetrate sufficiently for photosynthesis to occur.
apical
dominance
(ay-pik-ul)
[L. apex, top]
Concentration of growth at the tip of a plant shoot, where a terminal bud partially inhibits axillary bud growth.
Concentration of growth at the tip of a plant shoot, where a terminal bud partially inhibits axillary bud growth.
apical
meristem
(ay-pik-ul mare-eh-stem)
[L. apex, top + Gk. meristos, divided]
Embryonic plant tissue in the tips of roots and in the buds of shoots that supplies cells for the plant to grow in length.
Embryonic plant tissue in the tips of roots and in the buds of shoots that supplies cells for the plant to grow in length.
apomorphic
character
(ap-oh-more-fik)
A derived phenotypic character, or homology, that evolved after a branch diverged from a phylogenetic tree.
A derived phenotypic character, or homology, that evolved after a branch diverged from a phylogenetic tree.
apoplast
(ap-oh-plast)
In plants, the nonliving continuum formed by the extracellular pathway provided by the continuous matrix of cell walls.
In plants, the nonliving continuum formed by the extracellular pathway provided by the continuous matrix of cell walls.
apoptosis
Programmed
cell death brought about by signals that trigger the activation of a cascade of
"suicide" proteins in the cells destined to die.
aposematic
coloration
(ap-oh-so-mat-ik)
The bright coloration of animals with effective physical or chemical defenses that acts as a warning to predators.
The bright coloration of animals with effective physical or chemical defenses that acts as a warning to predators.
aquaporin
A
transport protein in the plasma membranes of a plant or animal cell that
specifically facilitates the diffusion of water across the membrane (osmosis).
aqueous
solution
(ay-kwee-us)
A solution in which water is the solvent.
A solution in which water is the solvent.
arboreal
[L. arbor, tree]
Tree-dwelling.
Tree-dwelling.
Archaea
One of
two prokaryotic domains, the other being the Bacteria.
archegonium pl. archegonia
(ar-kih-go-nee-um)
[Gk. archegonos, first of a race]
In plants, the female gametangium, a moist chamber in which gametes develop.
In plants, the female gametangium, a moist chamber in which gametes develop.
archenteron
(ark-en-ter-on)
[Gk. arch, first, or main + enteron, gut]
The endoderm-lined cavity, formed during the gastrulation process, that develops into the digestive tract of an animal.
The endoderm-lined cavity, formed during the gastrulation process, that develops into the digestive tract of an animal.
Archezoa
Primitive
eukaryotic group that includes diplomonads, such as Giardia; some systematists
assign kingdom status to archezoans.
arteriole
A very
small artery. See also artery.
artery
A vessel
that carries blood away from the heart to organs throughout the body.
arteriosclerosis
A cardiovascular
disease caused by the formation of hard plaques within the arteries.
artificial
selection
The
selective breeding of domesticated plants and animals to encourage the
occurrence of desirable traits.
ascus pl. asci
A saclike
spore capsule located at the tip of the ascocarp in dikaryotic hyphae; defining
feature of the Ascomycota division of fungi.
asexual
reproduction
A type of
reproduction involving only one parent that produces genetically identical
offspring by budding or by the division of a single cell or the entire organism
into two or more parts.
assimilation
The
energy-requiring process by which plant cells convert nitrate ions (NO3–)
taken up by the roots of plants into ammonium ions (NH4+),
which can then be used in the synthesis of amino acids and other nitrogenous
compounds.
associative
learning
The
acquired ability to associate one stimulus with another; also called classical
conditioning.
assortative
mating
A type of
nonrandom mating in which mating partners resemble each other in certain phenotypic
characters.
asymmetric
carbon
A carbon
atom covalently bonded to four different atoms or groups of atoms.
atmospheric
pressure
[Gk. atmos, vapor
+ sphaira, globe]
The weight of the Earth's atmosphere over a unit area of the Earth's surface.
The weight of the Earth's atmosphere over a unit area of the Earth's surface.
atom
[Gk. atomos, indivisible]
The smallest unit of matter that retains the properties of an element.
The smallest unit of matter that retains the properties of an element.
atomic
number
The
number of protons in the nucleus of an atom, unique for each element and
designated by a subscript to the left of the elemental symbol.
atomic
weight
The total
atomic mass, which is the mass in grams of one mole of the atom.
ATP
Abbreviation
of adenosine triphosphate, the principal
energy-carrying compound of the cell.
ATP
synthase
A cluster
of several membrane proteins found in the mitochondrial cristae (and bacterial
plasma membrane) that function in chemiosmosis with adjacent electron
transport chains, using the energy of a hydrogen-ion concentration gradient to
make ATP. ATP synthases provide a port through which hydrogen ions diffuse into
the matrix of a mitrochondrion.
atrioventricular
node
[L. atrium, yard,
court, hall + ventriculus, the stomach + nodus, knot]
A group of slow-conducting fibers in the atrium of the vertebrate heart that are stimulated by impulses originating in the sinoatrial node (the pacemaker) and that conduct impulses to the bundle of His, a group of fibers that stimulate contraction of the ventricles.
A group of slow-conducting fibers in the atrium of the vertebrate heart that are stimulated by impulses originating in the sinoatrial node (the pacemaker) and that conduct impulses to the bundle of His, a group of fibers that stimulate contraction of the ventricles.
atrioventricular
valve
A valve
in the heart between each atrium and ventricle that prevents a backflow of
blood when the ventricles contract.
atrium pl. atria
(a-tree-um)
[L. yard, court, hall]
A chamber that receives blood returning to the vertebrate heart.
A chamber that receives blood returning to the vertebrate heart.
autogenesis
model
According
to this model, eukaryotic cells evolved by the specialization of internal
membranes originally derived from prokaryotic plasma membranes.
autoimmune
disease
An
immunological disorder in which the immune system turns against itself.
autonomic
nervous system
(awt-uh-nahm-ik)
[Gk. autos, self + nomos, usage, law]
A subdivision of the motor nervous system of vertebrates that regulates the internal environment; consists of the sympathetic and parasympathetic divisions.
A subdivision of the motor nervous system of vertebrates that regulates the internal environment; consists of the sympathetic and parasympathetic divisions.
autopolyploid
(aw-toe-pol-ee-ploid)
A type of polyploid species resulting from one species doubling its chromosome number to become tetraploid, which may self-fertilize or mate with other tetraploids.
A type of polyploid species resulting from one species doubling its chromosome number to become tetraploid, which may self-fertilize or mate with other tetraploids.
autosome
(awtuh-some)
[Gk. autos, self + soma, body]
A chromosome that is not directly involved in determining sex, as opposed to the sex chromosomes.
A chromosome that is not directly involved in determining sex, as opposed to the sex chromosomes.
autotroph
(aw-toh-trohf)
[Gk. autos, self + trophos, feeder]
An organism that obtains organic food molecules without eating other organisms. Autotrophs use energy from the sun or from the oxidation of inorganic substances to make organic molecules from inorganic ones.
An organism that obtains organic food molecules without eating other organisms. Autotrophs use energy from the sun or from the oxidation of inorganic substances to make organic molecules from inorganic ones.
auxin
(awk-sins)
[Gk. auxein, to increase + in, of, or
belonging to]
A class of plant hormones, including indoleacetic acid (IAA), having a variety of effects, such as phototropic response through the stimulation of cell elongation, stimulation of secondary growth, and the development of leaf traces and fruit.
A class of plant hormones, including indoleacetic acid (IAA), having a variety of effects, such as phototropic response through the stimulation of cell elongation, stimulation of secondary growth, and the development of leaf traces and fruit.
auxotroph
(awk-soh-trohf)
A nutritional mutant that is unable to synthesize and that cannot grow on media lacking certain essential molecules normally synthesized by wild-type strains of the same species.
A nutritional mutant that is unable to synthesize and that cannot grow on media lacking certain essential molecules normally synthesized by wild-type strains of the same species.
Aves
The
vertebrate class of birds, characterized by feathers and other flight
adaptations.
axillary
bud
[Gk. axilla, armpit]
An embryonic shoot present in the angle formed by a leaf and stem.
An embryonic shoot present in the angle formed by a leaf and stem.
axis
An
imaginary line passing through a body or organ around which parts are
symmetrically aligned.
axon
(aks-on)
[Gk. axon, axle]
A typically long extension, or process, from a neuron that carries nerve impulses away from the cell body toward target cells.
A typically long extension, or process, from a neuron that carries nerve impulses away from the cell body toward target cells.