A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

ABC transporter proteins Large superfamily of membrane transport proteins

that use the energy of hydrolysis of ATP to transfer peptides and a variety of

small molecules across membranes.

acetyl Chemical group derived from acetic acid. Acetyl groups are important

in metabolism and are added covalently to some proteins as a posttranslational

modification.

acetyl CoA Small water-soluble molecule that carries acetyl groups in cells. It

consists of an acetyl group linked to coenzyme A (CoA) by an easily

hydrolyzable thioester bond. (2 62.)

acetylcholine receptor Ion channel that opens in response to binding of

acetylcholine, thereby converting a chemical signal into an electrical one. Best

understood example of a transmitter-gated channel. Sometimes called the

nicotinic acetylcholine receptor to distinguish it from a muscarinic

acetylcholine receptor, which is a G-protein-linked cell-surface receptor.

acetylcholine Neurotransmitter that functions at a class of chemical synapses

known as cholinergic synapses. Found both in the brain and in the peripheral

nervous system. It is the neurotransmitter at vertebrate neuromuscular

junctions. (15 9.)

acid Substance that releases protons when dissolved in water, forming a

hydronium ion (H3O+).

acid hydrolase Any of a group of diverse hydrolytic enzymes (including

proteases, nucleases, glycosidases, etc.) that have their optimal activity at acid

pH (around 5.0) and are found in lysosomes.

acquired immunological tolerance Unresponsiveness of the immune system to a given foreign antigen that can develop in some circumstances.

acrosomal vesicle Region at the head end of a sperm cell that contains a sac of

hydrolytic enzymes used to digest the protective coating of the egg.

acrosome reaction Reaction that occurs when a sperm starts to enter an egg,

in which the contents of the acrosomal vesicle are released, helping the sperm

to penetrate the zona pellucida.

actin Abundant protein that forms actin filaments in all eucaryotic cells. The

monomeric form is sometimes called globular or G-actin; the polymeric form

is filamentous or F-actin.

actin-binding protein Protein that associates with either actin monomers or

actin filaments in cells and modifies their properties. Examples include

myosin, -actinin, and profilin.

actin filament (microfilament) Helical protein filament formed by the

polymerization of globular actin molecules. A major constituent of the

cytoskeleton of all eucaryotic cells and part of the contractile apparatus of

skeletal muscle. (See Panel 16 1, p. 909.)

action potential Rapid, transient, self-propagating electrical excitation in the

plasma membrane of a cell such as a neuron or muscle cell. Action potentials,

or nerve impulses, make possible long-distance signaling in the nervous

system.

activated carrier Small diffusible molecule in cells that stores easilyexchangeable

energy in the form of one or more energy-rich covalent bonds.

Examples are ATP and NADPH. Also called a coenzyme.

activation energy Extra energy that must be possessed by atoms or molecules

in addition to their ground-state energy in order to undergo a particular

chemical reaction. (9 1.)

active site Region of an enzyme surface to which a substrate molecule binds in

order to undergo a catalyzed reaction.

active transport Movement of a molecule across a membrane or other barrier

driven by energy other than that stored in the electrochemical gradient of the

transported molecule.

acyl group Functional group derived from a carboxylic acid ( ). (R

represents an alkyl group, such as methyl.)

adaptation Adjustment of sensitivity following repeated stimulation. This is

the mechanism that allows a neuron, a photodetector, or a bacterium to react to

small changes in stimuli even against a high background level of stimulation.

adaptin Protein that binds clathrin to the membrane surface in clathrin-coated

vesicles.

adaptive immune response Response of the vertebrate immune system to a

specific antigen that typically generates immunological memory.

adaptor protein General term for proteins in intracellular signaling pathways

that link different proteins in the pathway directly together.

adenomatous polyposis coli (APC) Tumor suppressor protein that forms part

of a protein complex that recruits free cytoplasmic -catenin and degrades it.

adenylyl cyclase (adenylate cyclase) Membrane-bound enzyme that catalyzes

the formation of cyclic AMP from ATP. An important component of some

intracellular signaling pathways.

adherens junction Cell junction in which the cytoplasmic face of the plasma

membrane is attached to actin filaments. Examples include the adhesion belts

linking adjacent epithelial cells and the focal contacts on the lower surface of

cultured fibroblasts.

adhesion belt Beltlike adherens junction that encircles the apical end of an

epithelial cell and attaches it to the adjoining cell. Also known as the zonula

adipocyte A fat cell.

ADP (adenosine 5 -diphosphate) Nucleotide that is produced by hydrolysis of

the terminal phosphate of ATP. It regenerates ATP when phosphorylated by an

energy-generating process such as oxidative phosphorylation. (See Figure

2 57.)

adrenaline (epinephrine) Hormone released by chromaffin cells (in the

adrenal gland) and by some neurons in response to stress. Produces "fight or

flight" responses, including increased heart rate and blood sugar levels.

aerobic Describes a process that requires, or occurs in the presence of, gaseous

oxygen (O2).

affinity chromatography Type of chromatography in which the protein

mixture to be purified is passed over a matrix to which specific ligands for the

required protein are attached, so that the protein is retained on the matrix.

affinity constant (association constant) (Ka) Measure of the strength of

binding of the components in a complex. For components A and B and a

binding equilibrium A + B AB, the association constant is given by [AB]/

[A][B], and is larger the tighter the binding between A and B. (See also

dissociation constant.)

affinity maturation Progressive increase in the affinity of antibodies for the

immunizing antigen with the passage of time after immunization.

Akt see protein kinase B

alcohol Polar organic molecule that contains a functional hydroxyl group

( OH) bound to a carbon atom that is not in an aromatic ring. An example is

ethyl alcohol (CH3CH2OH).

aldehyde Organic compound that contains the group. An example is

glyceraldehyde. Can be oxidized to an acid or reduced to an alcohol.

alga (algae) Informal term used to describe a wide range of simple unicellular

and multicellular eucaryotic photosynthetic organisms. Examples include

Nitella, Volvox, and Fucus.

alkaloid Small but chemically complex nitrogen-containing metabolite

produced by plants as a defense against herbivores. Examples include caffeine,

morphine, and colchicine.

alkane (adjective aliphatic) Compound of carbon and hydrogen that has only

single covalent bonds. An example is ethane (CH3CH3).

alkene Hydrocarbon with one or more carbon-carbon double bonds. An

example is ethylene (CH2CH2).

alkyl group General term for a group of covalently linked carbon and

hydrogen atoms such as methyl ( CH3) or ethyl ( CH2CH3) groups. These

groups can be formed by removing a hydrogen atom from an alkane.

allele One of a set of alternative forms of a gene. In a diploid cell each gene

will have two alleles, each occupying the same position (locus) on homologous

chromosomes.

allelic exclusion The expression of an immunoglobulin chain (or T cell

receptor chain) gene from only one of the two homologous loci present for that

gene in the lymphocyte.

allosteric protein Protein that changes from one conformation to another

when it binds another molecule or when it is covalently modified. The change

in conformation alters the activity of the protein and can form the basis of

directed movement.

alpha helix (a-helix) Common folding pattern in proteins in which a linear

sequence of amino acids folds into a right-handed helix stabilized by internal

hydrogen bonding between backbone atoms.

alternative RNA splicing The production of different proteins from the same

RNA transcript by splicing it in different ways.

amide Molecule containing a carbonyl group linked to an amine.

amine Chemical group containing nitrogen and hydrogen. It becomes

positively charged in water.

amino acid Organic molecule containing both an amino group and a carboxyl

group. Those that serve as the building blocks of proteins are alpha amino

acids, having both the amino and carboxyl groups linked to the same carbon

atom. (See Panel 3 1, pp. 132 133.)

aminoacyl-tRNA synthetase Enzyme that attaches the correct amino acid to a

tRNA molecule to form an aminoacyl-tRNA. (6 57.)

amino group Weakly basic functional group derived from ammonia (NH3) in

which one or more hydrogen atoms are replaced by another atom. In aqueous

solution it can accept a proton and carry a positive charge.

amino terminus (N terminus) The end of a polypeptide chain that carries a

free -amino group.

aminoacyl tRNA Activated form of amino acid used in protein synthesis.

Consists of an amino acid linked through a labile ester bond from its carboxyl

group to a hydroxyl group on tRNA. (6 57.)

AMP (adenosine 5 -monophosphate) One of the four nucleotides in an RNA

molecule. Two phosphates are added to AMP to form ATP. (See Panel 2 6,

pp. 120 121.)

amphipathic Having both hydrophobic and hydrophilic regions, as in a

phospholipid or a detergent molecule.

anabolism System of biosynthetic reactions in a cell by which large molecules

are made from smaller ones.

anaerobic Describes a cell, organism, or metabolic process that functions in

the absence of air or, more precisely, in the absence of molecular oxygen (O2).

anaphase Stage of mitosis during which the two sets of chromosomes separate

and move away from each other. Composed of anaphase A (chromosomes

move toward the two spindle poles) and anaphase B (spindle poles move

apart).

anaphase-promoting complex (APC) Ubiquitin ligase that promotes the

destruction of a set of proteins, some of which initiate the separation of sister

chromatids during the metaphase-to-anaphase transition during mitosis.

anchorage dependence Dependence of cell growth on attachment to a

substratum.

anchoring junction Type of cell junction that attaches cells to neighboring

cells or to the extracellular matrix.

angiogenesis Growth of new blood vessels by sprouting from existing ones.

Ångstrom (Å) Unit of length used to measure atoms and molecules. Equal to

10 10 meter or 0.1 nanometer (nm).

animal pole In yolky eggs, that end free of yolk that cleaves more rapidly than

the vegetal pole.

ankyrin Protein mainly responsible for attaching the spectrin cytoskeleton to

the red blood cell plasma membrane.

antenna complex Part of a photosystem that captures light energy and

channels it into the photochemical reaction center. It consists of protein

complexes that bind large numbers of chlorophyll molecules and other

pigments.

anterior Situated toward the head end of the body.

anteroposterior Describes the axis running from the head to the tail of the

animal body.

antibiotic Substance such as penicillin or streptomycin that is toxic to

microorganisms. Usually a product of a particular microorganism or plant.

antibody (immunoglobulin) Protein produced by B cells in response to a

foreign molecule or invading microorganism. Often binds to the foreign

molecule or cell extremely tightly, thereby inactivating it or marking it for

destruction by phagocytosis or complement-induced lysis.

anticodon Sequence of three nucleotides in a transfer RNA molecule that is

complementary to a three-nucleotide codon in a messenger RNA molecule.

antigen Molecule that is able to provoke an immune response.

antigenic determinant (epitope) Specific region of an antigenic molecule that

binds to an antibody or a T cell receptor.

antigenic variation The ability to change the antigens displayed on the cell

surface; a property of some pathogenic microorganisms that enables them to

evade attack by the immune system.

antigen-presenting cell Cell that displays foreign antigen complexed with

MHC molecules on its surface.

antiparallel Describes the relative orientation of the two strands in a DNA

double helix; the polarity of one strand is oriented in the opposite direction to

that of the other.

antiporter Carrier protein that transports two different ions or small molecules

across a membrane in opposite directions, either simultaneously or in sequence.

antisense RNA RNA complementary to a specific RNA transcript of a gene

that can hybridize to the specific RNA and block its function.

APC see adenomatous polyposis coli; anaphase-promoting complex

apical Describes the tip of a cell, a structure, or an organ. The apical surface of

an epithelial cell is the exposed free surface, opposite to the basal surface. The

basal surface rests on the basal lamina that separates the epithelium from other

tissue.

apoptosis Form of cell death, also known as programmed cell death, in which

a 'suicide' program is activated within the cell, leading to fragmentation of the

DNA, shrinkage of the cytoplasm, membrane changes and cell death without

lysis or damage to neighboring cells. It is a normal phenomenon, occurring

frequently in a multicellular organism.

aqueous Pertaining to water, as for example, in an aqueous solution.

archea (singular archeon) Members of one of the two major divisions of

procaryotes (the Archea), the other being the Bacteria.

ARF protein Monomeric GTPase responsible for regulating both COPI coat

assembly and clathrin coat assembly at Golgi membranes.

aromatic Describes a molecule that contains carbon atoms in a ring, commonly drawn as linked through alternating single and double bonds. Often a molecule related to benzene.

ARP complex (ARP2/3 complex) Complex of proteins that nucleates actin

filament growth from the minus end.

asexual reproduction Any type of reproduction (such as budding in Hydra,

binary fission in bacteria, or mitotic division in eucaryotic microorganisms)

that does not involve gamete formation and fusion. It produces an individual

genetically identical to the parent.

association constant see affinity constant

aster Star-shaped system of microtubules emanating from a centrosome or

from a pole of a mitotic spindle.

astral microtubule In the mitotic spindle, any of the microtubules radiating

from the aster which are not attached to a kinetochore of a chromosome.

asymmetric cell division Cell division that produces two daughter cells that

differ, for example in size or in the presence or absence of some cytoplasmic

constituent.

atomic weight Mass of an atom relative to the mass of a hydrogen atom.

Essentially equal to the number of protons plus neutrons.

ATP (adenosine 5 -triphosphate) Nucleoside triphosphate composed of

adenine, ribose, and three phosphate groups that is the principal carrier of

chemical energy in cells. The terminal phosphate groups are highly reactive in

the sense that their hydrolysis, or transfer to another molecule, takes place with

release of a large amount of free energy. (2 26.)

ATP synthase Enzyme complex in the inner membrane of a mitochon-drion

and the thylakoid membrane of a chloroplast that catalyzes the formation of

ATP from ADP and inorganic phosphate during oxidative phosphorylation and

photosynthesis, respectively. Also present in the plasma membrane of bacteria.

ATPase Enzyme that catalyzes a process involving the hydrolysis of ATP. A

large number of different proteins have ATPase activity.

autoantibody Antibody produced by an individual against a protein, or other

potential antigen, of its own cells and tissues. Autoantibodies can cause

autoimmune disease.

autocatalysis Reaction that is catalyzed by one of its products, creating a

positive feedback (self-amplifying) effect on the reaction rate.

autocrine signaling Type of cell signaling in which a cell secretes signal

molecules that act on itself or on other adjacent cells of the same type.

autoimmune disease A pathological state in which the body mounts an

immune response against one or more of its own potential antigens.

autophagy Digestion of worn-out organelles by the cell's own lysosomes.

autoradiography Technique in which a radioactive object produces an image

of itself on a photographic film. The image is called an autoradiograph or

autoradiogram.

autosome Any chromosome other than a sex chromosome.

avidity Total binding strength of a polyvalent antibody with a polyvalent

antigen.

Avogadro's number 6 × 1023. This is the number of atoms in 1 gram of

hydrogen, and thus in the atomic or molecular weight equivalent in grams of

any element or molecule.

axon Long nerve cell process that is capable of rapidly conducting nerve

impulses over long distances so as to deliver signals to other cells.

axonal transport Directed transport of organelles and molecules along a nerve

cell axon. It can be anterograde (outward from the cell body) or retrograde

(back toward the cell body).

axoneme Bundle of microtubules and associated proteins that forms the core

of a cilium or flagellum in a eucaryotic cell and is responsible for their

movements.