Essential Cell Biology 5th edition

Glossary
acetyl CoA
Activated carrier that donates the carbon atoms in its readily
transferable acetyl group to many metabolic reactions,
including the citric acid cycle and fatty acid biosynthesis;
the acetyl group is linked to coenzyme A (CoA) by a
thioester bond that releases a large amount of energy when
hydrolyzed.
acid
A molecule that releases a proton when dissolved in water;
this dissociation generates hydronium (H 3 O + ) ions, thereby
lowering the pH.
actin filament
Thin, flexible protein
filament made from
a chain of globular actin molecules; a major constituent of
all eukaryotic cells, this cytoskeletal element is essential for
cell movement and for the contraction of muscle cells.
actin-binding protein
Protein that interacts with actin monomers or filaments
to control the assembly, structure, and behavior of actin
filaments and networks.
action potential
Traveling wave of electrical excitation caused by rapid,
transient, self-propagating depolarization of the plasma
membrane in a neuron or other excitable cell; also called
a nerve impulse.
activated carrier
A small molecule that stores energy or chemical groups
in a form that can be donated to many different metabolic
reactions. Examples include ATP, acetyl CoA, and NADH.
activation energy
The energy that must be acquired by a molecule to undergo
a chemical reaction.
active site
Region on the surface of an enzyme that binds to a substrate
molecule and catalyzes its chemical transformation.
active transport
The movement of a solute across a membrane against its
electrochemical gradient; requires an input of energy, such
as that provided by ATP hydrolysis.
adaptation
Adjustment of sensitivity following repeated stimulation;
allows a cell or organism to register small changes in a
signal despite a high background level of stimulation.
adenylyl cyclase
Enzyme that catalyzes the formation of cyclic AMP from
ATP; an important component in some intracellular
signaling pathways.
adherens junction
Cell junction that helps hold together epithelial cells in a
sheet of epithelium; actin filaments inside the cell attach to
its cytoplasmic face.
ADP
Nucleoside diphosphate produced by hydrolysis of the
terminal phosphate of ATP. (See Figure 3–31.)
allele
An alternative form of a gene; for a given gene, many alleles
may exist in the gene pool of the species.
allosteric
Describes a protein that can exist in multiple conformations
depending on the binding of a molecule (ligand) at a
site other than the catalytic site; such changes from one
conformation to another often alter the protein’s activity or
ligand affinity.
alpha helix (α helix)
Folding pattern, common in many proteins, in which a
single polypeptide chain twists around itself to form a rigid
cylinder stabilized by hydrogen bonds between every fourth
amino acid.
alternative splicing
The production of different mRNAs (and proteins) from the
same gene by splicing its RNA transcripts in different ways.
Alu sequence
Family of mobile genetic elements that comprises about
10% of the human genome; this short, repetitive sequence is
no longer mobile on its own, but requires enzymes encoded
by other elements to transpose.
amino acid
Small organic molecule containing both an amino group
and a carboxyl group; it serves as the building block of
proteins.
amino acid sequence
The order of the amino acid subunits in a protein chain.
Sometimes called the primary structure of a protein.
aminoacyl-tRNA synthetase
During protein synthesis, an enzyme that attaches the
correct amino acid to a tRNA molecule to form a “charged”
aminoacyl-tRNA.
amphipathic
Having both hydrophobic and hydrophilic regions, as in a
phospholipid or a detergent molecule.
anabolic pathway
Series of enzyme-catalyzed reactions by which large
biological molecules are synthesized from smaller subunits;
usually requires an input of energy.
anabolism
Set of metabolic pathways by which large molecules are
made from smaller ones.
anaphase
Stage of mitosis during which the two sets of chromosomes
separate and are pulled toward opposite ends of the dividing
cell.
anaphase-promoting complex (APC/C)
A protein complex that triggers the separation of sister
chromatids and orchestrates the carefully timed destruction
of proteins that control progress through the cell cycle; the
complex catalyzes the ubiquitylation of its targets.
antenna complex
In chloroplasts and photosynthetic bacteria, the part of
the membrane-bound photosystem that captures energy
from sunlight; contains an array of proteins that bind
hundreds of chlorophyll molecules and other photosensitive
pigments.