Essential Cell Biology 5th edition

Glossary G:17
stroma
In a chloroplast, the large interior space that contains the
enzymes needed to incorporate CO 2 into sugars during the
carbon-fixation stage of photosynthesis; equivalent to the
matrix of a mitochondrion.
substrate
A molecule on which an enzyme acts to catalyze a chemical
reaction.
substrate
A molecule on which an enzyme acts to catalyze a chemical
reaction.
subunit
A monomer that forms part of a larger molecule, such as an
amino acid residue in a protein or a nucleotide residue in
a nucleic acid. Can also refer to a complete molecule that
forms part of a larger molecule. Many proteins, for example,
are composed of multiple polypeptide chains, each of which
is called a protein subunit.
sugar
A substance made of carbon, hydrogen, and oxygen with
the general formula (CH 2 O) n . A carbohydrate or saccharide.
The “sugar” of everyday use is sucrose, a sweet-tasting
disaccharide made of glucose and fructose.
survival factor
Extracellular signal molecule that must be present to
suppress apoptosis.
symport
A transporter that transfers two different solutes across a
cell membrane in the same direction.
synapse
Specialized junction where a nerve cell
communicates with another cell (such as
a nerve cell, muscle cell, or gland cell),
usually via a neurotransmitter secreted by
the nerve cell.
synaptic vesicle
Small membrane-enclosed sac filled with neurotransmitter
that releases its contents by exocytosis at a synapse.
telomerase
Enzyme that elongates telomeres, synthesizing the
repetitive nucleotide sequences found at the ends of
eukaryotic chromosomes.
telomere
Repetitive nucleotide sequence that caps the ends of
linear chromosomes. Counteracts the tendency of the
chromosome otherwise to shorten with each round of
replication.
telophase
Final stage of mitosis in which the two sets of separated
chromosomes decondense and become enclosed by a
nuclear envelope.
template
A molecular structure that serves as a pattern for the
production of other molecules. For example, one strand
of DNA directs the synthesis of the complementary DNA
strand.
tertiary structure
Complete three-dimensional structure of a fully folded
protein.
tethering protein
Filamentous transmembrane protein involved in the
docking of transport vesicles to target membranes.
thylakoid
In a chloroplast, the flattened, disclike sac whose membranes
contain the proteins and pigments that convert light energy
into chemical-bond energy during photosynthesis.
tight junction
Cell–cell junction that seals adjacent epithelial cells together,
preventing the passage of most dissolved molecules from
one side of the epithelial sheet to the other.
tissue
Cooperative assembly of cells and matrix woven together
to form a distinctive multicellular fabric with a specific
function.
transcription
Process in which RNA polymerase uses one strand of
DNA as a template to synthesize a complementary RNA
sequence.
transcription regulator
Protein that binds specifically to a regulatory DNA sequence
to switch a gene either on or off.
transcriptional activator
A protein that binds to a specific regulatory region of DNA to
stimulate transcription of an adjacent gene.
transcriptional repressor
A protein that binds to a specific regulatory region of DNA
to prevent transcription of an adjacent gene.
transfer RNA (tRNA)
Small RNA molecule that serves as an adaptor that “reads”
a codon in mRNA and adds the correct amino acid to the
growing polypeptide chain.
transformation
Process by which cells take up DNA molecules from their
surroundings and then express genes present on that DNA.
transgenic organism
A plant or animal that has stably incorporated into its
genome one or more genes derived from another cell or
organism.
transition state
Transient structure that forms during the course of a
chemical reaction; in this configuration, a molecule has the
highest free energy; it is no longer the substrate, but is not
yet the product.
translation
Process by which the sequence of nucleotides in a messenger
RNA molecule directs the incorporation of amino acids into
protein.
translation initiation factor
Protein that promotes the proper association of ribosomes
with mRNA and is required for the initiation of protein
synthesis.
transmitter-gated ion channel
Transmembrane receptor protein or protein complex that
opens in response to the binding of a neurotransmitter,
allowing the passage of a specific inorganic ion; its
activation can trigger an action potential in a postsynaptic
cell.
transport vesicle
Membrane vesicle that carries proteins from one
intracellular compartment to another—for example, from
the endoplasmic reticulum to the Golgi apparatus.
transporter
Membrane transport protein that moves a solute across a
cell membrane by undergoing a series of conformational
changes.
transposon
General name for short segments of DNA that can move
from one location to another in the genome. Also known as
mobile genetic elements.