Essential Cell Biology 5th edition

G:10 Glossary
iron–sulfur center
Tightly bound metal complex that carries electrons in
proteins that operate early in the electron-transport chain;
has a relatively weak affinity for electrons.
K + leak channel
Ion channel permeable to K + that randomly flickers between
an open and closed state; largely responsible for the resting
membrane potential in animal cells.
karyotype
An ordered display of the full set of chromosomes of a cell,
arranged with respect to size, shape, and number.
keratin filament
Class of intermediate filament abundant in epithelial
cells, where it provides tensile strength; main structural
component of hair, feathers, and claws.
kinesin
A large family of motor proteins that uses
the energy of ATP hydrolysis to move
toward the plus end of a microtubule.
kinetochore
Protein complex that assembles on the centromere of a
condensed mitotic chromosome; the site to which spindle
microtubules attach.
L1 element
Type of retrotransposon that constitutes 15% of the human
genome; also called LINE-1.
lagging strand
At a replication fork, the DNA strand that is made
discontinuously in short fragments that are later joined
together to form one continuous new strand.
lamellipodium
Dynamic sheetlike extension on the surface of an animal
cell, especially one migrating over a surface.
law of independent assortment
Principle that, during gamete formation, the alleles for
different traits segregate independently of one another;
Mendel’s second law of inheritance.
law of segregation
Principle that the maternal and paternal alleles for a trait
separate from one another during gamete formation and
then reunite during fertilization; Mendel’s first law of
inheritance.
leading strand
At a replication fork, the DNA strand that is made by
continuous synthesis in the 5′-to-3′ direction.
ligand
General term for a small molecule that binds to a specific
site on a macromolecule.
ligand-gated channel
An ion channel that is stimulated to open by the binding of
a small molecule such as a neurotransmitter.
light reactions
In photosynthesis, the set of reactions that converts the
energy of sunlight into chemical energy in the form of ATP
and NADPH (stage 1 of photosynthesis).
lipid
An organic molecule that is insoluble in water but dissolves
readily in nonpolar organic solvents; typically contains
long hydrocarbon chains or multiple rings. One class, the
phospholipids, forms the structural basis for biological
membranes.
lipid bilayer
Thin pair of closely juxtaposed sheets, composed mainly of
phospholipid molecules, that forms the structural basis for
all cell membranes.
local mediator
Secreted signal molecule that acts at a short range on
adjacent cells.
long noncoding RNA
Class of RNA molecules more than 200 nucleotides in length
that does not encode proteins. Often used to regulate gene
expression.
loss-of-function mutation
A genetic alteration that reduces or eliminates the activity of
a gene. Such mutations are usually recessive: the organism
can function normally as long as it retains at least one
normal copy of the affected gene.
lysosome
Membrane-enclosed organelle that breaks down worn-out
proteins and organelles and other waste materials, as well
as molecules taken up by endocytosis; contains digestive
enzymes that are typically most active at the acid pH found
inside these organelles.
lysozyme
Enzyme that severs the polysaccharide chains that form the
cell walls of bacteria; found in many secretions including
saliva and tears, where it serves as an antibiotic.
M cyclin
Regulatory protein that binds to mitotic Cdk to form M-Cdk,
the protein complex that triggers the M phase of the cell
cycle.
M phase
Period of the eukaryotic cell cycle during which the nucleus
and cytoplasm divide.
M-Cdk
Protein complex that triggers the M phase of the cell cycle;
consists of an M cyclin plus a mitotic cyclin-dependent
protein kinase (Cdk).
macromolecule
Polymer built from covalently linked subunits; includes
proteins, nucleic acids, and polysaccharides with a
molecular mass greater than a few thousand daltons.
MAP kinase
Mitogen-activated protein kinase. Signaling molecule that
is the final kinase in a three-kinase sequence called the
MAP-kinase signaling module.
MAP-kinase signaling module
Set of three functionally interlinked protein kinases that
allows cells to respond to extracellular signal molecules
that stimulate proliferation; includes a mitogen-activated
protein kinase (MAP kinase), a MAP kinase kinase, and a
MAP kinase kinase kinase.
mass spectrometry
Sensitive technique that enables the determination of the
exact mass of all of the molecules in a complex mixture.
matrix
Large internal compartment within a mitochondrion.
mechanically-gated channel
An ion channel that allows the passage of select ions across
a membrane in response to a physical perturbation.
meiosis
Specialized type of cell division by which eggs and sperm
cells are made. Two successive nuclear divisions with only
one round of DNA replication generate four haploid cells
from an initial diploid cell.
membrane domain
Functionally and structurally specialized region in the