Essential Cell Biology 5th edition

G:12 Glossary
N-terminus
The end of a polypeptide chain that carries a free α-amino
group.
Na + pump (or Na + -K + ATPase)
Transporter found in the plasma membrane of most animal
cells that actively pumps Na + out of the cell and K + in using
the energy derived from ATP hydrolysis.
NAD +
A molecule that accepts a hydride ion (H – ) from a donor
molecule, thereby producing the activated carrier NADH.
Widely used in the energy-producing breakdown of sugar
molecules. (See Figure 3–34.)
NADH
Activated carrier of electrons that is widely used in the
energy-producing breakdown of sugar molecules. (See
Figure 3–34.)
NADP +
Molecule that accepts a hydride ion (H – ) from a donor
molecule, thereby producing the activated carrier NADPH;
widely used as an electron donor in biosynthetic pathways.
NADPH
Activated carrier closely related to NADH and used as an
electron donor in biosynthetic pathways. In the process it is
oxidized to NADP + .
Nernst equation
An equation that relates the concentrations of an inorganic
ion on the two sides of a permeable membrane to the
membrane potential at which there would be no net
movement of the ion across the membrane.
nerve terminal
Structure at the end of an axon that signals to another
neuron or target cell.
neuron
An electrically excitable cell that integrates and transmits
information as part of the nervous system; a nerve cell.
neurotransmitter
Small signaling molecule secreted by a nerve cell at a
synapse to transmit information to a postsynaptic cell.
Examples include acetylcholine, glutamate, GABA, and
glycine.
nitric oxide (NO)
Locally acting gaseous signal molecule that diffuses
across cell membranes to affect the activity of intracellular
proteins.
nitrogen fixation
Conversion of nitrogen gas from the atmosphere into
nitrogen-containing molecules by soil bacteria and
cyanobacteria; requires a great deal of energy.
noncovalent bond
Chemical association that does not involve the sharing
of electrons; singly they are relatively weak, but they can
sum together to produce strong, highly specific interactions
between molecules. Examples are hydrogen bonds and van
der Waals attractions.
nonhomologous end joining
An error-prone mechanism for repairing double-strand
breaks in DNA by rejoining the two broken ends; often
results in a loss of information at the site of repair.
nuclear envelope
Double membrane surrounding the nucleus. Consists of
outer and inner membranes, perforated by nuclear pores.
nuclear lamina
Fibrous layer on the inner surface of the inner nuclear
membrane formed as a network of intermediate filaments
made from nuclear lamins.
nuclear magnetic resonance (NMR) spectroscopy
Technique used for determining the three-dimensional
structure of a protein in solution.
nuclear pore
Channel through which selected large
molecules move between the nucleus
and the cytoplasm.
nuclear receptor
Protein inside a eukaryotic cell that, on binding to a signal
molecule, enters the nucleus and regulates transcription.
nucleolus
Large structure within the nucleus where ribosomal RNA is
transcribed and ribosomal subunits are assembled.
nucleosome
Beadlike structural unit of a eukaryotic chromosome
composed of a short length of DNA wrapped around an
octameric core of histone proteins; includes a nucleosomal
core particle (DNA plus histone protein) along with a
segment of linker DNA that ties the core particles together.
nucleotide
Basic building block of the nucleic acids, DNA and RNA; a
nucleoside linked to a phosphate.
nucleus
In biology, refers to the prominent, rounded structure that
contains the DNA of a eukaryotic cell. In chemistry, refers to
the dense, positively charged center of an atom.
Okazaki fragment
Short length of DNA, including an RNA primer, produced on
the lagging strand during DNA replication. Following primer
removal, adjacent fragments are rapidly joined together by
DNA ligase to form a continuous DNA strand.
oncogene
A gene that, when activated, can potentially make a cell
cancerous. Typically a mutant form of a normal gene (protooncogene)
involved in the control of cell growth or division.
open reading frame (ORF)
Long sequence of nucleotides that contains no stop codon;
used to identify potential protein-coding sequences in DNA.
optogenetics
Technique that uses light to control the activity of neurons
into which light-gated ion channels have been artificially
introduced.
organelle
A discrete structure or subcompartment of a eukaryotic
cell that is specialized to carry out a particular function.
Examples include mitochondria and the Golgi apparatus.
organic molecule
Chemical compound that contains carbon and hydrogen.
organoid
A miniature, three-dimensional collection of tissues formed
from the proliferation, differentiation, and self-assembly of
pluripotent cells in culture.
osmosis
Passive movement of water across a cell membrane from
a region where the concentration of water is high (because
the concentration of solutes is low) to a region where the
concentration of water is low (and the concentration of
solutes is high).
oxidation
Removal of electrons from an atom, as occurs during the
addition of oxygen to a carbon atom or when a hydrogen
is removed from a carbon atom; can also refer to a partial
shift of electrons between atoms linked by a covalent bond.