N
NAD + (nicotinamide adenine dinucleotide)
Abbreviation of nicotinamide adenine dinucleotide, a coenzyme present in all cells that
helps enzymes transfer electrons during the redox reactions of metabolism.
NADP
Abbreviation of nicotinamide adenine dinucleotide phosphate, a coenzyme that functions as
an electron acceptor in the lightdependent reactions of photosynthesis.
Natural killer cell
A nonspecific defensive cell that attacks tumor cells and destroys infected body cells,
especially those harboring viruses.
Natural logarithm
The logarithm to base e where e ≈ 2.718. The natural logarithm is represented by the
symbol ln.
Natural selection
Differential success in the reproduction of different phenotypes resulting from the
interaction of organisms with their environment. Evolution occurs when natural selection
causes changes in relative frequencies of alleles in the gene pool.
Nectar
A sugary fluid that attracts insects to plants.
Negative feedback
A primary mechanism of homeostasis, whereby a change in a physiological variable that is
being monitored triggers a response that counteracts the initial fluctuation.
Nematocyst
A threadlike stinger, containing a poisonous or paralyzing substance, found in the cnidocyte
of cnidarians.
Nemertea
An equivalent name for Rhynchocoela.
Nephridium
A tubular excretory structure found in many invertebrates.