Encyclopedia of Biology

nitrogenase AnENZYMEcomplex from bacteria that
catalyzes the reduction of dinitrogen to ammonia: N 2

• 8e–+10H+Ô 2NH 4 + H 2 with the simultaneous
  HYDROLYSISof at least 16 ATP molecules. The electron
  donor is reduced FERREDOXINorflavodoxin. Dihydro-
  gen is always a coproduct of the reaction. Ethyne
  (acetylene) can also be reduced to ethene (ethylene)
  and in some cases ethane. All nitrogenases are IRON-
  SULFUR PROTEINS. Three different types, which differ in
  the type of COFACTORpresent, have been identified:
  molybdenum-nitrogenase (the most common, which
  contains the iron-molybdenum cofactor), vanadium-
  nitrogenase, and iron-only nitrogenase.
  See alsoFEMO-COFACTOR;REDUCTION.

nitrogen fixation The natural process where atmo-
spheric nitrogen, N 2 , is converted to compounds that
can be easily utilized by plants. All organisms require
nitrogen compounds, but few are able to utilize N 2 , a
relatively inert and unreactive form and, unfortunately,
the most readily available. Most organisms require
fixed forms such as NH 3 , NO 3 , NO 2 , or organic N.
Bacteria performnitrogen fixation by combining the
nitrogen with hydrogen to form nitrates (NH 3 ) in the

soil, which plants can then use. Cyanobacteria (blue-

green algae) and bacteria (e.g., Rhizobiumspp.; Azoto-

bacterspp.) associated with legumes (like peas) can fix

N 2 by reducing it to ammoniacal (ammonialike) N,

mostly in the form of amino acids.

The ASSIMILATIONof dinitrogen through microbial

reduction to ammonia and conversion into organoni-

trogen compounds such as amino acids. Only a limited

number of microorganisms are able to fix nitrogen.

See alsoNITROGENASE.

NMR See NUCLEAR MAGNETIC RESONANCE SPEC-

TROSCOPY.

nocturnal An animal or plant that is active during

the night rather than the day.

node A point on a plant stem where a leaf is attached.

nodes of Ranvier These are gaps or segments

between the neuron’s myelin sheath wrapped around

nodes of Ranvier 245

ANIMALS

(protein)

PLANTS

(protein)

N 2

free nitrogen in

atmosphere

NO 3 –

nitrates

NO 4

ammonium

compounds

decay

decay

fixation

by

lightning

denitrification

denitrification

nitrification

bacteria

leguminous

nodules

feeding

assimilation

industrial

fertilizers

(Haber process)

The circulation of nitrogen between organisms and the environment.