2. • Nitrogen present in proteins and nucleic
acids
• Organisms cannot use nitrogen gas in the
air (N≡N)
• Must be converted from N2 into more
reactive form (e.g. NH3 or NO3-) – nitrogen
fixation
ALBIO9700/2006JK
3. Nitrogen fixation
• Fixation by living organisms
– Prokaryotes (Rhizobium) - mutualism
– Lives freely in soil and roots of plants (e.g.
leguminous plants).
– Plant roots produce protein (lectins) which bind to
polysaccharides on cell surface. Bacteria invade roots
and stimulate cells in the root to divide and develop
into small lumps or nodules, inside which the bacteria
form colonies.
– Bacteria fix nitrogen with nitrogenase
– Nitrogen gas (N2) to ammonium ions (NH4+)
• Supply of H (comes from reduced NADP)
• Supply of ATP (metabolism of sucrose)
• Anaerobic conditions (maintained through production of
leghaemoglobin – gets rid of oxygen which diffuses into
nodules)
ALBIO9700/2006JK
5. • Fixation in the
atmosphere
– Lightning can cause
nitrogen molecules to
react with oxygen to form
nitrogen oxides
– Dissolves in rain and
carried to ground
ALBIO9700/2006JK
6. • Fixation by the Haber
process
– Nitrogen and hydrogen
gases are reacted
together to produce
ammonia
– Ammonia often
converted to ammonium
nitrate – inorganic
fertiliser
ALBIO9700/2006JK
7. • Use of fixed nitrogen by plants
– In legumes:
• to make amino acids
• transported into xylem, distributed to all
parts and used within cells to synthesise
proteins
– Other plants:
• Rely on fixed nitrogen in soil
• Root hairs take up nitrate ions by active
transport
• Nitrate converted in roots to nitrite (NO2-)
then ammonia then amino acids which are
transported to other parts in xylem
ALBIO9700/2006JK
8. • Assimilation of nitrogen by animals
– Nitrogen supply comes from proteins in
diet and nucleic acids
– Proteins broken down to amino acids
(digestion), absorbed into blood and
distributed to cells, then built up again
into proteins
– Excess amino acids are deaminated
in liver, where nitrogen becomes part of
urea molecules and excreted in urine
ALBIO9700/2006JK
9. • Return of nitrate to the soil from living
organisms
– Protein in cells broken down to amino acids upon
death (decomposers – protease enzymes)
– Decomposers use some amino acids, some are
broken down and nitrogen released as ammonia
– Ammonia also produced from urea
(ammonification )
– Ammonia in soil is converted to nitrite ions (NO2-) and
then nitrate ions (NO3-) by nitrifying bacteria
(Nitrosomonas and Nitrobacter)
– Only occurs freely in well-aerated soils
– Some plants adapt by supplementing their nitrogen
intake using animal protein
ALBIO9700/2006JK
11. • Denitrification
– Denitrifying bacteria provide themselves with
energy by reversing nitrogen fixation and
converting nitrate to nitrogen gas, which is
returned to the air
– Common in sewage treatment plants, compost
heaps and wet soils
ALBIO9700/2006JK
