Recycling of NitrogenRecycling of Nitrogen

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• Nitrogen present in proteins and Nitrogen present in proteins and nucleic acidsnucleic acids

• Organisms cannot use nitrogen gas Organisms cannot use nitrogen gas in the air (Nin the air (N≡≡N)N)

• Must be converted from NMust be converted from N22 into more into more reactive form (e.g. NHreactive form (e.g. NH33 or NO or NO33

--) – ) – nitrogen fixationnitrogen fixation

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Nitrogen fixationNitrogen fixation• Fixation by living organismsFixation by living organisms

– Prokaryotes (Prokaryotes (RhizobiumRhizobium) - ) - mutualismmutualism– Lives freely in soil and roots of plants (e.g. Lives freely in soil and roots of plants (e.g.

leguminous plants).leguminous plants).– Plant roots produce protein (lectins) which bind Plant roots produce protein (lectins) which bind

to polysaccharides on cell surface. Bacteria to polysaccharides on cell surface. Bacteria invade roots and stimulate cells in the root to invade roots and stimulate cells in the root to divide and develop into small lumps or nodules, divide and develop into small lumps or nodules, inside which the bacteria form colonies.inside which the bacteria form colonies.

– Bacteria fix nitrogen with nitrogenaseBacteria fix nitrogen with nitrogenase– Nitrogen gas (NNitrogen gas (N22) to ammonium ions (NH) to ammonium ions (NH44

++))• Supply of H (comes from reduced NADP)Supply of H (comes from reduced NADP)• Supply of ATP (metabolism of sucrose)Supply of ATP (metabolism of sucrose)• Anaerobic conditions (maintained through production Anaerobic conditions (maintained through production

of leghaemoglobin – gets rid of oxygen which diffuses of leghaemoglobin – gets rid of oxygen which diffuses into nodules)into nodules)

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RhizobiumRhizobium

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• Fixation in the Fixation in the atmosphereatmosphere– Lightning can cause Lightning can cause

nitrogen molecules to nitrogen molecules to react with oxygen to react with oxygen to form nitrogen oxidesform nitrogen oxides

– Dissolves in rain and Dissolves in rain and carried to groundcarried to ground

ALBIO9700/2006JK

ALBIO9700/2006JK

• Fixation by the Fixation by the Haber processHaber process– Nitrogen and Nitrogen and

hydrogen gases are hydrogen gases are reacted together to reacted together to produce ammoniaproduce ammonia

– Ammonia often Ammonia often converted to converted to ammonium nitrate – ammonium nitrate – inorganic fertiliserinorganic fertiliser

• Use of fixed nitrogen by plantsUse of fixed nitrogen by plants– In legumes: In legumes:

•to make to make amino acidsamino acids•transported into xylem, distributed to transported into xylem, distributed to

all parts and used within cells to all parts and used within cells to synthesise proteinssynthesise proteins

– Other plants:Other plants:•Rely on fixed nitrogen in soilRely on fixed nitrogen in soil•Root hairs take up Root hairs take up nitrate ionsnitrate ions by by

active transportactive transport•Nitrate converted in roots to nitrite Nitrate converted in roots to nitrite

(NO(NO22--) then ammonia then amino acids ) then ammonia then amino acids

which are transported to other parts in which are transported to other parts in xylemxylem

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• Assimilation of nitrogen by animalsAssimilation of nitrogen by animals– Nitrogen supply comes from proteins Nitrogen supply comes from proteins

in diet and nucleic acidsin diet and nucleic acids– Proteins broken down to amino acids Proteins broken down to amino acids

(digestion), absorbed into blood and (digestion), absorbed into blood and distributed to cells, then built up distributed to cells, then built up again into proteinsagain into proteins

– Excess amino acids are Excess amino acids are deaminateddeaminated in liver, where nitrogen becomes part in liver, where nitrogen becomes part of urea molecules and excreted in of urea molecules and excreted in urineurine

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• Return of nitrate to the soil from living Return of nitrate to the soil from living organismsorganisms– Protein in cells broken down to amino acids Protein in cells broken down to amino acids

upon death (decomposers – protease enzymes)upon death (decomposers – protease enzymes)– Decomposers use some amino acids, some are Decomposers use some amino acids, some are

broken down and nitrogen released as ammoniabroken down and nitrogen released as ammonia– Ammonia also produced from urea Ammonia also produced from urea

((ammonificationammonification))– Ammonia in soil is converted to nitrite ions Ammonia in soil is converted to nitrite ions

(NO(NO22--) and then nitrate ions (NO) and then nitrate ions (NO33

--) by ) by nitrifying nitrifying bacteriabacteria ( (NitrosomonasNitrosomonas and and NitrobacterNitrobacter))

– Only occurs freely in well-aerated soilsOnly occurs freely in well-aerated soils– Some plants adapt by supplementing their Some plants adapt by supplementing their

nitrogen intake using animal proteinnitrogen intake using animal protein

ALBIO9700/2006JK

ALBIO9700/2006JK

• DenitrificationDenitrification– Denitrifying bacteria provide themselves Denitrifying bacteria provide themselves

with energy by reversing nitrogen with energy by reversing nitrogen fixation and converting nitrate to fixation and converting nitrate to nitrogen gas, which is returned to the airnitrogen gas, which is returned to the air

– Common in sewage treatment plants, Common in sewage treatment plants, compost heaps and wet soils compost heaps and wet soils

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