• 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
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
• 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
• 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
• 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
• 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
• 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
• 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
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