How do plants absorb nitrogen from the environment? Describe the mechanism of nitrate assimilation and synthesis of amino acids in plants.
Plants primarily absorb nitrogen from the environment through their roots as nitrate (NO3-) and ammonium (NH4+) ions. Some plants, via symbiotic relationships with microorganisms (e.g., Rhizobium in legumes), can also fix atmospheric nitrogen (N2) into usable forms.
Nitrate Assimilation: Once absorbed, nitrate (NO3-) is reduced to ammonium (NH4+) in a two-step process:
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Nitrate Reductase: Converts NO3- to nitrite (NO2-) in the cytoplasm.
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Nitrite Reductase: Converts NO2- to NH4+ in plastids (chloroplasts in leaves, amyloplasts in roots).
Amino Acid Synthesis: The newly formed ammonium (NH4+), which is toxic if accumulated, is rapidly incorporated into organic compounds, primarily amino acids:
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Glutamine Synthetase (GS): Catalyzes the reaction of NH4+ with glutamate to form glutamine.
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Glutamate Synthase (GOGAT): Transfers the amide nitrogen from glutamine to 2-oxoglutarate to form two molecules of glutamate.
These two enzymes (GS-GOGAT pathway) are central. Glutamate and glutamine then serve as donors for the synthesis of all other amino acids and nitrogenous compounds.
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