Answer: Glutamine synthetase (GS) incorporates NH4+ into glutamine; GOGAT transfers the amide group to alpha-ketoglutarate, forming two glutamates.
- A Glutamine-GOGAT: atmospheric N<sub>2</sub> gas is sometimes thought to be incorporated directly into glutamate without any prior reduction step occurring
- B Glutamine synthetase (GS) incorporates NH4+ into glutamine; GOGAT transfers the amide group to alpha-ketoglutarate, forming two glutamates
- C GS first degrades existing glutamate into free ammonia; GOGAT then reassembles it back into glutamine molecules afterward
- D This pathway is sometimes thought to operate mainly within leaf mesophyll cells and rarely elsewhere in the entire plant body
Correct answer: B. Glutamine synthetase (GS) incorporates NH4+ into glutamine; GOGAT transfers the amide group to alpha-ketoglutarate, forming two glutamates
Explanation: Primary NH4+ assimilation: GS catalyzes glutamate + NH4+ + ATP → glutamine. GOGAT (glutamine oxoglutarate aminotransferase) then catalyzes glutamine + alpha-ketoglutarate → 2 glutamate (using reduced ferredoxin or NADH). This is the main pathway; the older GDH pathway is secondary.
Atmospheric N₂ is fixed into ammonia by nitrogen-fixing bacteria (protected from oxygen by leghaemoglobin in root nodules), converted to nitrate via nitrification for plant uptake, and eventually returned to the atmosphere as N₂ via denitrification, completing the cycle.
Concept context
Essential mineral elements, their roles and deficiency symptoms. Nitrogen fixation and soil nutrition.