Nitrogen Assimilation in Plants
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Direct answer
Plants cannot touch the 78 per cent of the atmosphere that is nitrogen gas; they absorb nitrogen mainly as nitrate from soil, and only prokaryotes can crack the inert N≡N triple bond with the enzyme nitrogenase. Symbiotic bacteria such as Rhizobium in legume root nodules (and Frankia in Alnus) reduce N2 to ammonia, shielded from oxygen by the pink pigment leghaemoglobin. In soil, nitrifiers oxidise ammonia to nitrite (Nitrosomonas, Nitrosococcus) and nitrite to nitrate (Nitrobacter, Nitrococcus); inside the plant, nitrate reductase walks nitrate back down to nitrite and then ammonia. Ammonia is toxic and is immediately fixed into amino acids — chiefly glutamate — and redistributed by transamination, with amides such as asparagine and glutamine serving as storage and transport forms.
What you must remember
- Nitrogenase: the Mo-Fe enzyme complex of N2 fixation; extremely oxygen-sensitive, needs ATP (about 16 ATP per N2, per standard biochemistry), and reduces N2 to 2NH3.
- Symbiotic pairs: Rhizobium with legumes (pea, gram, beans); Frankia with non-legumes such as Alnus; cyanobacteria like Anabaena and Nostoc in various associations.
- Free-living fixers: aerobic Azotobacter and Beijerinckia, anaerobic Rhodospirillum — matching questions love this trio.
- Leghaemoglobin: the pink, oxygen-scavenging nodule pigment that protects nitrogenase while supplying respiring bacteria with just enough oxygen — a scavenger, not a carrier.
- Nitrification pair: Nitrosomonas and Nitrosococcus (ammonia to nitrite), then Nitrobacter and Nitrococcus (nitrite to nitrate).
- Denitrification: Pseudomonas and Thiobacillus return nitrate to atmospheric nitrogen, completing the cycle.
- Ammonia assimilation: NH4+ enters the alpha-ketoglutarate framework to form glutamate (glutamate dehydrogenase and the GS-GOGAT route), then transamination hands the amino group onward to other keto acids.
- Amides: asparagine and glutamine carry two nitrogen atoms each and are the long-distance nitrogen couriers of the plant body.
From one N2 molecule to grain protein: the full relay
Follow one nitrogen molecule from air to seed. Diatomic nitrogen diffuses into a legume nodule, where bacteroid nitrogenase — burning ATP from bacterial respiration — reduces it to ammonia. The plant absorbs that ammonia into glutamate, and from that moment the nitrogen is plant property: transamination reactions move the amino group to oxaloacetate (making aspartate) and pyruvate (making alanine), building the twenty proteinogenic amino acids. For export to fruits and seeds, the plant bolts extra nitrogen onto aspartate and glutamate, producing the amides asparagine and glutamine, which travel efficiently in the phloem because they are non-toxic and nitrogen-dense.
Meanwhile the soil tells its parallel story. A farmer's ammonium fertiliser is oxidised within days by Nitrosomonas to nitrite and by Nitrobacter to nitrate — which is why nitrate, not ammonium, dominates the root's menu, and why heavy rain leaches nitrogen from fields. Waterlogged, anaerobic soil flips the system: Pseudomonas and Thiobacillus strip the oxygen from nitrate and exhale N2, a farmer's nitrogen budget literally returning to the sky. NEET rewards candidates who can hold all three casts — fixers, nitrifiers, denitrifiers — in their heads simultaneously, because the matching question rarely tests one alone.
Where the confusion costs marks
The recurring slip is assigning the wrong job to a genus: Nitrosomonas makes nitrite, Nitrobacter makes nitrate — remember it as the "s" of Nitrosomonas starting the process (nitrite first). Second, leghaemoglobin questions: its credited function is oxygen scavenging to protect nitrogenase; options describing it as an oxygen carrier like muscle myoglobin are distractors. Third, direction of reactions: nitrate to ammonia inside the plant is reduction (the plant's own enzymes), while ammonia to nitrate in soil is oxidation (bacteria) — students who reverse these two pathways lose the reasoning question even when they know every genus. Finally, do not claim plants use N2 directly; the phrase "plants cannot utilise atmospheric nitrogen directly" is nearly a fixed NEET assertion.
Frequently asked questions
Why can plants not use atmospheric nitrogen directly?
The N≡N triple bond is inert, and only prokaryotic nitrogenase can break it, so plants depend on nitrate, ammonia or nitrogen fixed by bacteria.
What is the role of leghaemoglobin in root nodules?
It scavenges oxygen, creating the anaerobic micro-environment nitrogenase needs while still permitting respiration of the bacteroids.
Which bacteria convert ammonia to nitrate, and in how many steps?
Nitrification runs in two steps — Nitrosomonas and Nitrosococcus oxidise ammonia to nitrite, then Nitrobacter and Nitrococcus oxidise nitrite to nitrate.
How is toxic ammonia immediately handled in plant cells?
It is fixed into amino acids, chiefly glutamate, and redistributed to other keto acids by transamination; excess is stored as the amides asparagine and glutamine.
Name the free-living and symbiotic nitrogen fixers NEET commonly lists.
Free-living: Azotobacter, Beijerinckia, Rhodospirillum, Anabaena, Nostoc; symbiotic: Rhizobium (legumes) and Frankia (Alnus).