Nitrogen Metabolism — Important Questions

Alnus and actinomycete

Step 1: Symbiotic nitrogen fixation occurs in various plant-microbe associations. Step 2: The most well-known is legumes (angiosperms) with Rhizobium. Step 3: However, some non-leguminous angiosperms like Alnus, Myrica, and Purshia also fix nitrogen symbiotically — but their symbiont is an actinomycete, not Rhizobium. Step 4: Azolla-Anabaena is a pteridophyte-cyanobacteria association. Cycas-Cyanobacteria is a gymnosperm association. Digitaria-Azospirillum is another angiosperm-bacteria association. Step 5: Therefore, Alnus with actinomycete is the correct non-leguminous symbiotic system with

One pair of electrons is used to reduce 2H⁺ to H₂ instead of reducing N₂, indicating some energy loss.

Step 1: In the nitrogen fixation equation, 8 electrons (8e⁻) and 8 H⁺ are used, but only 6 electrons and 6 H⁺ are theoretically needed to reduce N₂ to 2NH₃. Step 2: The remaining 2e⁻ and 2H⁺ are used to produce one molecule of H₂. Step 3: This means nitrogenase always 'wastes' some electrons by producing H₂, making nitrogen fixation slightly inefficient. Step 4: This is an inherent characteristic of the nitrogenase enzyme — it cannot avoid this hydrogen evolution. Step 5: Options A, C, and D are factually incorrect — H₂ is a byproduct of energy wastage, not a reactant or pH regulator or reduce

Azotobacter has a very high respiration rate that consumes oxygen rapidly, protecting nitrogenase.

Step 1: Nitrogenase enzyme is extremely sensitive to oxygen (O₂) — oxygen inactivates it. Step 2: This creates a paradox for aerobic nitrogen fixers like Azotobacter — they need O₂ for respiration but must keep nitrogenase away from O₂. Step 3: Azotobacter overcomes this by maintaining an extremely high rate of aerobic respiration that rapidly consumes O₂ in its immediate surroundings. Step 4: This creates a locally oxygen-depleted zone around the nitrogenase enzyme. Step 5: Leghemoglobin is found only in legume nodules, not in Azotobacter. Heterocysts are structures found in cyanobacteria lik

Keto acid product: Oxaloacetic acid; Amino acid product: Glutamic acid

Step 1: In transamination, an amino group (–NH₂) is transferred from an amino acid to a keto acid. Step 2: The reaction is: α-Ketoglutaric acid + Aspartic acid → Glutamic acid + Oxaloacetic acid. Step 3: Aspartic acid (amino acid) donates its amino group to α-ketoglutaric acid (keto acid). Step 4: α-Ketoglutaric acid receives the amino group and becomes Glutamic acid (a new amino acid). Step 5: Aspartic acid, after losing its amino group, becomes Oxaloacetic acid (a keto acid). So the keto acid product is Oxaloacetic acid and the amino acid product is Glutamic acid. Options A, C, and D are inc