Describe the mechanism by which Rhizobium species fix atmospheric nitrogen in leguminous root nodules, including the symbiotic signaling pathway, the role of nodule organogenesis, and the energy cost for the plant.
Answer
Rhizobium infects root hairs releasing Nod factors that trigger nodule organogenesis; inside nodules, the bacteria convert N₂ to NH₃ via nitrogenase, producing H₂ as a by‑product; the plant supplies the required ATP (≈10 kJ per mole of N₂) and carbon skeletons, making the interaction energetically costly but ultimately beneficial.
Step-by-step solution
- Leguminous roots release flavonoids that attract Rhizobium; bacteria respond by producing Nod factors.
- Nod factors induce root hair curling and infection thread formation, initiating nodule organogenesis.
- Inside the nodule, nitrogenase converts atmospheric N₂ to NH₃, producing H₂ as a by‑product.
- The plant supplies ATP (≈10 kJ/mol N₂) and carbon sources; this high energy demand is compensated by the benefit of fixed nitrogen.
Common mistake: Confusing nitrogenase location or neglecting plant energy contribution
Concept tested: Symbiotic nitrogen fixation
Published · How this solution was prepared and checked
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