Discussion
Leguminous plants maintain a very low concentration of free oxygen in their root nodules because
Answer: B
Free oxygen stays low because of binding of oxygen to leghemoglobin.
Nitrogenase is irreversibly inactivated by oxygen, yet the bacteroids in the nodule need oxygen for respiration to supply the ATP that nitrogen fixation demands. Leghemoglobin, a plant haemoglobin made in large amounts in infected nodule cells (giving them their pink colour), binds oxygen tightly and delivers it to the respiring bacteroids while keeping the free oxygen concentration very low.
Rhizobia are aerobic, not anaerobic, so that option is wrong. Neither the reductase nor the nitrogenase component of the nitrogenase complex removes oxygen; both are themselves damaged by it.
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