Q.34 Leguminous plants maintain a very low concentration of free oxygen in their root nodules because
(A) the nitrogen fixing bacteria living in the root nodules are anaerobic
(B) of binding of oxygen to leghemoglobin
(C) reductase enzyme of the nitrogenase complex helps in removal of O2
(D) nitrogenase enzyme of the nitrogenase complex helps in removal of O2
Leguminous plants maintain low free oxygen in root nodules through leghemoglobin binding, protecting oxygen-sensitive nitrogenase while allowing respiration.
Correct Answer
Option (B) of binding of oxygen to leghemoglobin is correct. Leghemoglobin, a plant-produced hemoglobin-like protein in nodules, binds free O₂ to maintain microaerobic conditions (~10-50 nM), protecting nitrogenase while delivering O₂ for bacteroid respiration.
Option Analysis
(A) Nitrogen fixing bacteria are anaerobic: Incorrect
Rhizobia (e.g., Rhizobium, Bradyrhizobium) in nodules are microaerophilic, not strictly anaerobic; they require low O₂ for nitrogenase protection but need it for ATP-generating respiration.
(B) Binding of oxygen to leghemoglobin: Correct
Leghemoglobin buffers free O₂ at low levels in infected cells, giving nodules their pink color; it facilitates O₂ diffusion to bacteroids while preventing nitrogenase inactivation.
(C) Reductase enzyme removes O₂: Incorrect
No specific “reductase” in the nitrogenase complex directly removes O₂; respiration by bacteroids and plant cells consumes O₂, but leghemoglobin primarily controls free levels.
(D) Nitrogenase enzyme removes O₂: Incorrect
Nitrogenase is irreversibly inactivated by O₂ exposure; it does not remove O₂ but requires protection via leghemoglobin and nodule barriers to maintain activity.
| Option | Mechanism | Role in O₂ Control | Accurate? |
|---|---|---|---|
| (A) | Anaerobic bacteria | Respiration needs O₂ | No |
| (B) | Leghemoglobin binding | Buffers free O₂ | Yes |
| (C) | Reductase removal | Minor consumption | No |
| (D) | Nitrogenase removal | Gets inactivated | No |
