Answer: Scavenge O2 to very low levels, protecting nitrogenase from inactivation while supplying O2 for bacteroid respiration.
- A Fix atmospheric nitrogen gas directly into ammonia within the cytoplasm of nodule cells themselves in the majority of cases studied
- B Scavenge O2 to very low levels, protecting nitrogenase from inactivation while supplying O2 for bacteroid respiration
- C Transport carbon dioxide gas away from the immediate site of bacteroid cellular respiration activity as widely reported
- D Activate nod factor signaling molecules that initiate the curling of root hair cells nearby in standard practice
Correct answer: B. Scavenge O2 to very low levels, protecting nitrogenase from inactivation while supplying O2 for bacteroid respiration
Explanation: Leghemoglobin (plant hemoglobin in legume nodules): O2 buffer. Maintains O2 at very low concentration (O2 nM range) -- enough for bacteroid respiration but insufficient to inactivate nitrogenase (O2-labile).
Atmospheric N₂ is fixed into ammonia by nitrogen-fixing bacteria (protected from oxygen by leghaemoglobin in root nodules), converted to nitrate via nitrification for plant uptake, and eventually returned to the atmosphere as N₂ via denitrification, completing the cycle.
Concept context
Essential mineral elements, their roles and deficiency symptoms. Nitrogen fixation and soil nutrition.