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Question #46601
The Question
In a legume-Rhizobium symbiosis, the host plant may initiate 'biological decoupling'—downregulating nodule maintenance and nitrogen fixation—when subjected to prolonged photosynthetic stress, despite the soil being deficient in mobile nitrogen. Which of the following statements best explains the metabolic necessity of this downregulation?
Options
- A.
The host plant cannot sustain the supply of 8 ATP per NH3 produced, which is normally derived from the respiration of host-provided sugars.
- B.
The energy deficit prevents the nitrogenase enzyme from oxidizing atmospheric N2 into nitrate (NO3-) for plant assimilation.
- C.
The reduced availability of ATP directly inhibits the synthesis of α-ketoglutaric acid, which acts as the primary electron donor for the nitrogenase complex.
- D.
The plant must conserve energy by halting the synthesis of leghemoglobin, which is required to maintain an aerobic environment for the bacteroids.
Check Your Answer
Correct Answer
Step-by-Step Solution
Explanation
How the Examiner Thinks
Regulatory_Failure
Understand how NOT, EXCEPT, and logical constraints are used to manipulate exam results.
Concepts Behind This Question
Symbiotic Nitrogen Fixation|Nutrient Immobility|Ecosystem Productivity
Why This Question Is Tricky
Common Mistakes
Conceptual Pivot: Distinguishing the exact metabolic requirements and products of the nitrogenase reaction from downstream assimilation.
The trap relies on Functional Confusion and Word-Play: students often confuse the end-product of biological fixation (NH3) with nitrification (NO3-), misunderstand the role of α-ketoglutaric acid (assimilation vs fixation), and misread 'aerobic' for 'anaerobic' regarding leghemoglobin's function.
Quick Facts About This Question
Question Information
| Subject | Biology |
| Chapter | Microbes in Human Welfare - Class 12 |
| Concept | Symbiotic Nitrogen Fixation |
| Difficulty | 8 |
| Cognitive Load | 5 |
| Target Solve Time | 120 sec |
| Access | Free |