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Question #46948
The Question
During a severe and prolonged thermal spike in a freshwater ecosystem, the nitrogen-fixing capacity of the cyanobacterium Anabaena collapses significantly earlier than its general photosynthetic survival. Which of the following best explains this physiological shift?
Options
- A.
The bioenergetic cost of maintaining the thick, oxygen-impermeable wall of heterocysts becomes unsustainable, leading to structural breakdown and oxygen-induced inactivation of Nitrogenase.
- B.
Thermal stress triggers oxygen evolution by Photosystem II within the heterocysts, which irreversibly inactivates the oxygen-sensitive Nitrogenase enzyme.
- C.
Increased temperature enhances oxygen solubility in the aquatic medium, forcing vegetative cells to rapidly differentiate into heterocysts and depleting cellular ATP.
- D.
The thermal spike denatures the leghemoglobin present in the heterocysts, destroying its ability to scavenge oxygen and protect Nitrogenase.
Check Your Answer
Correct Answer
Step-by-Step Solution
Explanation
How the Examiner Thinks
Comparative_Analysis
Sharpen your ability to evaluate multiple scenarios and distinguish between competing potential answers.
Concepts Behind This Question
Cyanobacteria|Heterocyst|Thermal_Tolerance_Ranges
Why This Question Is Tricky
Common Mistakes
Conceptual Pivot: Students often memorize that heterocysts fix nitrogen but fail to integrate the bioenergetic cost of maintaining their structural adaptations (thick walls, lack of PSII).
The distractors exploit high-frequency NEET traps: confusing cyanobacterial adaptations with legume adaptations (leghemoglobin), forgetting that heterocysts lack PSII entirely, and misunderstanding the physical relationship between temperature and gas solubility.
Quick Facts About This Question
Question Information
| Subject | Biology |
| Chapter | Biological Classification - Class 11 |
| Concept | Cyanobacteria Heterocyst Function |
| Difficulty | 8 |
| Cognitive Load | 5 |
| Target Solve Time | 120 sec |
| Access | Free |