Before You Begin
Question #46950
The Question
In a culture of Anabaena actively fixing atmospheric nitrogen, light intensity is experimentally reduced to a point where it becomes the limiting factor according to Blackman's Law. Despite the continuous bubbling of abundant N2 gas, nitrogenase activity in the heterocysts rapidly declines. Which of the following best explains this observation?
Options
- A.
A. The heterocysts lose their thick, oxygen-impermeable walls under low light, leading to nitrogenase degradation.
- B.
B. Reduced photolysis of water in the heterocysts directly limits the supply of protons and electrons for nitrogen reduction.
- C.
C. Decreased photosynthetic yield in vegetative cells restricts the translocation of carbon skeletons required for ATP and reductant generation in heterocysts.
- D.
D. The nitrogenase enzyme shifts its substrate affinity from N2 to O2 due to a drop in the intracellular ATP/ADP ratio.
Check Your Answer
Correct Answer
Step-by-Step Solution
Explanation
How the Examiner Thinks
Evolutionary_Exception
Tackle problems where standard rules evolve or change based on specific environmental constraints.
Concepts Behind This Question
Cyanobacteria|Archaebacteria_Cell_Wall_Composition|Peptidoglycan
Why This Question Is Tricky
Common Mistakes
Conceptual Pivot: Students often memorize the spatial separation of nitrogen and carbon fixation in cyanobacteria but fail to synthesize their metabolic interdependence.
A common misconception is that heterocysts are entirely self-sufficient for energy via PSI cyclic photophosphorylation.
While PSI provides some ATP, the bulk of energy and all reductants come from the catabolism of sugars imported from vegetative cells.
The question tests the application of Blackman's Law across two interdependent cell types, requiring high-level integrative reasoning.
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 |