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Question #47623

Difficulty: 8/10 | 🟢 Free

The Question

Despite possessing Kranz anatomy and a robust $CO_2$ -concentrating mechanism, $C_4$ plants like sugarcane experience a severe decline in photosynthetic yield under extreme heat stress (e.g., above 45°C). Which of the following is the primary physiological reason for this metabolic collapse?

Options

A.
PEP carboxylase in mesophyll cells begins to exhibit oxygenase activity at extreme temperatures, initiating photorespiration.
B.
Extreme heat denatures RuBisCO activase in bundle sheath cells and disrupts the ATP-dependent regeneration of PEP.
C.
RuBisCO in the bundle sheath cells switches to oxygenase activity because high temperatures increase its affinity for $O_2$ over $CO_2$ .
D.
The agranal chloroplasts of bundle sheath cells lose their ability to perform non-cyclic photophosphorylation, halting $NADPH$ synthesis.

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

PEP_Carboxylase|RuBisCO|Kranz_Anatomy|Compensation_Point_Logic

Why This Question Is Tricky

Common Mistakes

Conceptual Pivot: Students often memorize that ' $C_4$ plants are adapted to high temperatures and do not undergo photorespiration.' The common misconception is assuming that if a $C_4$ plant fails under heat, it must be because photorespiration was somehow triggered (Options A and C).

The pivot requires understanding that extreme heat causes metabolic collapse via direct enzyme denaturation (RuBisCO activase and PEP regeneration enzymes), NOT by inducing RuBisCO oxygenase activity, because the spatial separation and $CO_2$ pump physically prevent $O_2$ competition until the enzymatic machinery itself denatures.

Quick Facts About This Question

Question Information

Subject	Biology
Chapter	Photosynthesis in Higher Plants - Class 11
Concept	C4 Pathway Efficiency
Difficulty	8
Cognitive Load	5
Target Solve Time	120 sec
Access	Free