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Question #47629

Difficulty: 8/10 | 🟢 Free

The Question

Scientists successfully introduce the genes for PEPCase and Pyruvate phosphate dikinase (PPDK) into a $C_3$ crop using biolistics, aiming to reduce photorespiration. Despite high expression of these enzymes, the transgenic plant exhibits a lower overall biomass yield compared to the wild-type $C_3$ plant. Which of the following best explains this physiological discrepancy?

Options

A.
The regeneration of PEP by PPDK consumes 2 ATP equivalents per cycle, creating a metabolic load that offsets any carbon gain due to the lack of Kranz anatomy.
B.
PEPCase exhibits a higher affinity for $O_2$ than RuBisCO, inadvertently increasing the rate of photorespiration in the transgenic mesophyll cells.
C.
The transgenic plant now requires 30 ATP and 12 NADPH to fix a single molecule of $CO_2$ , rapidly depleting the stroma's energy pool.
D.
The introduced $C_4$ enzymes competitively inhibit RuBisCO in the bundle sheath cells, preventing the synthesis of 3-phosphoglycerate (3-PGA).

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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

Biolistics/Gene_Gun_Physics|PEP_Regeneration_Energy_Cost|GEAC_Oversight

Why This Question Is Tricky

Common Mistakes

Conceptual Pivot: Students often memorize that ' $C_4$ plants are more efficient' without understanding the energetic cost of that efficiency.

The core misconception tested is failing to link the 12 extra ATP required for PEP regeneration to the necessity of Kranz anatomy.

Furthermore, Option C serves as a high-yield quantitative trap, testing the student's ability to distinguish between the ATP/NADPH requirements per $CO_2$ fixed versus per Glucose synthesized.

Quick Facts About This Question

Question Information

Subject	Biology
Chapter	Photosynthesis in Higher Plants - Class 11
Concept	PEP Regeneration Energy Cost
Difficulty	8
Cognitive Load	5
Target Solve Time	120 sec
Access	Free