Biology Bioenergetics NUMS 2024
PMDC Verified Question 22 of 67
Presence of PEP carboxylase in \( C_4 \) plants enable them to avoid photorespiration due to:
A
Presence of enzyme in high concentration
B
Absence of RuBisCO
C
PEP carboxylase being strictly carboxylase
D
High yield of enzyme
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option C: PEP carboxylase being strictly carboxylase
Concept:

\( C_4 \) plants have evolved a unique biochemical pump to ensure RuBisCO only ever interacts with \( CO_2 \), completely bypassing photorespiration.

Solution:

  • In mesophyll cells of \( C_4 \) plants, the initial carbon fixation is performed by PEP carboxylase, not RuBisCO.
  • The key evolutionary advantage is that PEP carboxylase has absolutely no affinity for oxygen. It is strictly a carboxylase.
  • It locks \( CO_2 \) into malate, safely shuttles it to the bundle-sheath cells, and floods RuBisCO with \( CO_2 \) so photorespiration cannot happen.


Why other options are incorrect:

  • \( C_4 \) plants definitely still possess RuBisCO (it operates in the bundle-sheath cells to run the Calvin cycle).
  • Concentration and yield of the enzyme are irrelevant; its structural specificity (zero oxygenase activity) is the mechanism.

Quality & Fidelity Assurance: Every question on BeambePrep is rigorously curated against the official PMDC syllabus with zero filler, zero out-of-syllabus content, and zero typos. When an authentic past paper originally contained a historical mistake or ambiguity from the examining board (such as UHS or NUMS), BeambePrep faithfully reflects the original paper while detailing the nuance and scientific consensus in the autopsy above.

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