Question

Photosynthesis in C₄ plants is relatively less limited by atmospheric CO₂ levels because :

• A. four carbon acids are the primary initial CO₂ fixation products
• B. the primary fixation of CO₂ is mediated via PEP carboxylase
• C. effective pumping of CO₂ into bundle sheath cells
• D. RUBISCO in C₄ plants has higher affinity for CO₂

Answer 1

The Correct Option is B

The question addresses the mechanism of CO₂ fixation in C₄ plants. To understand why photosynthesis in C₄ plants is less limited by atmospheric CO₂ levels, we need to examine the process of CO₂ fixation in these plants and the role of specific enzymes and cell structures:

1. Understanding CO₂ Fixation in C₄ Plants:
   • C₄ plants possess a specialized photosynthesis process that efficiently fixes CO₂, which is better adapted to high temperatures and low CO₂ concentrations.
2. Role of PEP Carboxylase:
   • The primary fixation of CO₂ in C₄ plants occurs in the mesophyll cells. Here, CO₂ is initially fixed into a four-carbon compound, oxaloacetate, by the enzyme phosphoenolpyruvate (PEP) carboxylase.
   • PEP carboxylase has a higher affinity for CO₂ than RUBISCO (ribulose bisphosphate carboxylase-oxygenase), making it more efficient under low CO₂ concentrations.
   • This initial fixation forms a four-carbon compound, which is then transported to the bundle sheath cells, where CO₂ is released for the Calvin cycle.
3. Explanation of Incorrect Options:
   • Option: "four carbon acids are the primary initial CO₂ fixation products": While it is true that four carbon acids are formed, the efficiency gain in CO₂ fixation is primarily due to the enzyme PEP carboxylase, not the product itself.
   • Option: "effective pumping of CO₂ into bundle sheath cells": This is part of the process, but the key reason these plants can efficiently fix CO₂ at low concentrations is the initial action of PEP carboxylase.
   • Option: "RUBISCO in C₄ plants has higher affinity for CO₂": RUBISCO in C₄ plants does not have a higher affinity for CO₂ compared to C₃ plants. The higher CO₂ concentration in bundle sheath cells ensures efficient functioning of RUBISCO.

In conclusion, the primary fixation of CO₂ in C₄ plants is mediated via PEP carboxylase, which allows these plants to effectively fix CO₂ even at lower atmospheric concentrations, making it the correct answer.