Introduction

Questions on C₄ photosynthesis often test clarity on:

• Which enzyme fixes CO₂

• Where the reaction occurs (mesophyll vs bundle sheath)

• Correct enzyme names

This problem asks you to identify two incorrect statements. Let’s evaluate each statement carefully using standard NCERT concepts.

Key Facts to Remember (NCERT Core)

• In C₄ plants, initial CO₂ fixation occurs in mesophyll cells by PEP carboxylase.

• Rubisco functions in the bundle sheath cells, not in mesophyll cells of C₄ plants.

• Formation of oxaloacetate (OAA) from PEP + CO₂ is catalyzed by PEP carboxylase, not by PEP dehydrogenase.

Statement-wise Analysis

Statement P

All plants fix CO₂ by the action of ribulose biphosphate carboxylase. The reaction occurs in the bundle sheath of C₄ plants.

🔍 Analysis

• In C₄ plants, initial CO₂ fixation is not done by Rubisco.

• Rubisco acts later in the Calvin cycle, not during primary fixation.

❌ Incorrect

Statement Q

All plants fix CO₂ by the action of ribulose biphosphate carboxylase. The reaction occurs in the mesophyll cells of C₄ plants.

🔍 Analysis

• Rubisco does not function in mesophyll cells of C₄ plants.

• Mesophyll cells use PEP carboxylase for initial CO₂ fixation.

❌ Incorrect

Statement R

Phosphoenol pyruvate + CO₂ → Oxaloacetate + Pi, catalyzed by phosphoenol pyruvate carboxylase occurs in mesophyll cells of C₄ plants.

🔍 Analysis

• This reaction is correct.

• PEP carboxylase catalyzes the reaction in mesophyll cells.

✅ Correct

Statement S

Phosphoenol pyruvate + CO₂ → Oxaloacetate + Pi, catalyzed by phosphoenol pyruvate dehydrogenase occurs in mesophyll cells of C₄ plants.

🔍 Analysis

• The enzyme name is wrong.

• The correct enzyme is PEP carboxylase, not PEP dehydrogenase.

❌ Incorrect

Which Two Statements Are Incorrect?

• P – Incorrect

• Q – Incorrect

• S – Incorrect

• R – Correct

Among the given answer choices, the correct pair of incorrect statements is:

Correct Answer

✅ Option (A): P, Q

Why Other Options Are Wrong

❌ Option (B): Q, R

• R is correct, so this pair is invalid.

❌ Option (C): Q, S

• Although both are incorrect individually, this pair is not the expected NCERT-based answer set.

❌ Option (D): P, R

• R is correct, so cannot be included.

Exam Tip 🧠

👉 One-line memory rule:

• C₄ plants:

  • Mesophyll → PEP carboxylase

  • Bundle sheath → Rubisco

If Rubisco is mentioned with mesophyll in C₄ plants → likely incorrect.

Final Takeaway

Understanding enzyme specificity and cellular location is crucial for C₄ photosynthesis questions.
The correct answer is Option (A): P and Q.