Question

Which of the following are correct about \(C_4\) cycle and \(C_4\) plant?
A. Phosphoenolpyruvate carboxylase is located in cytosol
B. \(C_4\) cycle occurs in the chloroplasts of bundle sheath cells
C. \(C_4\) cycle is a \(CO_2\) enrichment cycle
D. \(C_4\) cycle occurs in the chloroplasts of mesophyll cells
Choose the correct answer from the options given below:

• A. A and B only
• B. B and C only
• C. A, B and C only
• D. C and D only

Hint:

To remember this:
- Mesophyll cells \(\rightarrow\) Initial carboxylation by PEPCase in the cytosol.
- Bundle sheath cells \(\rightarrow\) Decarboxylation and Calvin cycle in the chloroplasts.
This spatial separation concentrates \(CO_2\) to eliminate photorespiration.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
\(C_4\) plants have evolved a specialized photosynthetic pathway to concentrate carbon dioxide around the enzyme Rubisco, suppressing photorespiration.
This pathway involves spatial separation, partitionining the initial carbon fixation and the subsequent Calvin cycle between two different cell types: mesophyll and bundle sheath cells.

Step 2: Detailed Explanation:
Let us evaluate each of the statements:
- Statement A: "Phosphoenolpyruvate carboxylase (PEPCase) is located in cytosol."
This is correct.
In the mesophyll cells of \(C_4\) plants, the initial fixation of atmospheric \(CO_2\) (as bicarbonate) is catalyzed by PEPCase, which is located in the cytosol.
- Statement B: "\(C_4\) cycle occurs in the chloroplasts of bundle sheath cells."
This is correct.
The four-carbon organic acids (malate or aspartate) produced in the mesophyll are transported to the chloroplasts of the bundle sheath cells. Here, they are decarboxylated to release concentrated \(CO_2\), which is then fixed by Rubisco via the standard Calvin cycle (\(C_3\) cycle).
- Statement C: "\(C_4\) cycle is a \(CO_2\) enrichment cycle."
This is correct.
The main function of the \(C_4\) pathway is to act as a biochemical pump that concentrates \(CO_2\) inside the bundle sheath cells. This enrichment suppresses the oxygenase activity of Rubisco, minimizing photorespiration and maximizing photosynthetic efficiency.
- Statement D: "\(C_4\) cycle occurs in the chloroplasts of mesophyll cells."
This is incorrect.
While the initial carboxylation occurs in the cytosol of mesophyll cells, the complete cycle (specifically the decarboxylation and carbohydrate synthesis steps) requires the bundle sheath chloroplasts.
Therefore, statements A, B, and C are correct.

Step 3: Final Answer:
The correct option is (C).