Question

Which of the following statements are true about the bundle sheath cells of \( C_4 \) plants?
(A). In comparison to mesophyll cells, they contain large number of agranal chloroplasts
(B). They have very thin cell wall, as compared to mesophyll cells, to facilitate gaseous exchange
(C). They have no intercellular spaces between them
(D). They generate less/no oxygen because of the low activity/lack of photosystem II

Choose the correct answer from the options given below:

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

Hint:

To prevent photorespiration, RuBisCO must be kept in an environment with high \( \text{CO}_2 \) and low \( \text{O}_2 \).
\( C_4 \) plants achieve this by making bundle sheath cell walls thick (blocking gas exit) and removing PSII (preventing local \( \text{O}_2 \) production).

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
\( C_4 \) plants adapt to hot and dry environments by utilizing Kranz anatomy, which spatially separates carbon assimilation from carbon fixation.
Initial \( \text{CO}_2 \) fixation occurs in mesophyll cells, where PEP carboxylase forms a four-carbon compound. This compound is transported to the bundle sheath cells, where it is decarboxylated to release \( \text{CO}_2 \) directly around RuBisCO, minimizing photorespiration.

Step 2: Detailed Explanation:
Let us evaluate each anatomical and physiological statement:
- Statement (A) is true: \( C_4 \) plants exhibit dimorphic chloroplasts.
The mesophyll cells contain standard granal chloroplasts (with grana stacks).
In contrast, the bundle sheath cells contain larger, agranal chloroplasts that lack distinct grana stacks.
- Statement (B) is false: To build up and maintain a high partial pressure of \( \text{CO}_2 \) inside the bundle sheath cells, these cells must prevent the gas from leaking back into the mesophyll.
Consequently, the cell walls of bundle sheath cells are exceptionally thick, highly reinforced, and often suberized, making them highly impermeable to gases.
- Statement (C) is true: The bundle sheath cells are tightly packed around the vascular bundles in a continuous ring, with no intercellular air spaces between them.
This tight cellular arrangement is another adaptation designed to prevent gas leakage.
- Statement (D) is true: The agranal chloroplasts of bundle sheath cells lack functional Photosystem II (PSII) complexes.
Since PSII is the site of light-driven photolysis of water, these cells generate very little to no oxygen.
This keeps the local concentration of oxygen low in the bundle sheath, preventing oxygen from competing with \( \text{CO}_2 \) for the active site of RuBisCO.
Thus, statements (A), (C), and (D) are correct, while statement (B) is false.

Step 3: Final Answer:
The correct option is (B) (A), (C) and (D) only.