Question

Characteristics of bundle sheath cells in $\text{C}_4$ plants}
(A) Have thin cell walls
(B) Contain centrifugally arranged chloroplasts
(C) Contain stacked thylakoid membranes and do not have starch granules
(D) Carry out light-independent reactions of photosynthesis

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

Hint:

To minimize photorespiration, bundle sheath cells perform the Calvin cycle under elevated $\text{CO}_2$ levels. Their chloroplasts are agranal (lack grana to avoid producing oxygen) but accumulate abundant starch.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
$\text{C}_4$ plants exhibit Kranz anatomy, characterized by two concentric photosynthetic cell layers: outer mesophyll cells and inner bundle sheath cells surrounding the vascular bundle.
This structural division of labor minimizes photorespiration by concentrating carbon dioxide around the enzyme RuBisCO in the bundle sheath cells.

Step 2: Detailed Explanation:
Let us analyze each of the stated characteristics of bundle sheath cells:
- (A) Have thin cell walls: This is incorrect.
Bundle sheath cells possess very thick cell walls that are suberized.
This thick wall acts as a barrier that prevents carbon dioxide from leaking back out into the mesophyll cells.
- (B) Contain centrifugally arranged chloroplasts: This is correct.
In certain $\text{C}_4$ plants (such as NADP-malic enzyme species like maize), the chloroplasts within bundle sheath cells are positioned centrifugally, meaning they are arranged toward the outer cell wall facing the mesophyll cells.
- (C) Contain stacked thylakoid membranes and do not have starch granules: This is incorrect.
Bundle sheath chloroplasts in NADP-ME $\text{C}_4$ plants are agranal, meaning they lack stacked thylakoids (grana) to minimize photosynthetic oxygen production.
However, they serve as the primary site for starch synthesis and contain abundant starch granules.
- (D) Carry out light-independent reactions of photosynthesis: This is correct.
The bundle sheath cells contain RuBisCO and the complete enzymatic machinery of the Calvin cycle (light-independent reactions) to fix carbon dioxide into carbohydrates.
Thus, only statements (B) and (D) are correct.

Step 3: Final Answer:
The correct characteristics of bundle sheath cells are (B) and (D) only.