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 remember this, recall that the Calvin cycle occurs inside the bundle sheath cells of $\text{C}_4$ plants. To prevent oxygen from competing with $\text{CO}_2$ at RuBisCO, these chloroplasts lack grana (are agranal, so they cannot perform oxygen-producing light reactions) but are packed with starch grains.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
$\text{C}_4$ plants exhibit Kranz anatomy, characterized by two concentric layers of photosynthetic cells: outer mesophyll cells and inner bundle sheath cells surrounding the vascular bundle.
This spatial separation allows $\text{C}_4$ plants to concentrate carbon dioxide around RuBisCO, minimizing photorespiration.

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 have exceptionally thick, suberized cell walls.
These thick walls form a physical barrier that is highly impermeable to gases, preventing concentrated carbon dioxide from escaping back into the mesophyll cells.
- (B) Contain centrifugally arranged chloroplasts: This is correct.
Depending on the $\text{C}_4$ subgroup, chloroplasts in bundle sheath cells can be arranged centrifugally (positioned toward the outer cell wall, as in NADP-malic enzyme types like maize) or centripetally (toward the inner vascular side).
- (C) Contain stacked thylakoid membranes and do not have starch granules: This is incorrect.
In many common $\text{C}_4$ plants, bundle sheath chloroplasts are agranal, meaning they lack stacked thylakoids (grana) to minimize oxygen production from light reactions.
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 the enzyme RuBisCO and the complete enzymatic machinery of the Calvin cycle (light-independent reactions) to fix carbon dioxide into sugars.
Thus, only statements (B) and (D) are correct.

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