Question

Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A) : The CAM plants undergo night time acidification and day time deacidification.
Reason (R) : The CAM plants take up $\text{CO}_2$ during day time, store it in the form of malate in vacuoles and break it down to release $\text{CO}_2$ in the night time.

• A. Both (A) and (R) are true and (R) is the correct explanation of (A).
• B. Both (A) and (R) are true but (R) is NOT the correct explanation of (A).
• C. (A) is true but (R) is false.
• D. (A) is false but (R) is true.

Hint:

To avoid confusion, remember that desert CAM plants must keep their stomata closed during the hot day-time to survive. Thus, they can only open stomata and take up carbon dioxide during the cool night-time, accumulating malic acid (acidification).

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Crassulacean Acid Metabolism (CAM) is a specialized photosynthetic adaptation in desert plants to minimize water loss (transpiration).
CAM plants separate carbon dioxide capture and the Calvin cycle temporally (by night and day) rather than spatially.

Step 2: Detailed Explanation:
Let us analyze the Assertion and the Reason individually:
- Assertion (A) is true: CAM plants display a diurnal rhythm of acid accumulation.
At night, they capture atmospheric carbon dioxide, converting it into organic acids (predominantly malic acid) which are stored in the vacuole, causing a sharp drop in pH (night-time acidification).
During the day, stomata close to conserve water, and the stored malic acid is transported out of the vacuoles and decarboxylated to release carbon dioxide for the Calvin cycle, causing the pH to rise (day-time deacidification).
- Reason (R) is false: The statement describes the reverse of the actual CAM cycle.
CAM plants open their stomata and take up carbon dioxide during the night (not day-time) to prevent excessive transpiration in hot climates.
This carbon dioxide is fixed by phosphoenolpyruvate carboxylase (PEPC) to form oxaloacetate, which is then reduced to malated and stored in vacuoles.
During the day (not night-time), they close their stomata and break down this stored malate to release carbon dioxide within the bundle sheath cells for the light-independent reactions.
Therefore, Assertion (A) is true but Reason (R) is false.

Step 3: Final Answer:
The correct option is: (A) is true but (R) is false.