Question

In CAM plants, a metabolite 'X' is stored in vacuoles at night for its future use as a source of carbon dioxide. Which gets fixed by Rubisco in chloroplasts during the day time. 'X' is -

• A. Pyruvate
• B. Oxaloacetate
• C. Malate
• D. Phosphoenol pyruvate

Hint:

CAM = Temporal separation (Time-based).
Night = Fixation into Malate (Storage in Vacuole).
Day = Release of $CO_2$ from Malate (Fixation by Rubisco).
Remember: C4 and CAM both use a "Malate bridge" to provide $CO_2$ to Rubisco.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
Crassulacean Acid Metabolism (CAM) is an adaptation in succulent plants (like cacti or pineapples) to minimize water loss in arid environments. The question focuses on the temporal separation of carbon fixation steps and the specific molecule used for intermediate storage.

Step 2: Detailed Explanation:

• Night-time Activity (Stomata Open): CAM plants open their stomata at night when temperatures are lower. Atmospheric $CO_2$ enters the leaf and is initially fixed by the enzyme PEP carboxylase.

• Initial Fixation Product: The $CO_2$ reacts with Phosphoenolpyruvate (PEP) to form a four-carbon compound called Oxaloacetate.

• Reduction and Storage: Oxaloacetate is immediately reduced to Malate (malic acid) by the enzyme malate dehydrogenase. This malic acid is then actively transported into the large central vacuole of the cell.

• Purpose of Storage: Accumulating malate in vacuoles allows the plant to "harvest" carbon while preventing water loss. The pH of the vacuole drops significantly overnight as the acid concentration increases.

• Day-time Activity (Stomata Closed): During the day, stomata are tightly shut to conserve water. The stored malate is transported back out of the vacuole into the cytosol and then into the chloroplasts.

• Decarboxylation: The malate is decarboxylated (broken down) to release $CO_2$ and Pyruvate. This internal release creates a high concentration of $CO_2$ around the enzyme Rubisco, which then fixes it into the Calvin cycle (C3 pathway) to produce sugars using solar energy.

Step 3: Final Answer:
The intermediate metabolite 'X' that is stored in the vacuole at night and provides $CO_2$ during the day in CAM plants is Malate.