Following are certain statements regarding Crassulacean Acid Metabolism (CAM) plants:

A. The HCO₃⁻ concentration is enriched in the cytosol during night by the CO₂ coming from the external atmosphere through the open stomata and the mitochondrial respiration.
B. Oxaloacetate produced by the action of PEPCase is stored in the vacuole during the dark.
C. During light, oxaloacetate produces malate that provides CO₂ for the Calvin-Benson cycle in the chloroplast.
D. During dark, phosphoenolpyruvate is produced by the breakdown of starch present in the chloroplast.
E. CAM is a mechanism of concentrating CO₂ around Rubisco by keeping stomata closed during the day.

Which one of the following options represents the combination of all correct statements?

1. A, B, and E
2. B, C, and E
3. A, C, and D
4. A, D, and E

Correct option is D

Statement A: Correct

"The HCO₃⁻ concentration is enriched in the cytosol during the night by the CO₂ coming from the external atmosphere through the open stomata and the mitochondrial respiration."
 CAM plants open their stomata at night to absorb CO₂ from the atmosphere. This CO₂ is converted to bicarbonate (HCO₃⁻) in the cytosol. Additionally, CO₂ released by mitochondrial respiration contributes to this process.

Statement B: Incorrect

"Oxaloacetate produced by the action of PEPCase is stored in the vacuole during the dark."
 In CAM plants ,PEPCase fixes CO₂ to produce oxaloacetate during the night. However, oxaloacetate is not stored in the vacuole; it is converted into malate, which is then stored in the vacuole during the night. Therefore, oxaloacetate itself is not stored in the vacuole. Instead, it is malate that gets stored in the vacuole, and later, malate is decarboxylated during the day to release CO₂ for the Calvin cycle.

Statement C: Incorrect

"During light, oxaloacetate produces malate that provides CO₂ for the Calvin-Benson cycle in the chloroplast."
This statement is incorrect in its description of timing. The production of malate from oxaloacetate occurs during the night, not the day. During the day, malate (which was stored in the vacuole) is decarboxylated to release CO₂ for the Calvin cycle.

Statement D: Correct

"During dark, phosphoenolpyruvate is produced by the breakdown of starch present in the chloroplast."
This statement is correct. In CAM plants, starch stored in the chloroplast is broken down at night, and phosphoenolpyruvate (PEP) is produced from the breakdown products. PEP is used by PEPCase to fix CO₂ into oxaloacetate.

Statement E:Correct

"CAM is a mechanism of concentrating CO₂ around Rubisco by keeping stomata closed during the day."
This statement is correct. CAM plants concentrate CO₂ around the enzyme Rubisco by keeping their stomata closed during the day, preventing water loss. They take in CO₂ during the night and store it as malate for use during the day.