Question

Desert plants absorb CO2 at night (stomata are closed during the day to prevent water from leaving the plant). CO2 is converted into malate, which is stored in vacuoles until morning. Why would plants not be able to directly store CO2 inside of vacuoles?

Answer 1

CAM plants exhibit scotoactive stomata i.e. open during the night and closed during the day time.
It is an adaptation to reduce water loss by transpiration.

CAM plants exhibit temporal separation of primary and secondary carboxylation events.
Primary carboxylation occurs during the night to produce organic acid.
Primary carboxylation occurs during the day time to produce carbohydrates.

The organic acid produced by primary carboxylation is stored in the vacuoles. Plants cannot store CO2 in the vacuoles.
This is due to the fact that CO2 can easily diffuse across the biological membrane.

Biological membranes are lipid bilayers composed of phospholipids and proteins. They are predominantly hydrophobic. They allow the free movement of small non-polar molecules and gases only.

Since CO2 is converted Malate, it cannot diffuse across the membrane.