Q.Succulents are known to keep their stomata closed during the day to check transpiration. How do they meet their photosynthetic CO2 requirements?

Succulents that keep their stomata shut during the day instead open them at night to take in and store CO2, using a special adaptation called Crassulacean Acid Metabolism (CAM).

• At night, when their stomata are open, these plants take in atmospheric CO2 and fix it into an organic acid — chiefly malic acid — using the enzyme PEP carboxylase, storing this acid in their large cell vacuoles.
• During the day, the stomata close (to minimise water loss through transpiration), but the stored malic acid is broken down, releasing CO2 within the leaf cells. …

Succulents and other plants adapted to arid conditions face a real conflict: they need CO2 for photosynthesis, but opening their stomata during the hot, dry daytime would cause excessive water loss through transpiration. Such plants resolve this conflict by separating CO2 UPTAKE from CO2 USE in time, rather than in space (unlike C4 plants, which separate the two steps between mesophyll and bundle sheath cells within the same daytime period).

At night, when temperatures are lower and humidity is higher, these plants open their stomata and take in atmospheric CO2. This CO2 is fixed by the enzyme PEP carboxylase onto phosphoenolpyruvate (PEP), producing an organic acid — predominantly malic acid — which accumulates and is stored in the plant's large cell vacuoles overnight (this nighttime acid build-up is why such plants often taste distinctly sour in the early morning). …

Method 1 — Work out how a plant can fix CO2 without daytime stomata being open

1. Note the constraint: stomata are closed during the day (to limit transpiration), so CO2 cannot enter the leaf by day in the ordinary way.
2. Reason that if CO2 is still needed for daytime photosynthesis, the plant must acquire and store it at a DIFFERENT time — logically, at night, when opening stomata risks far less water loss (lower temperature, higher humidity).
3. Recall the specific adaptation such succulents use: at night they open their stomata, take in CO2, and fix it into an organic acid (mainly malic acid) using the enzyme PEP carboxylase, storing the acid in their (often enlarged) cell vacuoles — this pathway is called Crassulacean Acid Metabolism (CAM). …