CAM (Crassulacean Acid Metabolism) plants use a specialized photosynthetic pathway to survive in arid environments. This pathway involves a temporal separation of carbon dioxide uptake and the Calvin cycle.
Here's a breakdown of how CAM photosynthesis works:
CAM Photosynthesis: A Two-Step Process
CAM photosynthesis cleverly separates the steps of carbon fixation to minimize water loss.
Nighttime: CO2 Uptake and Storage
- Stomata open: Unlike most plants, CAM plants open their stomata at night to reduce water loss during the hot, dry daytime.
- CO2 enters: CO2 diffuses into the leaf.
- Carbon fixation: The CO2 is combined with phosphoenolpyruvate (PEP) to form malate. This process is facilitated by the enzyme PEP carboxylase.
- Malate storage: The malate (an acid) is then stored in large central vacuoles within the plant cells. This creates a reservoir of stored carbon.
Daytime: Calvin Cycle
- Stomata close: During the day, the stomata close to conserve water.
- Malate release: Malate is released from the vacuoles.
- Decarboxylation: The malate is decarboxylated, releasing CO2.
- Calvin Cycle: This CO2 is then used in the Calvin cycle, where it is fixed into sugars, just like in C3 photosynthesis.
Here's a table summarizing the differences between Night and Day in CAM Photosynthesis:
| Stage | Time | Stomata | CO2 Source | Carbon Fixation |
|---|---|---|---|---|
| Step 1 | Night | Open | Atmosphere | CO2 + PEP -> Malate (stored in vacuole) |
| Step 2 | Day | Closed | Malate (released from vacuole) | CO2 -> Sugars (Calvin Cycle) |
In essence, CAM plants open their stomata at night to take in CO2 and store it as malate. During the day, they close their stomata to conserve water and use the stored CO2 to carry out photosynthesis. This temporal separation of carbon fixation and the Calvin cycle is the key to their survival in harsh environments.
