How do CAM plants limit photorespiration?

CAM (Crassulacean Acid Metabolism) plants limit photorespiration by separating the initial carbon fixation and the Calvin cycle temporally, performing these processes at different times of the day.

Here's a breakdown of how this temporal separation works:

• Nighttime Carbon Fixation:

  • CAM plants open their stomata at night to take in CO₂. This is crucial because it minimizes water loss, a major concern for plants in arid environments where CAM plants are commonly found.
  • The CO₂ is fixed by an enzyme called PEP carboxylase, which combines CO₂ with phosphoenolpyruvate (PEP) to form oxaloacetate.
  • Oxaloacetate is then converted to malate, which is stored in vacuoles within the mesophyll cells. This effectively "stores" the carbon.

• Daytime Calvin Cycle:

  • During the day, the stomata close to conserve water.
  • The stored malate is transported from the vacuoles to the cytoplasm.
  • Malate is decarboxylated, releasing CO₂.
  • The high concentration of CO₂ around the RuBisCO enzyme favors carboxylation (carbon fixation in the Calvin cycle) over oxygenation (photorespiration). This effectively saturates RuBisCO with CO₂, minimizing its ability to bind to oxygen.
  • The released CO₂ then enters the Calvin cycle, where it's used to produce sugars.

In essence, CAM plants create a high concentration of CO₂ near RuBisCO during the day, when the Calvin cycle operates, effectively minimizing photorespiration by making CO₂ readily available and reducing the likelihood of RuBisCO binding to oxygen instead. This clever adaptation allows them to thrive in hot, dry climates where photorespiration would otherwise be a significant problem.