Plants overcome photorespiration primarily through two key adaptations: C4 photosynthesis and CAM photosynthesis. These mechanisms enhance CO2 concentration around RuBisCO, the enzyme responsible for carbon fixation, thereby minimizing oxygen binding and photorespiration.
C4 Photosynthesis
C4 plants have evolved a spatial separation of initial CO2 fixation and the Calvin cycle. This process involves two distinct cell types: mesophyll cells and bundle sheath cells.
- Initial CO2 Fixation (Mesophyll Cells): In mesophyll cells, CO2 is initially fixed by the enzyme PEP carboxylase (PEPcase), which has a higher affinity for CO2 than RuBisCO and does not bind to oxygen. PEPcase combines CO2 with phosphoenolpyruvate (PEP) to form oxaloacetate, a four-carbon compound (hence "C4").
- Transfer to Bundle Sheath Cells: Oxaloacetate is then converted to malate or aspartate and transported to bundle sheath cells, which are located deeper within the leaf and surround the vascular bundles.
- Decarboxylation and Calvin Cycle (Bundle Sheath Cells): In the bundle sheath cells, malate or aspartate is decarboxylated, releasing CO2. This elevates the CO2 concentration around RuBisCO, allowing the Calvin cycle to proceed efficiently with minimal photorespiration. The pyruvate that is produced from the decarboxylation process is then transported back to the mesophyll cells to regenerate PEP.
| Feature | C3 Plants | C4 Plants |
|---|---|---|
| Initial Fixation | RuBisCO | PEP Carboxylase (in mesophyll cells) |
| Location | Mesophyll cells | Mesophyll cells & Bundle sheath cells |
| Photorespiration | Significant | Minimal |
| CO2 Efficiency | Lower | Higher |
| Examples | Wheat, Rice, Soybeans | Corn, Sugarcane, Sorghum |
CAM Photosynthesis
CAM (Crassulacean Acid Metabolism) plants utilize a temporal separation of CO2 fixation and the Calvin cycle. These plants open their stomata at night, allowing CO2 to enter the leaves, which is then fixed into organic acids and stored.
- Nighttime CO2 Fixation: At night, CAM plants open their stomata, allowing CO2 to enter. PEP carboxylase fixes CO2 into oxaloacetate, which is then converted to malate and stored in vacuoles.
- Daytime Calvin Cycle: During the day, the stomata close to conserve water. Malate is transported from the vacuoles and decarboxylated, releasing CO2. This CO2 is then used in the Calvin cycle, while the pyruvate is recycled to eventually regenerate PEP. The high CO2 concentration in the leaf during the day ensures that RuBisCO primarily fixes CO2 rather than oxygen, minimizing photorespiration.
| Feature | C3 Plants | CAM Plants |
|---|---|---|
| Stomata Opening | Day | Night |
| CO2 Fixation | During the day | During the night |
| Calvin Cycle | During the day | During the day |
| Water Use Efficiency | Lower | Higher |
| Examples | Most plants | Cacti, Succulents, Pineapples |
In summary, C4 and CAM plants have evolved sophisticated mechanisms to concentrate CO2 around RuBisCO, reducing its affinity for oxygen and effectively minimizing photorespiration. C4 plants achieve this through spatial separation, while CAM plants do so through temporal separation.
