C4 plants and CAM plants are the primary types of plants that avoid photorespiration.
Photorespiration is a process that occurs in plants when the enzyme RuBisCO, responsible for carbon fixation in photosynthesis, binds to oxygen instead of carbon dioxide. This reduces the efficiency of photosynthesis, particularly in hot and dry conditions. Plants have evolved different strategies to minimize or avoid photorespiration. The two major groups of plants employing such strategies are C4 and CAM plants.
C4 Plants
C4 plants, such as maize (corn), sugarcane, and sorghum, have developed a spatial separation of carbon fixation and the Calvin cycle.
- Spatial Separation: The initial carbon fixation occurs in mesophyll cells, where CO2 is captured by PEP carboxylase, an enzyme with a higher affinity for CO2 than RuBisCO. This forms a 4-carbon compound (hence "C4").
- Bundle Sheath Cells: This 4-carbon compound is then transported to bundle sheath cells, where it is decarboxylated, releasing CO2.
- Calvin Cycle in Bundle Sheath Cells: The high concentration of CO2 in the bundle sheath cells ensures that RuBisCO primarily binds to CO2, minimizing photorespiration.
CAM Plants
CAM (Crassulacean Acid Metabolism) plants, like cacti and succulents, use a temporal separation of carbon fixation and the Calvin cycle.
- Temporal Separation: CAM plants open their stomata at night to take in CO2, which is then fixed into organic acids and stored.
- Daytime Calvin Cycle: During the day, when the stomata are closed to conserve water, the organic acids are broken down to release CO2, which is then used in the Calvin cycle.
Summary Table
| Plant Type | Mechanism | Separation Type | Examples | Advantage |
|---|---|---|---|---|
| C4 | Spatial Separation | Spatial | Maize, Sugarcane, Sorghum | Minimizes photorespiration in hot environments |
| CAM | Temporal Separation | Temporal | Cacti, Succulents | Conserves water in arid environments |
In conclusion, C4 and CAM plants have evolved unique mechanisms to overcome the limitations imposed by photorespiration, allowing them to thrive in specific environmental conditions.
