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12.6: C4 and CAM Photosynthesis

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  • The enzyme RuBisCO needs a high \(\ce{CO2}\) environment to function efficiently. If the ratio of oxygen to \(\ce{CO2}\) gets too high, RuBisCO will bind oxygen instead and waste energy in the process. This is called photorespiration and accounts for a large amount of yield loss for crops in hot areas. Why does photorespiration happen?

    If it is too hot or dry, plants often close their stomata to prevent water loss. This prevents \(\ce{CO2}\) from entering the leaf, as well as prevents \(\ce{O2}\) from exiting. Oxygen builds up inside the leaf and photorespiration happens instead of the Calvin cycle. Though this wastes energy for the plant, preventing water loss is often a larger priority. However, some plants have evolved special ways of performing photosynthesis that prevent or limit photorespiration.

    C4 Photosynthesis is for Plants Adapted to Hot Environments

    In most photosynthesis, the first product of the Calvin cycle is a 3 carbon compound, so this type of photosynthesis is called C3. For plants adapted to particularly hot environments, the first compound formed has 4 carbon atoms, hence C4 photosynthesis. In these plants, RuBisCO is restricted to the bundle sheath cells of the leaf. Carbon dioxide is converted into an acid and transported into the bundle sheath cells where it will be converted back into \(\ce{CO2}\). This keeps the concentration high where RuBisCO is active, preventing photorespiration.

    CAM Photosynthesis is for Plants Adapted to Dry Environments

    CAM plants are often found in desert environments. It is too hot and/or dry to keep stomata open during the day, so they only open them at night. However, there is no light at night to do photosynthesis. To solve this, CAM plants have evolved to take in \(\ce{CO2}\) at night and store it in the central vacuole in the form of an acid. This is where CAM gets its name: Crassulacean Acid Metabolism. During the day, the acid is converted back into \(\ce{CO2}\), and the Calvin cycle can take place alongside the electron transport chain.

    In both of these types of photosynthesis, compounds must be formed, transported, and broken back apart again. Each of these tasks costs energy to perform, but it outweighs the energy lost by photorespiration.

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