VARIATIONS IN PHOTORESPIRATION

Photorespiration is a metabolic pathway that occurs in photosynthetic organisms and releases carbon dioxide, consumes oxygen, and produces no chemical energy or food. This process limits the growth rates of plants. Photorespiration generally occurs on hot, dry, sunny days causing plants to close their stomata and the oxygen (O2) concentration in the leaf to be higher than the carbon dioxide (CO2) concentration.

When a plant leaf contains a high concentration of oxygen, instead of carbon dioxide, this poses a problem for the normal photosynthetic process, including the light reactions and the Calvin cycle. As a plant's environment becomes hot, dry, and bright, its stomata, where carbon dioxide enters the leaf, tend to close. This lowers the concentration of carbon dioxide into the Calvin cycle, causing the cycle to accept oxygen in place of the carbon dioxide and makes a new product. This new product splits, and part of it is then exported out of the chloroplast and must be broken down by other parts of the cell into carbon dioxide.

This seems like a useless process, and is a result of inefficient rubisco which evolved in an aquatic environment. Most plants, however, are affected by this phenomenon when the conditions are hot and dry because they undergo the Calvin cycle directly and create a 3-carbon first product (3-phosphoglycerate), thus they are called C3 plants. These include rice, wheat, peas, and pepper. Besides C3 carbon fixation, some plants have adapted alternate modes to fix carbon and minimize photorespiration.

One very important adaptation is C4 photosynthesis. C4 plants are unique because they incorporate another type of carbon fixation that forms a four-carbon sugar (hence their name) before they undergo the Calvin cycle. Some C4 plants include sugarcane, corn, Sorghum, and pigweed. C4 plants have a unique leaf anatomy including two types of photosynthetic cells: bundle-sheath cells and mesophyll cells, where as C3 plants contain only mesophyll cells. In the C4 plants, the Calvin cycle occurs in the bundle-sheath cells (in C3 plants this occurs in the mesophyll cells). The mesophyll cells contain a unique enzyme (phosphoenolpyruvate carboxylase) that can collect carbon dioxide using organic compound intermediates to complete the Calvin cycle when the conditions are too hot and dry, the stomata close, and can no longer collect CO2 as normal. In other words, the mesophyll cells act to minimize photorespiration and supply a steady flow of carbon dioxide to the Calvin cycle even when the stomata are closed.

Another plant adaptation to minimize photorespiration is CAM (crassulacean acid metabolism). Plants that utilize CAM use the identical process as the C4 plants, including the organic acid process and the Calvin cycle. However, the C4 plants operate these two processes in separate structures and the CAM plants operate these two pathways at different times. These special plants close their stomata during the day and open them at night. When the stomata are closed, it helps the plant prevent water loss as well as prevent CO2 from entering the leaves. The carbon dioxide collected at night through the stomata is stored as organic acids until the day. During the day, the CO2 is released from these organic acids and functions to drive the Calvin cycle. Some examples of CAM plants are pineapples, and many varieties of cacti.

 


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