Explain how photorespiration is avoided in C-4 plants.

C4 plants have developed a specialized mechanism to minimize photorespiration, which is a wasteful process that occurs in plants when they take in oxygen instead of carbon dioxide during photosynthesis. This process can be detrimental to plant growth and productivity. C4 plants have evolved a structural and biochemical adaptation that allows them to concentrate carbon dioxide around the enzyme responsible for fixing carbon, thereby reducing the chances of oxygen interfering with the process.

The key feature of C4 plants is the presence of two distinct types of photosynthetic cells: mesophyll cells and bundle sheath cells. The mesophyll cells are located in the outer layers of the leaf, while the bundle sheath cells are located deeper within the leaf. These two types of cells work together to carry out photosynthesis more efficiently.

The first step in the C4 pathway takes place in the mesophyll cells, where an enzyme called phosphoenolpyruvate carboxylase (PEP carboxylase) captures carbon dioxide and combines it with a three-carbon molecule called phosphoenolpyruvate (PEP). This reaction forms a four-carbon compound called oxaloacetate, which is then converted into malate or aspartate.

The four-carbon compounds are transported from the mesophyll cells to the bundle sheath cells, which surround the veins of the leaf. Within the bundle sheath cells, the four-carbon compounds release carbon dioxide, which is then available for the Calvin cycle, the process that converts carbon dioxide into carbohydrates. Importantly, the Calvin cycle takes place in the bundle sheath cells, where the concentration of carbon dioxide is high and the concentration of oxygen is low. This spatial separation between the two processes helps to prevent photorespiration by reducing the chances of oxygen interfering with the fixation of carbon dioxide.

The release of carbon dioxide from the four-carbon compounds within the bundle sheath cells is made possible by an enzyme called phosphoenolpyruvate carboxykinase (PEP carboxykinase). This enzyme releases carbon dioxide and regenerates the original three-carbon molecule, PEP, which is then transported back to the mesophyll cells to initiate the C4 pathway again.

Overall, the C4 pathway in plants allows for efficient carbon dioxide capture and concentration around the enzyme responsible for fixing carbon dioxide, thereby minimizing the occurrence of photorespiration. This adaptation is particularly advantageous for plants growing in hot and dry environments, where photorespiration is more likely to occur due to the increased likelihood of oxygen competing with carbon dioxide for the enzyme's active sites.