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Aim: What is photorespiration? Photorespiration Rubisco (ribulose biphosphate carboxylase) is an enzyme that adds CO2 to ribulose biphosphate during the Calvin cycle. (1st step) Rubisco has an active site that can hold either oxygen or carbon dioxide. Under normal conditions, when CO2 is in high concentration, rubisco begins the Calvin cycle. When the concentration of O2 is high in the mesophyll of the leaf, rubisco binds to it and not CO2. The Calvin cycle cannot occur and ribulose bi phosphate is broken down (oxidized). Photorespiration When rubisco adds O2 to ribulose biphosphate, it splits into a three-carbon piece and a two-carbon piece in a process called photorespiration. Unlike normal cellular respiration in the mitochondria, this process produces no ATP, nor additional organic molecules. Note: Oxygen can poison a plant by inhibiting the Calvin cycle. Rice, wheat and soybeans are plants that are easily poisoned by high levels of oxygen. They are called C3 plants and shut down the Calvin cycle during xeric conditions (hot, dry periods) What is C3 leaf anatomy? C3 leaf anatomy is found in most plants. Layers include the epidermis, palisade and spongy mesophyll. Guard cells close on hot days to prevent dehydration. Oxygen builds up as CO2 decreases and photorespiration may result. What is C4 leaf anatomy? Kranz C4 Kranz leaves were discovered in 1904. Such leaves are found on plants that live in hot, dry environments Bundle sheath cells contain numerous chloroplasts in C4 leaves but not C3 leaves. Mesophyll cells in C4 leaves contain few chloroplasts. What is the advantage of C4 leaf anatomy? C4 photosynthesis is carried out by many monocots such as corn, sugarcane and other grasses. C4 photosynthesis involves two forms of carbon fixation: Calvin cycle and the Hatch-Slack cycle. What is the advantage of C4 leaf anatomy? Mesophyll cells absorb CO2 from air spaces within the leaf and use the Hatch-Slack cycle to create malate (malic acid). The PEP enzymes involved in the Hatch-Slack cycle are not affected by oxygen like rubisco is. Malate (malic acid) is chemically transported to the bundle sheath cells and CO2 is chemically removed from it. This keeps the CO2 concentration high and the Calvin cycle can occur. What is Crassulacean Acid Metabolism? CAM CAM photosynthesis is performed by cacti, pineapple, and Spanish moss. These plants, known as CAM plants for crassulacean acid metabolism (CAM), open stomata during the night and close them during the day. During the night, these plants fix CO2 into a variety of organic acids (malic and isocitric) in mesophyll cells. During the day, the light reactions supply ATP and NADPH to the Calvin cycle and CO2 is released from the organic acids even when the stomates are closed.