AEBI 210 Lecture Notes - C4 Carbon Fixation, Crassulacean Acid Metabolism, Vascular Bundle
8 Jan 2013
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Photorespiration can drain up to 50% of the co2 fixed on a hot, dry day. An alternative carbon fixing pathway has developed to avoid this. The c4 pathway is found in many grasses, and plants adapted to hot, dry conditions. Leaves of c4 plants have wreaths" of mesophyll cells surrounding vascular bundles. The choroplasts of the bundle sheath have no grana thylakoids or ps2. C4 reactions use co2 more efficiently than rubisco (2-3 times faster than c3 reactions) C4 reactions are effective at a range of temperatures. Pep carboxylase fixes co2 (as hco3-) into a 4 carbon oxaloacetate. Malate or aspartate is decarboxylated releasing co2 and pyruvate. The co2 enters the calvin cycle, while the pyruvate is converted back to pep. There is a physical separation of the c4 pathway and calvin cycle. Is a combination of c3 and c4 reactions in the same cell. Stomata open at night, and co2 is fixed by pep carboxylase resulting in malate.
