Photosynthesis Part 3

CAM Plants

These are desert plants *The stomata open at night to take in CO2 and release O2 --- transpiration- water loss through the stomata

Photorespiration

*Photorespiration occurs when the CO2 levels inside a leaf become low. This happens on hot dry days when a plant is forced to close its stomata to prevent excess water loss. If the plant continues to attempt to fix CO2 when its stomata are closed, the CO2 will get used up and the O2 ratio in the leaf will increase relative to CO2 concentrations. *When the CO2 levels inside the leaf drop to around 50 ppm, Rubisco starts to combine O2 with RuBP instead of CO2. *The net result of this is that instead of producing 2 3C PGA molecules, only one molecule of PGA is produced and a toxic 2C molecule called

Calvin Cycle

Carbon fixation. ​​ C, O, molecule combines with a five-carbon acceptor molecule, ribulose-1,5-bisphosphate (RuBP). This step makes a six-carbon compound that splits into two molecules of a three-carbon compound, 3-phosphoglyceric acid (3-PGA). This reaction is catalyzed by the enzyme RuBP carboxylase/oxygenase, or rubisco. Simplified diagram (showing carbon atoms but not full molecular structures) illustrating the reaction catalyzed by rubisco. Rubisco attaches a carbon dioxide molecule to an RuBP molecule, and the six-carbon intermediate thus produced breaks down into two 3-phosphoglycerate (3-PGA) molecules. the unstable six-carbon intermediate formed when they combine, and the two 3-PGA molecules produced by the intermediate's breakdown. Reduction. In the second stage, ATP and NADPH are used to convert the 3-PGA molecules into molecules of a three-carbon sugar, glyceraldehyde-3-phosphate (G3P). This stage gets its name because NADPH donates electrons to, or reduces, a three-carbon intermediate to make G3P. Simplified diagram of the reduction stage of the Calvin cycle, showing carbon atoms but not full molecular structures. A molecule of 3-PGA first receives a second phosphate group from ATP (generating ADP). Then, the doubly phosphorylated molecule receives electrons from NADPH and is reduced to form glyceraldehyde-3-phosphate. This reaction generates NADP+ and also releases an inorganic phosphate. Reactions of the reduction stage of the Calvin cycle, showing the molecular structures of the molecules involved. Regeneration. Some G3P molecules go to make glucose, while others must be recycled to regenerate the RuBP acceptor. Regeneration requires ATP and involves a complex network of reactions, which my college bio professor liked to call the "carbohydrate scramble." ^1 ​