In CAM Plants, CO2 Capture and Rubisco Action Are Temporally Separated Succulent plants such as cactus and pineapple, which are native to very hot, very dry environments, have another variation on photosynthetic CO2 fixation, which reduces loss of water vapor through the pores (stomata) by which CO2 and O2 must enter leaf tissue. Instead of separating the initial trapping of CO2 and its fixation by rubisco across space (as do the C4 plants), they separate these two events over time. At night, when the air is cooler and moister, the stomata open to allow entry of CO2, which is then fixed into oxaloacetate by PEP carboxylase. The oxaloacetate is reduced to malate and stored in the vacuoles, to protect cytosolic and plastid enzymes from the low pH produced by malic acid dissociation. During the day the stomata close, preventing the water loss that would result from high daytime temperatures, and the CO2 trapped overnight in malate is released as CO2 by the NADP-linked malic enzyme. This CO2 is now assimilated by the action of rubisco and the Calvin cycle enzymes. Because this method of CO2 fixation was first discovered in stonecrops, perennial flowering plants of the family Crassulaceae, it is called crassulacean acid metabolism, and the plants are called CAM plants. 770 Chapter 20 Carbohydrate Biosynthesis in Plants and Bacteria FIGURE 20–23 Carbon assimilation in C4 plants. The C4 pathway, involving mesophyll cells and bundle-sheath cells, predominates in plants of tropical origin. (a) Electron micrograph showing chloroplasts of adjacent mesophyll and bundle-sheath cells. The bundle-sheath cell contains starch granules. Plasmodesmata connecting the two cells are visible. (b) The C4 pathway of CO2 assimilation, which occurs through a four-carbon intermediate. NADPH + H+ Ribulose 3-Phosphoglycerate 1,5-bisphosphate AMP ATP Pi Mesophyll cell PEP Pyruvate pyruvate phosphate dikinase PPi + + Malate malic NADP+ enzyme Triose phosphates Bundle-sheath cell Plasma membranes Oxaloacetate Malate malate dehydrogenase NADPH + H+ Pi NADP+ Pyruvate – Plasmodesmata PEP carboxylase H2O (in air) H+ CO2 HCO3 CO2 (b) (a) Mesophyll cell Plasmodesmata Bundlesheath cell
Posted on: Thu, 04 Jul 2013 02:55:20 +0000
Trending Topics
Recently Viewed Topics
© 2015