Photosynthetic Carbohydrate Synthesis:- A Transport System Exports Triose Phosphates from the Chloroplast and Imports Phosphate
The inner chloroplast membrane is impermeable to most phosphorylated compounds, including fructose 6-phosphate, glucose 6-phosphate, and fructose 1,6-bisphosphate. It does, however, have a specific antiporter that catalyzes the one-for-one exchange of Pi with a triose phosphate, either dihydroxyacetone phosphate or 3 phosphoglycerate . This antiporter simultaneously moves Pi into the chloroplast, where it is used in photophosphorylation, and moves triose phosphate into the cytosol, where it can be used to synthesize sucrose, the form in which the fixed car bon is transported to distant plant tissues. Sucrose synthesis in the cytosol and starch synthesis in the chloroplast are the major pathways by which the excess triose phosphate from photosynthesis is “harvested.” Sucrose synthesis (described below) releases four Pi molecules from the four triose phosphates re quired to make sucrose. For every molecule of triose phosphate removed from the chloroplast, one Pi is trans ported into the chloroplast, providing the ninth Pi mentioned above, to be used in regenerating ATP. If this exchange were blocked, triose phosphate synthesis would quickly deplete the available Pi in the chloroplast, slowing ATP synthesis and suppressing assimilation of CO2 into starch. The Pi–triose phosphate antiport system serves one additional function. ATP and reducing power are needed in the cytosol for a variety of synthetic and energy requiring reactions. These requirements are met to an as yet undetermined degree by mitochondria, but a second potential source of energy is the ATP and NADPH generated in the chloroplast stroma during the light reactions. However, neither ATP nor NADPH can cross the chloroplast membrane. The Pi–triose phosphate antiport system has the indirect effect of moving ATP equivalents and reducing equivalents from the chloroplast to the cytosol (Fig. 1). Dihydroxyacetone phosphate formed in the stroma is transported to the cytosol, where it is converted by glycolytic enzymes to 3-phosphoglycerate, generating ATP and NADH. 3 Phosphoglycerate reenters the chloroplast, completing the cycle.
FIGURE 1 Role of the Pi–triose phosphate antiporter in the transport of ATP and reducing equivalents. Dihydroxyacetone phosphate leaves the chloroplast and is converted to glyceraldehyde 3-phosphate in the cytosol. The cytosolic glycer aldehyde 3-phosphate dehydrogenase and phosphoglycerate kinase reactions then produce NADH, ATP, and 3-phosphoglycerate. The latter reenters the chloroplast and is reduced to dihydroxyacetone phosphate, completing a cycle that effectively moves ATP and reducing equivalents (NADPH/NADH) from chloroplast to cytosol.
