Transfer of substrates across the chloroplast envelope.

The chloroplast represents a relatively autonomous metabolic compartment within the plant cell. It is surrounded by an envelope consisting of two membranes of which the inner membrane is the functional barrier. Utilizing the energy of light the chloroplast is able to synthesize dihydroxyacetonephosphate from carbon dioxide and water. To provide the cell with this substrate, inorganic phosphate is required. In the case of phosphate deficiency the product of CO2 fixation may be temporarily stored within the chloroplast as starch. Specific transport processes across the inner envelope membrane permit the transfer of metabolites between the chloroplast and the cytosol. The phosphate translocator facilitates the export of dihydroxyacetone phosphate in exchange for inorganic phosphate. It also catalyzes a shuttle for inorganic phosphate with 3-phosphoglycerate, permitting the indirect transfer of reducing equivalents and of ATP from the chloroplast to the cytosol. The dicarboxylate carrier transporting various dicarboxylates may be suited for the transfer of reducing equivalents from the cytosol into the chloroplast. The ATP translocator, catalyzing a transport of ATP into the chloroplast in exchange for ADP, appears to be important for providing the chloroplast with ATP during the night phase, as required for the mobilization of starch.