After its formation in the chloroplast stroma, glyceraldehyde 3-phosphate is transported to the cytosol in exchange for phosphate. The final steps of sucrose synthesis (Figure 12-48, bottom) occur in the cytosol of leaf cells.

574

A phosphate/glyceraldehyde 3-phosphate antiporter in the inner chloroplast membrane brings fixed CO₂ (as glyceraldehyde 3-phosphate) into the cytosol when the cell is exporting sucrose vigorously. No fixed CO₂ leaves the chloroplast unless phosphate is fed into it to replace the phosphate carried out of the stroma in the form of glyceraldehyde 3-phosphate. During the synthesis of sucrose from glyceraldehyde 3-phosphate, inorganic phosphate groups are released (Figure 12-48, bottom left). Thus the synthesis of sucrose facilitates the transport of additional glyceraldehyde 3-phosphate from the chloroplast to the cytosol by providing phosphate for the antiporter. It is worth noting that glyceraldehyde 3-phosphate is a glycolytic intermediate and that the mechanism of the conversion of glyceraldehyde 3-phosphate to hexoses is almost the reverse of that in glycolysis.

The synthesis of starch is more complex. The key monomer substrate used to build large starch polymers is ADP-glucose. This polymerization takes place in the stroma, and starch polymers are stored there in densely packed crystalline aggregates called granules (see Figure 12-37). The enzymes that generate ADP-glucose from glucose 1-phosphate and ATP are found in both the stroma and the cytosol, indicating that hexoses of various structures are imported from the cytosol into the stroma for starch synthesis.