How does glyceraldehyde 3phosphate formed in the chloroplast

Solution

Much of the glyceraldehyde 3-phosphate produced by calvin cycle in chloroplasts is exported to the cytosol and used to produce the disaccharide, sucrose. First the glyceraldehyde 3-phosphate is converted to fructose 6-phosphate and glucose 1-phosphate. The chemical reactions involved are essentially a reversal of glycolysis. The glucose 1-phosphate is then converted to UDP glucose and this reacts with fructose 6-phosphate to synthesize sucrose 6-phosphate. Hydrolysis of the sucrose 6-phosphate yields starch. Excess starch is produced in the stroma of chloroplasts are synthesized by the conversion of glucose 6-phosphate which in turn in used to synthesize the nucleotide sugar derivatives.