G3P

Glyceraldehyde-3-phosphate (G3P) is a phosphorylated three-carbon sugar derivative that serves as a central metabolic intermediate in both glycolysis and the Calvin cycle. It is a triose phosphate with an aldehyde group on carbon-1 and a phosphate ester on carbon-3.

In glycolysis, fructose-1,6-bisphosphate is cleaved by aldolase into dihydroxyacetone phosphate (DHAP) and G3P; DHAP is rapidly isomerized to G3P, so every glucose yields two molecules of G3P. G3P is oxidized by glyceraldehyde-3-phosphate dehydrogenase to 1,3-bisphosphoglycerate, generating NADH. This compound is then converted to 3-phosphoglycerate by phosphoglycerate kinase, producing ATP. Therefore, each molecule of G3P contributes to substrate-level ATP generation, with two G3P per glucose molecule enabling a net gain of two ATP and two NADH in glycolysis.

In the Calvin cycle, G3P is produced in the chloroplast stroma from 1,3-bisphosphoglycerate after reduction by NADPH and ATP. For every three CO2 fixed, one G3P molecule is formed; two G3P molecules can be used to synthesize hexoses such as glucose, while the remaining molecules regenerate ribulose-1,5-bisphosphate to perpetuate the cycle. G3P is transported from the chloroplast to the cytosol via the triose phosphate/phosphate translocator to support carbohydrate synthesis.

G3P is thus a key junction between energy production and biosynthesis. In plants, it serves as a precursor for sucrose, starch, and cellulose; in other organisms, it feeds glycolytic flux and energy metabolism.