3-Phosphoglyceric acid (3PG), or glycerate 3-phosphate (GP), is a biochemically significant 3-carbon molecule that is a metabolic intermediate in both glycolysis and the Calvin cycle. This chemical is often termed PGA when referring to the Calvin cycle. In the Calvin cycle, 3-Phosphoglycerate is the product of the spontaneous split of an unstable 6-carbon intermediate formed by CO2 fixation. Thus, two 3-phosphoglycerate molecules are produced for each molecule of CO2 fixed.
In the Calvin cycle, two glycerate 3-phosphate molecules are reduced to form two molecules of glyceraldehyde 3-phosphate (PGAL). This is the first compound formed during the C3 or Calvin cycle. It is a reactive biomolecule that is easily reduced.