Leaf cytosol contains non-phosphorylating and phosphorylating glyceraldehyde 3-phosphate dehydrogenase (np-GAPDH and p-GAPDH, respectively). From the viewpoint of carbon metabolism, np-GAPDH is redundant. However, mutants lacking np-GAPDH show significant metabolic adjustments and decreased growth suggesting that np-GAPDH has central functions in plant metabolism. Here, a cytosolic oxidation-reduction (COR) cycle is proposed. In its forward direction, np-GAPDH supplies NADPH. In its backward direction, phosphoglycerate kinase and p-GAPDH consume ATP and NADH. Thus, COR cycling may constitute a central hub in energy metabolism.