Substrate level phosphorylation refers to the metabolic reactions which result in the formation of ATP or GTP by the direct transfer of phosphoryl group from ADP or GDP from another phosphorylated compound. Substrate level phosphorylation provide a quicker but less efficient source of ATP.
Substrate level phosphorylation occur in cytoplasm of cells during glycolysis and in mitochondria during Kreb's cycle under both aerobic and anaerobic conditions.
In glycolysis, the first substrate level phosphorylation occur after the conversion of 3 phosphoglycealdehyde to 1,3 biphosphoglycerate.