If O 2 is not available to the cell, fermentation, an anaerobic process, occurs. During fermentation, glucose is incompletely metabolized to lactate or CO 2 and alcohol. Fermentation results in a net gain of only two ATP per glucose molecule. Occurs in the cytosol outside the mitochondria. Is the breakdown of glucose to two pyruvate molecules.
Glycolysis begins with addition of two phosphate groups, activating glucose to react.
Glucose, a C 6 molecule, splits into two C 3 molecules, each with a phosphate group. Enough energy is released from breakdown of glucose to generate four ATP molecules. Two of four ATP molecules produced are required to replace two ATP molecules used in the phosphorylation of glucose. There is a net gain of two ATP from glycolysis. If aerobic respiration follows glycolysis, pyruvate enters mitochondria.