In stage 1 (Glycolysis) each glucose molecule is broken down to form two
Pyruvate (pyruvic acid) molecules. This occurs in the cell cytoplasm and in the
process two ATP molecules are produced.
What happens next depends on the presence or absence of Oxygen:
If oxygen is absent then the second stage does not occur and the Pyruvate is
changed into either lactic acid or ethanol and carbon dioxide. This is called
Anaerobic Respiration and it produces only 2 ATP molecules.
If oxygen is present then Stage 2 takes place in the Mitochondrion of a cell.
The following Movie will take you to the mitochondrion of a cell.