Energy generation or cell respiration consists of 4 phases: glycolysis, conversion of pyruvic acid to Acetyl Coenzyme A, Krebs’ cycle and oxidative phosphorylation.
The last 3 phases of energy generation take place inside the mitochondrion while the first phase takes place in the cytoplasm of a cell.
Glycolysis is an anaerobic process (does not need oxygen to proceed) that involves breaking down of a sugar molecule (usually glucose) into 2 molecules of pyruvic acid. If oxygen is still not available after this process, then pyruvic acid is converted into lactic acid in animal cells and into ethyl alcohol in plant cells.
If oxygen is available however, the 2nd phase of energy generation takes place, that is, pyruvic acid is converted to Acetyl Coenzymne A in preparation for the 3rd phase of the process.
Glycolysis yields 4 molecules of ATP (the energy currency of cells) but the net yield is only 2 molecules of ATP because 2 molecules are used up to prepare glucose for the process. On the other hand, phase 2 or the conversion of pyruvic acid to AcetylCoA yields a net of 6 ATP molecules.
The last 2 phases of cell respiration are coupled. That is, one cannot occur without the other. We will thus talk about these 2 in another post.