Glycolysis is the breakdown of a molecule of glucose into two molecules of pyruvate, while the Krebs cycle is the conversion of the resulting pyruvate molecules into a compound known as acetyl CoA. Both of these steps occur before oxidative phosphorylation, which is the major energy-producing process of aerobic metabolism. Glycolysis is a process used by all forms of life, even those that do not use oxygen.
While glycolysis only produces a net two ATP, as opposed to 36 in full aerobic respiration, some conditions cause organisms to rely on it completely – even those that normally use oxygen. According to the University of Illinois at Chicago, human muscle cells are capable of continuing some function using glycolysis when they exceed their aerobic capacity. Some single-celled organisms use glycolysis exclusively. When this occurs, organisms use fermentation to replenish necessary molecules, producing lactic acid or alcohol as waste. Glycolysis takes place in the cytoplasm, away from the mitochondria.
The Krebs cycle, on the other hand, takes place in the mitochondria directly before oxidative phosphorylation. The Krebs cycle also generates two net ATP, along with carbon dioxide. The Krebs cycle is a relatively complex process with several steps, slowly altering the acetyl CoA until it is in a suitable form for oxidative phosphorylation.