If O2 is not available pyruvic acid undergoes anaerobic respiration/fermentation, but under aerobic condition the pyruvic acid enters into mitochondria and converted to Acetyl Co-A. Acetyl co-A functions as substrate entrant for Krebs cycle so, a connecting link between glycolysis and Krebs cycle. Glycolysis is the process of breakdown of glucose (hexose sugar) to two molecules of pyruvic acid through a series of enzymes mediated reactions. It occurs in cytoplasm and is common both to aerobic and anaerobic respiration. Last product is pyruvic acid. Krebs cycle or citric acid cycle or TCA cycle occurs in matrix of mitochondria. It is a stepwise oxidative and cyclic degradation of activated acetate derived from pyruvate.