Illustration showing the metabolic pathways of anaerobic and aerobic respiration. Both start with glycolysis, the conversion of glucose to pyruvic acid. In the absence of oxygen (anaerobic) either alcoholic fermentation will then convert pyruvic acid to energy and ethanol, or lactic acid fermentation will convert it to energy and lactic acid. In the presence of oxygen (aerobic) the link reaction takes pyruvic acid the product of glycolysis and converts it to acetyl coenzyme A (Acetyl-CoA). Acetyl-CoA is used by the citric acid, or Krebs, cycle in the matrix of the mitochondria to create electron carriers for the next stage, the electron transport system. This is a series of reactions in the membrane of the mitochondria that produces energy in the form of adenosine triphosphate (ATP).