Illustration showing the metabolic pathways of aerobic respiration in a eukaryotic cell. The first stage is glycolysis, which occurs in the cell cytoplasm. 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).