In the presence of oxygen the products of glycolysis go on to the Krebs cycle and the electron transport chain. If oxygen is not present fermentation takes place. When we inhale, we bring in oxygen for this process. When we exhale we expel the CO 2 that is formed as a by-product. The passage of electrons through the electron transport chain is an energy releasing process, and this energy is used to produce a proton gradient.
Protons build up between the inner and outer membranes of the mitochondria.
At the end of the electron transport chain oxygen receives the electrons. High yield of ATP . With abundant oxygen, 30something molecules of ATP per glucose molecule burned. The ATP produced are from glycolysis only. Pyruvate produced by glycolysis does not enter the Krebs cycle, but instead undergoes one or two chemical steps to become ethanol or lactic acid.