Substrate-level phosphorylation occurs during glycolysis and the Krebs cycle and yields a relatively small amount of ATP. The generation of ATP from chemiosmosis at the end of the Electron Transport Chain is referred to as oxidative phosphorylation because oxygen's oxidative property allows a large amount of free energy to be made available for the synthesis of ATP. This process generates a relatively large amount of ATP for the cell when compared to substrate-level phosphorylation.
This figure emphasizes several important concepts about cellular respiration. First, note the locations of glycolysis, the Krebs cycle, and the electron transport chain and oxidative phosphorylation. Second, note how the electron carriers transport electrons to the transport chain, and the net amount of ATP generated at each step. In particular, compare the amount of ATP generated by oxidative phosphorylation to the amount generated by substrate-level phosphorylation. The maximum net yield of 38 ATPs per molecule of glucose is merely an estimate. Much of the energy bound in a molecule of glucose is actually lost as heat during metabolism. While this heat is actually a waste product, homeotherms ("warm-blooded" animals) capitalize on this "waste" and use it to maintain constant body temperatures.