See an overview of Cellular Respiration here http://www.youtube.com/watch?v=1aYPo5xNVIA This is the final step in cellular respiration, also known as aerobic respiration. Electrons have been carried by compounds such as NAD+, FADH, and NADP. Now they are used to set off a chemical reaction that takes advantage many membrane proteins of the mitochondria to make ATP. The final membrane protein, an enzyme, is called ATP synthase or sometimes just synthase. The cells of almost all eukaryotes (animals, plants, fungi, algae, protozoa -- in other words, the living things except bacteria, archaea, and a few protists) contain intracellular organelles called mitochondria, which produce ATP. Energy sources such as glucose are initially metabolized in the cytoplasm. The products are imported into mitochondria. Mitochondria continue the process of catabolism using metabolic pathways including the Krebs cycle, fatty acid oxidation, and amino acid oxidation. The end result of these pathways is the production of two kinds of energy-rich electron donors, NADH and FADH2. Electrons from these donors are passed through an electron transport chain to oxygen, which is reduced to water. This is a multi-step redox process that occurs on the mitochondrial inner membrane. The enzymes that catalyze these reactions have the remarkable ability to simultaneously create a proton gradient across the membrane, producing a thermodynamically unlikely high-energy state with the potential to do work. Although electron transport occurs with great efficiency, a small percentage of electrons are prematurely leaked to oxygen, resulting in the formation of the toxic free-radical superoxide. See similar animations on Part I Glycolysis here http://www.youtube.com/watch?v=aCypoN3X7KQ and part II Krebs Cycle here http://www.youtube.com/watch?v=x-stLxqPt6E *Anaerobic Respiration/Fermentation can be seen here: http://www.youtube.com/watch?v=l8rEiWhejWo