0.22 Bis2a 06.2 oxidative phosphorylation v1.2

In the last module we discussed the various ways cells synthesize ATP and had a detailed discussion on substrate level phosphorylation. The second primary mechanism for ATP and energy formation is by oxidative phosphorylation. First and foremost, oxidative phosphorylation does not imply the use of oxygen, it can, but it does not have to use oxygen. It is called oxidative phosphorylation because it relies on red/ox reactions to generate a membrane potential that can then be used to do work. One of the "machines" that can be driven by the membrane potential, also referred to as the proton motive force or PMF , is the F ₁ F ₀ ATPase . Unlike SLP, which directly synthesizes ATP, Oxidative Phosphorylation is an indirect mechanism. It is derived from a process that begins with moving electrons through a series of electron transporters or carriers that undergo red/ox reactions. The energy released from these reactions leads to the movement of protons across a membrane. This accumulation of protons on ones side of the membrane "polarizes" or "charges" the membrane, with a net positive (protons) on one side of the membrane and a negative charge on the other side of the membrane. this is called an electrical potential due to the charge separation. In addition, the accumulation of protons also causes a pH gradient to form across the membrane and is referred to as the chemical potential . Together this is called an electro-chemical gradient across the membrane. Think of this as a cellular capacitor, as the charge and pH gradient grows more and more energy is stored and can be used to do work, such as driving the F ₁ F ₀ ATPase and generating ATP indirectly.

Below the basic concepts of oxidative phosphorylation are described. Remember for ATP synthesis to occur two criteria must be met, the first is the formation of the membrane potential via a series of red/ox reactions, referred to as an electron transport chain and second, a membrane bound, proton driven F ₁ F ₀ ATPase, that uses the potential energy from the PMF to drive the formation of ATP by allowing protons to move from the higher concentration on one side of the membrane to the other side of lower concentration. Nowhere is molecular oxygen required for this to happen. Oxygen is a terrific terminal electron acceptor and allows for a very efficient way to generate a large PMF, however, other compounds such as hydrogen sulfide can also act as terminal electron acceptors. The eukaryotic mitochondrion has evolved an incredibly efficient electron transport chain to maximize ATP production for every 2 high energy electrons that enter the chain. While this mitochondrial electron transport chain is what we (eukaryotes) use, the diversity of the electron transport chain in nature is one of the most amazing features of life on this planet. Think about all of the unique and inhospitable places there are on this planet, yet some form of life can survive there. It makes one think about the possibility of life on other worlds. Remember all that is required is a donor of high energy electrons, carriers to move the electrons by red/ox reactions in a membrane, and a terminal electron acceptor. As we will discuss below, as long as the terminal electron acceptor has a higher affinity for the electrons than the electron donor, the electrons will move, and the energy can be captured by the cell.