The Electron Transport Chain: ATP HarvestingAfter the Krebs cycle, the original glucose molecule has been completely oxidized, which means that its potential energy has been released. One glucose molecule theoretically contains enough potential energy to manufacture close to one hundred molecules of ATP. It is clear that we have not stored a significant amount of the released energy in this form yet. Where is the energy?
The energy is temporarily residing in the reduced coenzyme molecules NADH and FADH2. During the final stages of respiration, the electron transport chain, these coenzymes will donate electrons to a series, or chain, of carriers, and will become oxidized in the process, back to NAD+ and FAD (the coenzymes can then go back and participate in glycolysis and the Krebs cycle). The carriers are physically located in the inner mitochondrial membrane, and include cytochrome proteins, among other molecules. As electrons are passed down the chain, energy is released; the final electron acceptor is molecular oxygen (O2), which is reduced and converted to water as it accepts electrons (this is where the oxygen we inhale is actually used). The energy released is temporarily stored and ultimately used to make ATP by oxidative phosphorylation. The chemiosmotic model explains how ATP is produced this way (see Figure 4.4). The model highlights three major points:
- The reduced coenzymes NADH and FADH2 ultimately react with oxygen by donating electrons through a series of intermediaries. This reaction causes oxidation of the coenzymes to NAD+ and FAD, and reduction of oxygen to water. A large amount of energy is released in the process. If the intermediate carriers did not exist, all of the energy would be released at once, which would be difficult for the cell to control and manage.
- As energy is slowly released, it is used to pump protons (H+ ions) from the matrix into the intermembrane space by active transport (see Chapter 11). This establishes an electrochemical proton gradient which stores the energy that has been released. This gradient can be used to do work.
- The enzyme ATP synthase is embedded in the inner membrane, and protons diffuse through a channel in this protein back into the matrix. As the protons βfallβ through the enzyme, the energy they release is used to do work: the phosphorylation (addition of a phosphate group) of ADP to make ATP.
The electron transport chain and oxidative phosphorylation
When all is said and done, 32 ATP molecules are harvested by the electron transport chain from all of the reduced cofactors generated through glycolysis and the Krebs cycle. If we add this to the 2 ATP molecules obtained from glycolysis and the 2 produced in the Krebs cycle, we come up with a grand total of 38 molecules of ATP harvested. However, the net amount is 36 ATP molecules because some energy is used to transport pyruvic acid into the mitochondria.