Electron Transport

The last stage of aerobic respiration is the electron transport chain. The electron carriers are embedded in sequence along the inner mitochondrial membrane. This membrane is folded, providing space for many sets of these carriers. NADH and FADH2 carry high-energy electrons from the previous stages of respiration to the electron transport chain. The electrons are passed from carrier to carrier and gradually lose energy. Some of this energy is used to synthesize ATP; the rest is lost as heat. Oxygen is the final electron acceptor, combining with electrons and protons to produce water. How does the electron transport chain produce ATP?
The carrier molecules use the energy from the electrons to actively transport protons from the matrix to the intermembrane space. This creates a proton concentration gradient. Protons can diffuse back across the membrane, but only through channels created by the enzyme ATP synthase. This enzyme uses the energy of the diffusing protons to bind a phosphate group to ADP producing ATP. Each NADH yields 3 ATPs, FADH2 yields 2. The electron transport chain greatly increases the yield of ATP from the breakdown of glucose. Only 4 ATPs result from glycolysis and the Krebs cycle. Electron transport yields 32 more.

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