Electron Transport and ATP Synthesis

How does the electron transport chain use high-energy electrons from glycolysis and the Krebs cycle?

Products from both the Krebs cycle and glycolysis feed into the last step of cellular respiration, the electron transport chain, as seen in Figure 9–6. Recall that glycolysis generates high-energy electrons that are passed to NAD⁺, forming NADH. Those NADH molecules can enter the mitochondrion, where they join the NADH and FADH₂ generated by the Krebs cycle. The electrons are then passed from all those carriers to the electron transport chain. The electron transport chain uses the high-energy electrons from glycolysis and the Krebs cycle to convert ADP into ATP.

Electron Transport NADH and FADH₂ pass their high-energy electrons to the electron transport chain. In eukaryotes, the electron transport chain is composed of a series of electron carriers located in the inner membrane of the mitochondrion. In prokaryotes, the same chain is in the cell membrane. High-energy electrons are passed from one carrier to the next. At the end of the electron transport chain is an enzyme that combines these electrons with hydrogen ions and oxygen to form water. Oxygen serves as the final electron acceptor of the electron transport chain. Thus, oxygen is essential for getting rid of low-energy electrons and hydrogen ions, the wastes of cellular respiration. Without oxygen, the electron transport chain cannot function.

Every time 2 high-energy electrons pass down the electron transport chain, their energy is used to transport hydrogen ions (H⁺) across the membrane. During electron transport, H⁺ ions build up in the intermembrane space, making it positively charged relative to the matrix. Similarly, the matrix side of the membrane, from which those H⁺ ions have been taken, is now negatively charged compared to the intermembrane space.

ATP Production How does the cell use the potential energy from charge differences built up as a result of electron transport? As in photosynthesis, the cell uses a process known as chemiosmosis to produce ATP. The inner mitochondrial membrane contains enzymes known as ATP synthases. The charge difference across the membrane forces H⁺ ions through channels in these enzymes, actually causing the ATP synthases to spin. With each rotation, the enzyme grabs an ADP molecule and attaches a phosphate group, producing ATP.

The beauty of this system is the way in which it couples the movement of high-energy electrons with the production of ATP. Every time a pair of high-energy electrons moves down the electron transport chain, the energy is used to move H⁺ ions across the membrane. These ions then rush back across the membrane with enough force to spin the ATP synthase and generate enormous amounts of ATP. On average, each pair of high-energy electrons that moves down the full length of the electron transport chain provides enough energy to produce 3 molecules of ATP.

In Your Notebook Relate the importance of oxygen in cellular respiration to the reason you breathe faster during intense exercise.

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