ATP is made through chemiosmosis

During electron transport, protons are also actively transported across the membrane: electron transport within each of the three transmembrane complexes (I, III, and IV) results in the transfer of protons from the matrix to the intermembrane space (Figure 9.8). Since the lipid bilayer does not allow charged H+ to diffuse across it, transfer of H+ across it through the electron transport chain sets up a gradient, with the concentration of H+ in the intermembrane space higher than in the matrix. In addition, because H+ carries a charge, there is more positive charge in the intermembrane space. These two gradients—of concentration and charge—set up a proton-motive force which is a key factor in energy metabolism in cells: The gradient of H+ across the inner membrane is a source of potential energy.

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Figure 9.8 The Respiratory Chain and ATP Synthase Produce ATP by a Chemiosmotic Mechanism As electrons pass through the transmembrane protein complexes in the respiratory chain, protons are transferred from the mitochondrial matrix into the intermembrane space. As the protons return to the matrix through ATP synthase, ATP is formed.

Animation 9.1 Electron Transport and ATP Synthesis

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Activity 9.5 Electron Transport Simulation

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How can this energy be tapped for use by the cell? The answer is that another protein, ATP synthase, allows the H+ to diffuse back into the matrix down its concentration gradient. In the process, potential energy is captured and used for the formation of ATP. The coupling of the proton-motive force and ATP synthesis is called the chemiosmotic mechanism—or chemiosmosis—and is found in all respiring cells.

To summarize, the energy originally contained in glucose and other fuel molecules is ultimately captured in the cellular energy currency, ATP. For each pair of electrons passed along the chain from NADH to oxygen, about 2.5 molecules of ATP are formed. FADH2 oxidation produces about 1.5 ATP molecules because it enters the electron transport chain at a later step than NADH (see Figure 9.8).

ATP synthesis is a reversible reaction, and ATP synthase can also act as an ATPase, hydrolyzing ATP to ADP and Pi:

ATP ⇌ ADP + Pi + free energy

If the reaction goes to the right, free energy is released. In the mitochondrion, it is used to transfer H+ out of the mitochondrial matrix—not the usual mode of operation. If the reaction goes to the left, it uses the free energy from H+ diffusion into the matrix to make ATP. What makes it prefer ATP synthesis? There are two answers to this question:

  1. ATP leaves the mitochondrial matrix for use elsewhere in the cell as soon as it is made, keeping the ATP concentration in the matrix low, and driving the reaction toward the left.

  2. The H+ gradient is maintained by electron and proton transport.

Every day a person hydrolyzes about 1025 ATP molecules to ADP. This amounts to 9 kg, a significant fraction of the person’s entire body weight! The vast majority of this ADP is “recycled”—converted back to ATP—using free energy from the oxidation of glucose.