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This illustration shows an A T P synthase enzyme embedded in the inner mitochondrial membrane. A T P synthase allows protons to move from an area of high concentration in the intermembrane space to an area of low concentration in the mitochondrial matrix. The energy derived from this exergonic process is used to synthesize A T P from A D P and inorganic phosphate.
ATP synthase is a complex, molecular machine that uses a proton (H+) gradient to form ATP from ADP and inorganic phosphate (Pi). (Credit: modification of work by Klaus Hoffmeier)

OpenStax Biology 2e

In chemiosmosis, the free energy from the series of redox reactions just described is used to pump hydrogen ions (protons) across the mitochondrial membrane. The uneven distribution of H+ ions across the membrane establishes both concentration and electrical gradients (thus, an electrochemical gradient), owing to the hydrogen ions’ positive charge and their aggregation on one side of the membrane.

If the membrane were continuously open to simple diffusion by the hydrogen ions, the ions would tend to diffuse back across into the matrix, driven by the concentrations producing their electrochemical gradient. Recall that many ions cannot diffuse through the nonpolar regions of phospholipid membranes without the aid of ion channels. Similarly, hydrogen ions in the matrix space can only pass through the inner mitochondrial membrane by an integral membrane protein called ATP synthase. This complex protein acts as a tiny generator, turned by the force of the hydrogen ions diffusing through it, down their electrochemical gradient. The turning of parts of this molecular machine facilitates the addition of a phosphate to ADP, forming ATP, using the potential energy of the hydrogen ion gradient.

– What do you call the difference in charge and chemical concentration across a membrane?

Chemiosmosis is used to generate 90 percent of the ATP made during aerobic glucose catabolism; it is also the method used in the light reactions of photosynthesis to harness the energy of sunlight in the process of photophosphorylation. Recall that the production of ATP using the process of chemiosmosis in mitochondria is called oxidative phosphorylation. The overall result of these reactions is the production of ATP from the energy of the electrons removed from hydrogen atoms. These atoms were originally part of a glucose molecule. At the end of the pathway, the electrons are used to reduce an oxygen molecule to oxygen ions. The extra electrons on the oxygen attract hydrogen ions (protons) from the surrounding medium, and water is formed. Thus, oxygen is the final electron acceptor in the electron transport chain.

This illustration shows the electron transport chain, the A T P synthase enzyme embedded in the inner mitochondrial membrane, and the citric acid cycle occurring in the mitochondrial matrix. The citric acid cycle feeds N A D H and F A D H subscript 2 baseline to the electron transport chain. The electron transport chain oxidizes these substrates and, in the process, pumps protons into the intermembrane space. A T P synthase allows protons to leak back into the matrix and synthesizes A T P.
 In oxidative phosphorylation, the pH gradient formed by the electron transport chain is used by ATP synthase to form ATP.

Source: OpenStax Biology 2e
The third step within the entire process of rebuilding ATP is oxidative phosphorylation, where reduced coenzymes such as NADH and FADH2 are oxidized with the help of oxygen. This is why animals and humans have to respire. Oxidative phosphorylation also takes place in the mitochondria of a cell. Hydrogen is split into protons and electrons, and electrons are passed to oxygen and other inorganic compounds.


Clark, M., Douglas, M., Choi, J. Biology 2e. Houston, Texas: OpenStax. Access for free at:


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