6.7: Chemo-osmosis (an overview)

One of the most surprising discoveries about biological systems is the almost universal use of H⁺–based electrochemical gradients to generate ATP. Originally known as the chemiosmotic hypothesis, it was produced by the eccentric British scientist, Peter Mitchell (1920–1992).^[245] Before the significance of H⁺ transmembrane gradients was widely appreciated, Mitchell proposed that energy captured through the absorption of light (by phototrophs) or the breakdown of molecules (by chemotrophs) relied on the same basic homologous (evolutionarily-related) mechanism, the generation of H⁺ gradients across membranes (the plasma membrane in prokaryotes and the internal membranes of mitochondria and chloroplasts, intracellular eukaryotic organelles derived from bacteria.

What makes us think that these processes have a common evolutionary origin? It is the observation that in both captured energy is transferred through the movement of electrons through a structurally similar membrane-embedded “electron transport chain” (ETC). Electron transport chains are composed of a series of membrane and membrane-associated proteins and involve a series of reduction-oxidation (redox) reactions. Where an electron is within a molecule's electron orbital system influences the amount of energy present in the molecule. Adding a negative charge (an electron) to a molecule can increase electron-electron repulsion and raise the molecule’s potential energy. When an electron is added to a molecule, that molecule is said to have been "reduced". It is weird that adding an electron "reduces" a molecule (→). Generally, when an electron is removed, the molecule's energy is decreased and the molecule is said to have been "oxidized".^[246]Electrons, like energy, are neither created nor destroyed in biological systems, so the reduction of one molecule is always coupled to the oxidation of another. In a system of redox reactions, the energetic electrons removed from the reduced molecule are used to drive thermodynamically unfavorable reactions. Some of the energy difference is used to move H⁺ ions across a membrane, generating a H⁺ concentration gradient. Subsequently the thermodynamically favorable movements of H⁺s down their concentration gradient (across the membrane) is used to drive ATP synthesis, a thermodynamically unfavorable reaction. ATP synthesis itself involves the rotating ATP synthase. The reaction can be written: \[\mathrm{H}^{+}_{\text {outside }}+\mathrm{ADP}+\mathrm{Pi}+\mathrm{H}^{+} \rightleftharpoons \mathrm{ATP}+\mathrm{H}_2 \mathrm{O}+\mathrm{H}^{+}_{\text {inside,}}\nonumber\]

where “inside” and “outside” refer to compartments defined by the membrane containing the electron transport chain and the ATP synthase, with the ATP synthesis reaction occurring within the membrane-bound compartment. When run in reverse (as noted above) the ATP synthase acts as an ATP hydrolase, pumping H⁺s and other molecules against their concentration gradients. Such pumping ATPases establish most of the important ion gradients across membranes. In such a reaction: \[A T P+\mathrm{H}_2 \mathrm{O}+\text { molecule in low concentration region } \rightleftharpoons A D P+\text { Pi }+ \text { molecule in high concentration region. }\nonumber\]

The key difference between phototrophs and chemotrophs is, essentially, where do the high energy electrons come from - the absorption of light or the breakdown of unstable molecules.

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