6.8: Chemo-osmosis (an overview)

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One of the most surprising discoveries in biology was the wide spread, almost universal use of H⁺ gradients to generate ATP. What was originally known as the chemiosmotic hypothesis was produced by the eccentric British scientist, Peter Mitchell (1920–1992)¹⁷⁶. Before the significance of H+ membrane gradients was known, Mitchell proposed that energy captured through the absorption of light (by phototrophs) or the breakdown of molecules into more stable molecules (by various types of chemotrophs) relied on the same basic (homologous) mechanism, namely the generation of H⁺ gradients across membranes (the plasma membrane in prokaryotes or the internal membranes of mitochondria or chloroplasts (intracellular organelles, derived from bacteria – see below) in eukaryotes.

What makes us think that these processes might have a similar evolutionary root, that they are homologous? Basically, it is the observation that in both light- and chemical-based processes captured energy is transferred through the movement of electrons through a membrane-embedded “electron transport chain”. An electron transport chain involves a series of membrane and associated proteins and a series of reduction-oxidation or redox reactions (see below) during which electrons move from a high energy donor to a lower energy acceptor. Some of the energy difference between the two is used to move H⁺ ions across a membrane, generating a H⁺ concentration gradient. Subsequently the thermodynamically favorable movement of H⁺ down this concentration gradient (across the membrane) is used to drive ATP synthesis, a thermodynamically unfavorable process. ATP synthesis itself involves the rotating ATP synthase. The reaction can be written:

\[H^+_{outside} + ADP + P_i ⇌ ATP + H_2O + H^+_{inside}\]

where “inside” and “outside” refer to compartments defined by the membrane containing the electron transport chain and the ATP synthase. Again, this reaction can run backwards. When this occurs, the ATP synthase acts as an ATPase (ATP hydrolase) that can pump H⁺ (or other molecules) against its concentration gradient. Such pumping ATPases establishes most biologically important molecular gradients across membranes. In such a reaction:

ATP + H2O + molecule in low concentration region ⇌ ADP + Pi + molecule in low concentration region.

The most important difference between phototrophs and chemotrophs is how high energy electrons enter the electron transport chain.

Contributors and Attributions

Michael W. Klymkowsky (University of Colorado Boulder) and Melanie M. Cooper (Michigan State University) with significant contributions by Emina Begovic & some editorial assistance of Rebecca Klymkowsky.