Electrochemical gradient: Encyclopedia II - Electrochemical gradient - Proton gradients

Electrochemical gradient - Proton gradients

The proton gradient can be used as an intermediate energy storage for heat production and flagellar rotation. Additionally, it is an interconvertible form of energy in active transport, electron potential generation, NADPH synthesis, and ATP synthesis/hydrolysis.

The electrochemical potential difference between the two sides of the membrane in mitochondria, chloroplasts, bacteria and other membranous compartments that engage in active transport involving proton pumps, is at times called a chemiosmotic potential or proton motive force (see chemiosmotic hypothesis). In this context, protons are often considered separately using units either of concentration or pH.

Some archaea, most notably halobacteria, make proton gradients by pumping in protons from the environment with the help of the solar driven enzyme bacteriorhodopsin, here it is used for driving the molecular motor enzyme ATP synthase to make the necessary conformational changes required to synthesize ATP.

Proton gradients are also made by bacteria by running ATP synthase in reverse; this is used to drive flagellas.

The F₁F_O ATP synthase is a reversible enzyme. Large enough quantities of ATP cause it to create a transmembrane proton gradient. This is used by fermenting bacteria - which do not have an electron transport chain, and hydrolyze ATP to make a proton gradient - which they use for flagella and the transportation of nutrients into the cell.

In respiring bacteria under physiological conditions, ATP synthase generally runs in the opposite direction creating ATP while using the proton motive force created by the electron transport chain as a source of energy. The overall process of creating energy in this fashion is termed: oxidative phosphorylation. The same process takes place in mitochondria where ATP synthase is located in the inner mitochondrial membrane, so that F₁-part sticks into mitochondrial matrix, where ATP synthesis takes place.

Adapted from the Wikipedia article "Proton gradients", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki