Proton-motive force

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In all cells, the energy storage and utilization process involves the proton-motive force (PMF) in some step. This can be described as the storing of energy as a combination of a proton and voltage gradient across a plasma membrane. The chemical potential energy refers to the difference in concentration of the protons and the electrical potential energy a consequence of the charge separation (when the protons move without a counterion).

In eukaryotes the proton-motive force exists across the inner mithochondrial membrane. It is generated by energy from the citric acid cycle and is utilized in the synthesis of ATP via ATP synthase. In plants, the proton-motive force also exists in the thylakoid membranes of the chloroplast, where it is generated by the light reaction of photosynthesis and used to synthesize ATP for use in the fixation of CO₂. In prokaryotes, it exists across the cellular membrane of the organism.

Free Energy is released from the Electron Transport Chain that is utilized to move protons from the mitochondrial matrix to the intermembrane space or cytosol. Moving the protons to the outer parts of the mitochondrial creates a higher concentration of positively charged particles, resulting in a slightly positive, and slightly negative side. This charge difference results in an electrochemical gradient. This gradiant is composed of both the pH Gradiant and the chemical gradiant. The pH gradient is a result of the H+ ion concentration difference. Together the electrochemical gradient is both a concentration and charge difference.