In all cells, the energy storage and utilization process involves the Proton-motive force 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. 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.