The proton pump is a membrane-integrated enzymatic complex which is able to mobilize protons to generate a proton gradient across the cell membrane. This proton gradient constitutes a fundamental energy reservoir. The proton pump plays an important role in cell respiration and photosynthesis. The electron transport chain in cell respiration generates an electrochemical potential which is coupled to the proton pumps located in the membrane. This proton gradient is an energy reservoir because it is the driver for the generation of chemical energy (ATP), or any secondary transport system associated to it, such as the transport of nutrients, the maintenance of the ionic homeostasis of cells, or the movement of bacterial flagellum.