Essentially, the electron transport chain pumps protons out of the mitochondrial matrix. Note that all protein complexes pump protons away from the matrix and into the intermembrane space (the concentration of protons is there). Through diffusion, the protons move from an area of high concentration to an area of low concentration (spontaneous process). This spontaneous flow of protons from the outside to the inside of the mitochondrial matrix, carrying electrical potential, is known as the proton motive force. There are two components to the proton motive force: the concentration gradient and the electrochemical potential (where the positive charge builds up on one side of the im membrane and the negative charge on the other). Though both components contribute to the proton motive force, it is the membrane (electrical) potential that is driving the flow of protons. In 1961 peter mitchell proposed the chemiosmotic hypothesis that the proton motive force drives the synthesis of atp.