The electron transport chain pumps protons out of the mitochondrial matrix. As a result, under normal conditions the proton concentration in the intermembrane space is about. 10-fold higher than the concentration in the matrix. Furthermore, excess positive charge builds up on the outer surface of the inner mitochondrial membrane, because the protons are not pumped with neutralizing counterions. The proton concentration gradient and the electrical potential combine to produce a protonmotive force that causes protons to flow back into the matrix if they are given the opportunity (fig 14-10, p. 462). This force is used to generate atp (fig 14-11, p. 462). Peter mitchell originally proposed that atp synthesis was driven by the flow of protons down an electrochemical gradient. Although not initially accepted by biochemists, his proposal, called the chemiosmotic hypothesis, best explained the experimental data (see p. 468-469). For example, it was observed that intact membranes are required for atp synthesis.