As electrons travel down the electron transport chain free energy is released. Some of this free energy is used to translocate protons from the matix into the intermembrane space. The concentration of protons in the intermembrane space is greater than the concentration in the matrix. Thus a proton concentration gradient is formed. This proton concentration gradient is called an electrochemical gradient because there is a concentration gradient of protons and a potential difference across the inner mitochondrial membrane.