The matrix and intermembraneous space of mitochondria can be conceptualized as the two half-cells of a galvanic cell. The positive cell potential of such a galvanic cell represents the free energy available for the oxidation of NADH or FADH2 by O2. The 'load' of the cell involves pumping protons from the matrix into the intermembraneous space, converting the energy expended by the above reaction into an electropotential and diffusion gradient involving differences in H+ concentration across the inner mitochondrial membrane. The free energy of this H+ concentration gradient is ultimately used to carry out ATP generation.