 In 1961, the British biochemist Peter Mitchell (1961) suggested a mechanism for oxidative phosphorylation (as this process became known) that did not require a high-energy intermediate. According to Mitchell’s scheme, NADH is oxidized step-wise on the inner membranes of mitochondria (membrane-bounded intracellular organelles that are canonically described as the cell’s power stations). This process occurs asymmetrically with respect to the membrane such that a net transport of protons across the membrane occurs (later it was shown that the protons are actually transported across proteinaceous pores in the membrane). Thus, a concentration gradient of protons builds up. As protons are electrically charged, this gradient also creates a voltage difference across the membrane. How is this energy used? According to Mitchell, the membrane contains another enzyme that uses the energy of the proton gradient to directly phosphorylate ADP. Thus, it is the proton gradient that couples the reduction of NADH to the phosphorylation of ADP, not a chemical high-energy intermediate. This mechanism became known as the “chemi-osmotic mechanism”.