Cellular battery

Pumping out hydrogen ions from one side of die mitochondrial membrane to the other results in a change in the potential energy across die surface. First, since hydrogen ions are charged, the intermembrane space becomes plus-charged, and the matrix minus-charged. This results in an electric voltage of 140 millivolts (mV) across the membrane. This is similar to batteries where electrons are inclined to move from the pointed (minus charged) end to the barbed (plus charged) end, when the two ends are wired. Second, the H⁺ ions in the intermembrane space become ten times more concentrated than in the matrix. This also favors their return to the matrix, in much the same way as humans wish to migrate from densely populated areas to quieter places, to reach equilibrium. The concentration difference adds another 60 mV to the electric voltage, resulting in 200 mV of net potential. If we think that a typical AA battery has a 1.5 V charge, when eight of these membranes are stacked in serial order, they exceed the voltage of such a battery. However, H⁺ ions cannot freely pass across the lipid membrane since the membrane does not allow any particle with a net charge to pass. Neither can they use the electron transport chain proteins to go back. This is because they can only take protons from the carrier molecules and these molecules are not present in the intermembrane space. There remains only one place for their return – that is the ATP synthase complex, where energy conversion takes place.

Figure 2. ATP molecules are made by using a similar design of water mills which uses hydraulic potential to rotate the mill. Concentration difference of H⁺ ions across 100 nm wide lipid membrane of mitochondria create 200 mV of voltage which favors their return to the matrix. 4 H⁺ ions pass through the proton channel inside the stator of ATP synthase which causes 120 ⁰ of rotation of the whole machine. This movement leads to a conformational change of the lollipop shaped head region to make a single ATP molecule. Figure is modified from 1.