Hello! I am at a loss as to the cause of an energy potential from a concentration gradient. Take for example the H+ concentration gradient across the innermembrane in mitochondria. The greater concentration of H+ in the intermembrane space relative to the matrix results in an electrical potential and a chemical potential (acoording to wiki). I assume the chemical potential merely makes reference to the difference in concentrations. For example, an uncharged molecule with a higher concentration on one side of a membrane would have chemical potential, but not electrical potential. If we were to place a partition through the middle of a container filled with water and placed an uncharged species A in to only one of the two compartments, the individual molecules would simply go about their regular business of moving about, bumping in to each other and the water molecules, and would diffuse randomly throughout the section. If an apprture was introduced. Slowly the molecules of A would pass through the aperture in a random, undirected manner. There is no purposive movement to occupy the whole container and to distribute themselves evenly throughout the container. It is simply through random movements and the resulting (approximately) even distribution is simply due to probability. In this example there is a concentration gradient, but I do not see the potential to do work. I was thinking of a gas in a container (not partitioned), with a turbine located near an aperture. The container holds a molecule B which is not present outside the container. If the container is at the same pressure as the outside, then opening the aperture will not drive the turbine, yet the molecules will diffuse out and become evenly spread throughout the container and the outside. If the pressure inside the container is greater than outside the container, opening the aperture would then result in the movement of the molecules out of the container and their greater and directed kinetic energy may be used to drive the turbine. In both examples, there is a concentration gradient, yet only trhe second can be used to do work. Returning to the H+ difference in the mitochondria, I do not therefore see how the concentration gradient in and of itself can be used to do work. The difference in concentration is used to do work, however; but where does the potential energy come from? Potential energy, as far as I am aware, isn't an energy 'on it's own' (I apologise if this becomes a little obscure): you can have gravitational potential energy due to the force of gravity trying to pull two masses togeather. If this is impeded in someway, the objects have potential energy. But if one of the objects suddenly disappeared, the potential energy would also disappear: there has to be something to provide that potential energy. The only 'source' I can think of for the H+ gradient is the elecrostatic attraction. The H+ are pulled/pushed through ATP Synthase by the force produced by the electrostatic attraction (present due to differences in charge across the innermembrane). If the electrostatic attraction was taken away, the molecules would simply diffuse down ther concentration gradient randomly, if possible, and this could not be used to do work (in this case generate ATP) unless there was a difference in pressure, as with the gas example, but in this case liquid pressure. By my reasoning at least. Apologies if this is tortuous, but I wanted to explain my reasoning, as the explanations I have found do not seem to answer these issues. Any help appreciated, Many thanks, Nobahar.