Peter Mitchell, a British scientist, proposed the theory of chemiosmosis in 1961 to describe the way in which ATP is actually synthesized; he received the Nobel Prize for his work in 1978. In prokaryotes, it occurs at the cell membrane. In eukaryotic animal cells, chemiosmosis takes place in the crystae of the mitochondria, and in autotrophic cells, it occurs across the thylakoid membranes in the chloroplasts. Since chemiosmosis is the same for all three membranes, in the following discussion we will not need to differentiate between the membranes.
On one side of the membrane (we will call this side the "inside") is a supply of hydrogen atoms. Special carrier molecules use the energy released in the electron transport chain to bring hydrogen atoms close to the membrane and separate the hydrogen into an H+ ions (refer to Chapter Two is you need a refresher) and electrons. The electrons are brought back to the inside of the membrane while the H+ ions are forced to the other side (the "outside"). As more and more H+ ions accumulate on the outside of the membrane, two gradients are formed. First, a pH gradient is formed. This means that the outside is more acidic (because it has H+ ions) than the inside of the membrane. Also, an electrical gradient forms, since H+ ions have a positive charge.
When these gradients become sufficiently intense, they force the H+ ions through a channel (sometimes called the F0 channel) in the membrane in a tremendous gush. The ions end up in a large structure called the F1 unit where an enzyme called ATP synthetase is located. Also present in the F1 are ADP molecules and phosphate molecules. As the H+ ions rush by, they provide the energy which brings the ATP synthetase, ADP, and phosphates together. The ATP synthetase bonds the ADP and phosphate molecules, forming ATP. The H+ ions, now on the inside of the membrane, can be transported by the carrier molecules across the membrane so that the process may be repeated when enough energy is released from the electron transport chain.
