The present state of the chemiosmotic coupling theory.

Although the general principles of the chemiosmotic coupling theory have become widely accepted, the (degree of) loc(aliz)ation of electrochemical proton potential difference cannot yet be deduced from the existing experimental data. Many results are not in ready accordance with the idea that one protonic electrochemical potential difference, i.e. the one between a homogeneous inner and a homogeneous outer aqueous phase, would be the high-free-energy intermediate of membrane-linked free-energy transduction. Rather, free-energy transduction in an organelle like a mitochondrion or a chloroplast might take place in large number (about 1 per H+-ATPase) of miniature chemiosmotic systems. The energized protons produced in such a miniature system might be largely (but not totally) confined to a proton-domain belonging to it. Hence, there might be many (rather than one) different relevant proton gradients.