A common mechanism that cells use to generate energy. It works like this:



+ + + + + + + More Positive Charge + + + + +
--------------------------------------------
||||||||||||||||MEMBRANE||||||||||||||||||||
--------------------------------------------
- - - - - - - More Negative Charge - - - - -

A positive charge is built up on one side of the membrane, by pumping protons across it - an osmotic imbalance. Obviously, a crucial feature of the system is a proton-impermeable lipid, otherwise a short circuit will result, as shown by the use of chemical 'uncouplers'. The proton motive force (deltaP or pmf) is channelled back through an engine known as the FoF1 ATPase (or CFoCF1 in chloroplasts) which uses the flow of protons to drive the phosphorylation of the cellular currency molecule ATP.

+ + + + + + + + + + + + + + + + + + + + + +
----------------|    | |    |--------------
|||||||||||||||||    | |    |||||||||||||||
----------------|   \   /   |--------------
         ____________\ /____________
- - - - |Pi +  ADP ---------> ATP   | - - - 
         ---------------------------


This concept is a crucial part of Mitchell's famous chemiosmotic hypothesis, that the flow of charged ions ('vectorial metabolism' as he opaquely described it:) can be coupled to chemical energy (or vice versa). This allows organisms to both control the osmotic balance inside their cells (pumping ions out, for example is especially useful for halophiles) and derive energy from osmotic imbalance.

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