An amazing molecular machine, ATP synthase, can run use electrostatic potential energy stored in a proton gradient to generate ATP. Under different conditions, it can run in the opposite direction, using ATP to build a proton gradient. Created by Graham Johnson of fivth.com and Graham Johnson medical media www.grahamj.com, for Molecular Biology of the Cell, 4e by Alberts, Walter, et al, Gardland Publishers. ©2002  

Please visit www.grahamj.com for examples of other molecular machines.

Update:
Glad to hear my APT synthase movie has helped so many people learn an otherwise complicate subject.

I put a lot of time into making this with Peter Walter (UCSF) and Mike Morales (Garland Publisher) back in 2001. http://www.youtube.com/user/garlandscience/featured

A new software project I work on with Ludovic Autin in Art Olson's lab, can make it a lot easier to make a movie like this, and we distribute it for free.  Check out:  http://epmv.scripps.edu if you want to try it out.
-Graham