[Cooper]

Hi,

My question is about complex 2 of the electron transport chain. I've read that "the free energy for electron transfer from succinate to CoQ is insufficient to drive ATP synthesis"...but I am confused about what that really means.

ATP's are not directly synthesized via the complexes so I am confused as to why the free energy change matters. Isn't the only thing that matters the electrochemical gradient set up across the inner mitochondrial membrane? So isn't the reason complex 2 is insufficient to drive ATP synthesis merely because it does not span the membrane and thus is incapable of transferring protons across? 

Thanks,
Cooper

[Corribus]

ATP's are not directly synthesized via the complexes so I am confused as to why the free energy change matters. Isn't the only thing that matters the electrochemical gradient set up across the inner mitochondrial membrane?

How do you think you form the electrochemical gradient?  Where do you think the potential energy to do that comes from?

[Yggdrasil]

What's the context for that statement?  Perhaps it's just explaining why the electron transfer reactions don't directly power ATP synthesis and why the cell has evolved a more complicated scheme involving an electrochemical gradient.

[Babcock_Hall]

Do you know what the balanced equation is for complex II and how it differs from the other three complexes?  Could you calculate the free energy of the reaction if given the appropriate half-cell reduction potentials.  I don't think that the issue of spanning the membrane or not has anything to do with it.