[plat_num]

Hi, just a general question I've been pondering over for some time now.  We're taught various concepts where gradients play a role, for example the H+ atoms in ATP Synthesis drive the ATP pump because they are higher in concentration inside the cell vs the outside of the cell.  This causes the H+ atoms (protons) to move across the membrane driving the pump. 

My general question is how come it depends on the concentration of the atoms and not things like the overall pressure on one side vs the other, or the concentration of any other atoms in the mixture, etc?  Or does it depend on those other factors as well but for now we're suppose to assume that everything else is to be considered equal when dealing with things like diffusion and gradients in general?

[nj_bartel]

I believe it's due to the second law of thermodynamics - things trend toward the state of highest disorder.

[Borek]

Or does it depend on those other factors as well but for now we're suppose to assume that everything else is to be considered equal when dealing with things like diffusion and gradients in general?

I would say so.

There are processes - like reverse osmosis - in which you use high pressure to force diffusion in the opposite direction.