Question

how does a concentration gradient provide energy for the movement of molecules?

Answer 1

Osmotic pressure provide the energy also known as chemiosmosis in biology , this can happen without a gradient in some cases but cell usually exploit an electrochemical gradient.

Osmosis is the net movement of molecules from a region of high water potential to a region of lower potential, down a concentration gradient.


You can read more about here Osmosis and molecular diffusion here http://en.wikipedia.org/wiki/Molecular_diffusion