[ajb]

But it's not just about maintaining pressure. Osmotic pressure can do work, and pushing against it requires work.

Here is an experiment you can do in your kitchen to demonstrate this: Take an egg, and peel a bit of shell off one end so the membrane is visible but unbroken. Push a straw into the other end , and put the egg (straw upwards) so the membrane is in contact with water. Osmotic pressure will force the contents of the egg upwards against gravity out through the straw.

The osmotic pressure has done work (lifted the contents of the egg) and water has become mixed with the egg contents. To obtain the water back, you would need to put in at least the same energy.