Written by Tim Sheppard MBBS BSc
Created 8/11/09; last updated 11/1/10
What is osmotic pressure?
Osmotic pressure is considered to be the pressure across a semi-permeable membrane caused by a solution. Another way of thinking of it is the pressure that you would need to apply to stop water trying to cross a membrane because of osmosis. Confusing, hey? Of course, you need to understand osmosis, but once you've done that, how do you understand what this osmotic pressure is?
I tend to think of it a bit like a pulling effect that is attracting water across the membrane. If there is a lot of solute dissolved on one side of the membrane, and not much on the other, the effect of osmosis says that the water will transfer to the side where there is lots of solute, until eventually the two solutions have the same amount dissolved in them.
So the osmotic pressure is that pressure which forces the water to move from where there is little dissolved to where there is lots dissolved - the kind of pressure which is trying to make everything even. I often think
of it more like a pull - that where there is more dissolved in solution, the huge amount of solute is pulling the water (or other liquid) across to join it.
Importantly, this doesn't have to be the same solute - you might have sodium dissolved on one side, and potassium on the other. The important thing is comparing the amount dissolved on one side and the amount on the other, and the pressure that this difference creates.
Strictly, osmotic pressure should be defined in terms of the amount of pressure you'd need to apply to something in order to stop water from flowing into it by osmosis. however, this to my mind just complicates it. The simple way of thinking about it is remembering that the more there is dissolved in a solution, the more it will attract water to dilute it up again. Whenever you put a cell, surrounded by a cell membrane, into a solution with a much lower concentration of solute (e.g. pure water), water will try to leak into the cell by osmosis to try to balance things out. The pressure which achieves this leakage is called osmotic pressure.
What is osmotically active?