4. Semi-permeable membranes

Osmosis is in fact a kind of diffusion, and it's the nature of the membrane that makes the difference between diffusion and osmosis. If we now change the membrane separating the two compartments so that only water can pass through it, then the movement of glucose will be stopped. Only water molecules will move randomly through the membrane. And since there are fewer water molecules on the side with glucose, the water level will in fact begin to rise, as more water molecules move into the compartment, along with the water and glucose already in there. Soon, the number of water molecules on each side of the membrane will be equal, while the number of glucose molecules stays the same in their compartment. The net effect, is increased volume.

The term osmotic pressure then, refers to the amount of pressure that would be needed to stop the water from rising in the side of the compartment containing glucose.