there is no movement of sugar out of the cellophane sack into the beaker however because the cellophane is imperme able to molecules of sugar -the sugar molecules are too large to go through the pores of the membrane .as water moves into the cellophane sack the sugar solution becomes increaingly dilute and because the cellophane sack has expanded to its limit as a reslt of an increased volume of water water begins to move up the glass tube.in time the water that has accumulated in the cellophane sack and the glass tube exerts a downward pressure that forces water molecules out of the cellophane sack and back into the beaker. this movement of water through a selectively permeable membrane produces osmotic pressure.osmotic pressure is the pressure required to prevent the movement of pure water (water with no solutes) into a solution containing some solutes. in other words osmotic pressure is the pressure needed to stop the flow of water across the selectively permeable membrane (cellopane).when water molecules leave and .enter the cellophane sack at the same rate equilibrium is reached
a bacterial cell may be subjected to any of three kinds of osmotic solutions isotonic hypotonic or hypertonic.an isotonic solution is amedium in which the overall concentration of solutes equals that found inside a cell (iso means equal).water leaves and enters the cell at the same rate (no net change) the cell"s contents are in equilibrium with the solution outside the cell wall