Transmembrane proteins that form hydrophilic channels accelerate the process, but even without these, water is still able to get through. Water passes by diffusion from a region of higher to a region of lower concentration. Note that this refers to the concentration of water, NOT the concentration of any solutes present in the water. However, the concentration of water can be altered by the active transport of solutes and in this way the movement of water in and out of the cell can be controlled. If the concentration of water in the medium surrounding a cell is greater than that of the cytosol, the medium is said to be hypotonic .
Water enters the cell by osmosis.
Plant cells and bacterial cells avoid bursting in hypotonic surroundings by their strong cell walls. These allow the build-up of turgor within the cell. When the turgor pressure equals the osmotic pressure, osmosis ceases. Such a solution is said to be isotonic . These organic molecules exert an osmotic effect that, if not compensated for, would cause the cell to take in so much water that it would swell and might even burst.