If two solutions are separated by a selectively permeable membrane, which way will water move?
The solution will always move down the concentration gradient. This means that the water will always move from where there is more water to where there is less water. This diffusion of water is called osmosis.
Hypotonic is a term that applies to the solution that contain less solute (and more water) than the other to which it is being compared. Hypertonic is the solution that contains more solute (and less water).
Thus, moving down the concentration gradient can also mean that the water moves from the hypotonic solution towards the hypertonic solution.
If a cell is placed in a solution that is hypertonic in comparison to it, then it shrink because the water will leave the cell. If a cell is placed in a hypotonic solution in comparison to it, then the cell will lyse (burst) because the water will quickly fill the cell. The picture accessed via this hyperlink depicts this concept nicely.
If two solutions are isotonic, then they have the same concentration. The water will move at an equal rate between two isotonic solution. Thus, no swelling or shrinking will occur.
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