Water can cross a membrane in two ways. All membranes are permeable to water to a small degree. Although it is a polar molecule, it is very small, so water can slip between the phospholipids bilayer. However some membranes can be up to 100 times more permeable to water due to membrane channels. A solution with the same water potential as a cell is said to be isotonic with the cell.
A solution with a lower water potential than the cell is said to be hypertonic
A solution with a higher water potential than the cell is said to by hypotonic
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