When cells and their extracellular environments are isotonic, the concentration of water molecules is the same outside and inside the cells, and the cells maintain their normal shape (and function).
Osmosis occurs when there is an imbalance of solutes outside of a cell versus inside the cell.
A solution that has a higher concentration of solutes than another solution is said to be hypertonic, and water molecules tend to diffuse into a hypertonic solution (Figure 3.8).
Water also can move freely across the cell membrane of all cells, either through protein channels or by slipping between the lipid tails of the membrane itself.
Osmosis is the diffusion of water through a semipermeable membrane (Figure 3.7).
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