Osmosis across cell membranes

Many charged molecules can not freely diffuse across cell membranes. Thanks to channels that let water through the membrane, called aquaporins, water can easily move across cell membranes. The membrane is therefore said to be permeable to water.

When cells are exposed to a hypertonic solution, water will move from the cells into the solution, causing them to shrink. When cells are exposed to a hypotonic solution, water will move into the cells, causing them to swell and possibly burst. In an isotonic solution, no water moves across the membrane as the tonicity is the same on both sides.

A net movement of water flows out of red blood cells placed in a hypertonic solution, causing them to shrink. Equal amounts of water move into and out of red blood cells placed in an isotonic solution so there is no net movement of water and the cells stay the same size and shape. A net movement of water into red blood cells placed into a hypotonic solution causes them to swell.

Figure 1: Water movement across the cell membrane in hypertonic, isotonic, and hypotonic solutions.