Osmosis in red blood cells. Illustration of the different appearance of red blood cells when placed in solutions of differing osmotic potential. Osmosis (the flow of water across a semi-permeable membrane, such as a cell membrane) occurs when there is a difference in relative solute-water concentrations. At left, in a hyperosmotic solution, water flows out of the cells, causing them to shrivel. At centre, in an isomotic solution, the cells are normal. At right, in a hyposmotic solution, water flows into the cells and they swell and eventually burst. For osmosis in plant cells, see image C023/8785. For this artwork without labels, see image C023/8784.