Animation depicting the effects of osmosis on a red blood cell. Osmosis is the movement of water through a semi-permeable membrane due to differences in solute concentrations either side of it. Water molecules are red and white, and the solute is blue. Typical solutes include metal ions and sugars. At the start of the clip, the solution has the same concentration inside and outside the cell (isotonic), and the cell looks normal. As more solute arrives outside the cell, the solution becomes hypertonic, and water leaves the cell (arrow), leading to it becoming shrivelled, or crenated. In the alternative, with much more water and less solute outside the cell (hypotonic), water moves into the cell, causing it to swell. In the second part of the clip, we see a close-up of the membrane and its pores, showing the movement of water with regards to the cell in hypotonic, isotonic and finally hypertonic solutions.