Osmosis is the diffusion of water across a semi-permeable membrane. In cells, osmosis accounts for such important processes as root absorption of water and re-absorption of water in the kidneys. This reading is designed for high school biology. The reading explains how osmosis is similar to and different from diffusion and describes the cell's response in isotonic, hypotonic, and hypertonic situations. The reading also includes examples of ways in which osmosis is important in living things. Three open-response questions check understanding and reinforce important concepts.