Water flow in osmosis. Illustration of the phenomenon of osmosis, the movement of water across a semi-permeable membrane, caused by the presence of solute molecules. Here, the water molecules consist of two hydrogen atoms (blue, small spheres) and one oxygen atom (red). The large blue spheres (left) are molecules dissolved in the water (the solute). At left, water molecules form hydrogen bonds with the solute molecules. This reduces the number of 'free' water molecules (the free energy or water potential), resulting in a net flow from right to left to restore the balance. For this artwork with labels, see image C023/8781.