osmosis
...of higher concentration to a region of lower concentration is called diffusion. Diffusion occurs because of the random motion in molecules. When molecules are close together, the force pushing them apart is stronger, forcing the molecules to spread out over a large area. They continue to move and when the concentration is equal they stop spreading out to change the concentration. In cells with semi-permeable membrane, a special form of diffusion takes place called osmosis. Diffusion occurs when there is no barrier to block the movement of molecules and the cell membrane allows the form of diffusion called osmosis to occur by allowing certain things into the cell. Water, some small molecules, and many easily dissolvable substances are often able to move freely across the cytoplasm at different rates. Some faster, some slower, however water is an easily monitorable substance as it moves to and from the cytoplasm freely. As a result water is often used occasionally with an impermeable substance, to show the movement of water in theses circumstances. If the solution, which the cell lays, has higher concentration of solutes than the solution inside the cell, there will be a concentration gradient of water molecules out of the cell. If the situation is reversed the opposite is true and water will move into the cell. In animal cells, t...