Osmosis is the diffusion of water across the cell membrane. In osmosis, water moves from highest concentration to the lowest concentration down the concentration gradient. Water can move through the membrane without any additional help because the water molecule’s size is extremely small.
There are two types of Osmosis however, water movement directly through the bilayer or through an Aquaporin. (As shown below)
Osmosis occurs because of water potential. The movement of water occurs from the area with a higher water potential to that of a lower potential. In order to calculate water potential you must know solute potential and osmotic potential.