Osmosis

Osmosis is the net movement of water molecules across a partially permeable membrane, from a region of higher water potential to a region of lower water potential.It is a special case of diffusion, involving only water. It is passive: the energy comes from the kinetic energy of the water molecules, and no ATP is used.Why does water move? A partially permeable membrane lets water molecules through but not larger solute molecules such as sucrose. On the side with more solute, many water molecules are held around the solute particles, so fewer are free to move. More free water molecules hit the membrane from the dilute side, so there is a net movement of water towards the more concentrated side.So in terms of concentration, water moves:
• from a dilute solution (higher water potential)
• to a concentrated solution (lower water potential).Water molecules cross cell membranes in two ways. Some pass slowly through the phospholipid bilayer. Much faster movement happens through channel proteins called aquaporins. Cells that move a lot of water, such as those in kidney collecting ducts, have many aquaporins.Osmosis continues until the water potentials on both sides are equal, or until a pressure, such as a cell wall pushing back, stops further net movement.Example: two sucrose solutions, 0.1 mol dm⁻³ and 0.5 mol dm⁻³, are separated by a partially permeable membrane. The 0.1 mol dm⁻³ solution has the higher water potential, so water moves into the 0.5 mol dm⁻³ solution.