Osmosis (Cellular)

Osmosis is the movement of water across a membrane which is selectively permeable. In osmosis, water moves across a membrane from a region with low solute concentration to a region with high solute concentration. Thus, osmosis tends to equalize the solute concentrations in two separate membrane-enclosed regions.

In living cells, water moves by osmosis across membranes between cells or between membrane-enclosed compartments within an individual cell. All biological membranes are considered selectively permeable since they are highly permeable to water but much less permeable to other substances, such as ions, proteins, and other solutes dissolved in the cell. Osmosis is a passive process, in that it requires no expenditure of cellular energy.

Osmotic pressure is the pressure exerted by dissolved solutes in a solution of water. The stronger the concentration of the dissolved solutes, the greater the movement of water up the concentration gradient, and the greater the osmotic pressure. The significance of osmosis in biology is illustrated by two examples below.