Describe the process whereby water moves passively across a membrane.

Water moves across a cell membrane through the process of osmosis. In osmosis, water moves down its concentration gradient. Rarely do we think about a concentration gradient for water, but as more solutes are present in a solution, the concentration of water is decreased (pure water is a 55.5 M solution). Therefore, water will always move down its concentration gradient from a solution higher in water concentration (lower solute concentration) to a solution with lower water concentration (higher solute concentration). What drives water movement is the total number of solute particles in a solution, which is referred to as osmolarity. In fact, it only takes a very small change in osmolarity to cause a substantial amount of water to move. We tend to talk about the driving force for water movement as an osmotic pressure. The osmotic pressure of a solution increases as the total solute concentration (osmolarity) increases. However, water will always move toward the solution with the greatest osmotic pressure.

Because water is polar, its ability to pass the lipid bilayer is limited. Most cell membranes have aquaporins (water channels) in them, which allow water to move across the membrane by osmosis.