Plasma membranes are more permeable to water than to most other small molecules, ions, and macromolecules. This permeability is due partly to simple diffusion of water through the lipid bilayer and partly to protein channels (aquaporins) in the membrane that selectively permit the passage of water. Water will move out of a cell if cell is surrounded with hypertonic solution. Hypertonic solution is one with higher osmolarity than the cytosol, in other words, with higher solute’s concentration and lower water concentration. Water molecules tend to move from a region of higher water concentration (in this case, cytosol) to one of lower water concentration (in this case, extracellular space). The cell will shrink due to water flowing out.