A semipermeable membrane is a layer that only certain molecules can pass through. Semipermeable membranes can be both biological and artificial. Artificial semipermeable membranes include a variety of material designed for the purposes of filtration, such as those used in reverse osmosis, which only allow water to pass. The biological membranes of cells are created by two sheets of phospholipid, which contain a lipid tail attached to a polar head. The tail regions of each sheet cluster together, while the heads of the molecules point outward. The polar heads point both outward toward the environment of the cell and inward toward the cytosol. In this way, a hydrophobic region of lipid tails separates the two bodies of solution. While water and other small molecules can slip through the gaps between the phospholipid molecules, other molecules like ions and large nutrients cannot force their way into or out of the cell. This makes the phospholipid bilayer an excellent semipermeable membrane that allows cells to keep their contents separated from the environment and other cells. The concentration of the solution bound by a semipermeable membrane can be described by its tonicity as compared to the environment or other cells. Because biological membranes are permeable to water but not solutes, water tends to move into cells that are hypertonic to their environment, while water moves out of cells that are hypotonic.