These self-assembled structures remove the nonpolar tail from interaction with water while allowing the polar head group to interact with water. Amphipathic molecules with a single tail, e.g., detergents or fatty acid salts, usually form micelles, while molecules with two tails, e.g., phospholipids, form bilayers, which, in turn, form closed structures like vesicles. Two-tailed amphipathic molecules are the basis for the formation of Cellular Membranes . Colligative Properties are properties of the solution that depend on the number of solute molecules present. Examples are freezing point depression, boiling point elevation and osmotic pressure.
Osmotic pressure is particularly important in biological systems.
Osmotic pressure develops when a semipermeable membrane separates two solutions, one of which contains a solute that cannot pass through the membrane. A biological example would be the membrane of a cell. As illustrated below, this will cause a column of water to rise in the tube containing the solute. For a cell, if water moves into the cell from the surrounding solution, the solution is said to be hypotonic - the concentration of dissolved material is less in the solution than in the cell. In such a solution the cell will expand and probably burst.