Phospholipids are made from a glycerol molecule joined to two fatty acid chains and a phosphate group.The phosphate group is polar and forms a hydrophilic head, whilst the long fatty acid chains are non-polar and form a hydrophobic tail.When phospholipids are put in water, the hydrophobic tails orient away from the water so they either form a circular micelle or a bilayer.A cell membrane is formed from a self-sealing phospholipid bilayer. The hydrophilic heads are oriented towards the interior and external aqueous environment and the hydrophobic tails are on the inside oriented away from the water molecules.Lipid soluble molecules such as steroid hormones can dissolve in the bilayer and easily pass through.Water soluble molecules and ions are prevented from entering or leaving the cell through the bilayer and must pass through special channels in the membrane.
