Lipids are the esters of fatty acids with various alcohols. Lipids are made up of carbon, hydrogen and oxygen. Some lipids may also contain elements like phosphorous and nitrogen. A simple form of lipids is hydrocarbons with a carboxyl group at one end.
Lipids are nonpolar and hydrophobic (water hating) and are insoluble or partly soluble in water. They are soluble in organic solvents like ether, acetone etc.
A major type of lipid found in cell membrane is the phospholipids. Each phospholipid has 2 fatty acids and a glycerol. One end of the phospholipid has a phosphate group and this phosphate group is polar and hydrophilic (water loving) .The other end has two long chains of fatty acids and this end is non-polar and hydrophobic. In aqueous medium, the nonpolar tails of phospholipids segregate away from water and form a double layered structure called lipid bilayer. When exposed to an aqueous solution, the charged heads are attracted to the water phase and the non-polar tails are repelled from the water phase.
When two single layers of polar lipids come together to form a double layer, the outer hydrophilic face of each single layer will orient itself towards the solution and the hydrophobic portion will become immersed to the core of the bilayer. The structure of lipid bilayer helps the membrane to be selectively permeable and fluid nature. Phospholipids play an important role in absorption and transportation of fatty acids.