Dr. Ananya Mandal explains on News Medical that lipids are mainly composed of hydrocarbons, and they are highly reduced forms of carbon. They make up the building blocks of the structure and function of living cells.
According to About.com's Regina Bailey, lipids have diverse structures and functions. The main lipid groups are fats, phospholipids, steroids and waxes. Fats contain glycerol and three fatty acids. They can be either liquid or solid at room temperature. Those in liquid form are classified as oils, while those in solid form are called fats. Fatty acids comprise a long chain of carbons with a carboxyl group at one end. Fats serve various useful purposes, such as storing energy, insulating the body and protecting organs.
Phospholipids consist of a polar molecule, a phosphate group, a glycerol unit and two fatty acids. They are found in cell membranes, which enclose the cytoplasm and other parts of a cell.
Steroids include cholesterol, cortisone and sex hormones, such as estrogen, testosterone and progesterone. They have a carbon backbone that contains four fused ring-like structures. Waxes are composed of a fatty acid and an ester of a long-chain alcohol. Numerous plants have wax-coated fruits and leaves to avoid loss of water. Some animals also have wax-coated feathers and fur to repel water.