Role of Lipids

Roles of Lipids

Lipids have many roles in the body, from serving a structural role in membranes and storing energy for later use to helping the brain transmit nervous impulses and regulating the function of the body.

Structure

Lipids make up the majority of cell membranes' structure. Membranes consist of a bilayer of lipids, with their water-soluble (hydrophilic) head groups facing out and their hydrophobic (water-hating) fatty acid chains facing in. Proteins in the membrane have a variety of roles, but the lipids form the physical barrier that separates the inside of the cell from the outside of the cell.

Energy Source

Lipids are stored in adipose tissue (fat cells) when more fat in ingested than is needed immediately. In lean people 18% to 25% of the body mass is lipid (largely in cell membranes and adipose tissue). Muscles, fat cells and the liver metabolize lipids to form ATP during fasts, including overnight. After eating, fat cells and liver cells can use fatty acids from the diet to make new lipids to store in adipose cells.

Insulation

A layer of fat under the skin insulates animals from cold temperatures. This is especially true for mammals that live in the arctic (polar bears) and water (whales, seals, dolphins); all these animals rely on a thick layer of fat under their skin, blubber, to keep their body warm.

Nerve Function

Nerves have long axons that conduct signals from one part of the brain or body to another cell in the brain or spinal cord. Some axons are very long--from the bottom of the foot to the spine, for instance. A thick layer of myelin, a special membrane lipid, helps nerve impulses travel more quickly along axons.

Signaling

Steroid hormones (estrogens, testosterone, adrenal hormones) are lipids that transmit signals throughout the body. The hormones bind carrier proteins in the blood, then diffuse through cell membranes to bind receptors inside cells and change gene expression. Other lipids in the cell membrane either activate second messenger systems or are modified as part of a second messenger system that changes the cell's activities.

Protein Modification

Proteins can have lipids attached to them. These lipids can hold a water soluble protein in a membrane. In other cases, lipids and proteins form a complex that transports lipids (hydrophobic molecules) through the blood.

Other Functions of Lipids

Prostaglandins and leukotrienes are lipids that regulate inflammation. Fat soluble vitamins have many roles; vitamin K regulates blood clotting, vitamin E is an antioxidant, vitamin D helps the body use calcium and vitamin A has roles in eyesight and immunity.