Lipids

Lipids, diverse group of fatty substances found in all living organisms. Lipids are distinguished from other classes of organic compounds in that they do not dissolve in water but are soluble in alcohol, ether, or other organic solvents.

Among the most important lipids are the phospholipids, which are major components of the cell membrane (see Cell). Phospholipids limit the passage of water and water-soluble compounds through the membrane, enabling the cell to keep its contents separate from the outside environment.

Fats and oils, which are composed of triglycerides, serve as stored energy reserves in plant and animal cells. Each triglyceride is composed of three fatty acid molecules bonded to one glycerol molecule. When an organism has excess energy available from food or from photosynthesis, it may use that energy to form stores of triglycerides. These can later be broken down to yield energy when the organism needs it. Fats and oils contain twice as much stored energy, per unit of weight, as carbohydrates or proteins.

Other important lipids are the waxes, which form protective coatings on the leaves of plants and the skins of animals, and the steroids, which include vitamin D and several key hormones.