John DeFetto, age 15, of Portland, Ore., for his question:

WHAT ARE LIPIDS?

Lipids are a 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. Phospholipids limit the passage of water and water soluble compound though the membrane, enabling the cell to keep its contents separate from the outside environment.

Pate and oils, also known as triglycerides, serve as stored energy reserves in plant and animal cells. Each molecule of fat or oil is composed of building blocks called fatty acids linked with an alcohol called glycerin or glycerol.

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.