Lipids

Lipids are a group of organic compounds that include fats, oils, and waxes. Lipids are important because they are a concentrated source of energy. They also serve as an important building material for cells and have many industrial and commercial applications.

Lipids are organic or natural substances that are produced by animals and plants. Common lipids include butter, vegetable oil, and beeswax. Lipids are not soluble in water, meaning that they cannot be dissolved in it. In fact, lipids repel water. Fats and oils are both lipids, yet they are different. Fats are usually solid or semisolid at room temperature (like butter), while oils are liquid at room temperature. Lipids are classified as saturated or unsaturated depending on their chemical structure (the type of bonds between the atoms). It is these bonds that make fats solid and oils liquid at room temperature.

Animals and plants store fats in their cells to use as an energy reserve. Plants usually store lipids in their seeds, while animals store them in cells of their skin. When needed, both can convert them back into fatty acids (which are made up of carbon, hydrogen and oxygen and are therefore a basic energy source). Many mammals use this layer of fatty deposits below their skin to keep warm in cold weather. Body fat is an insulator against low temperatures and internal heat loss. It is also an excellent shock absorber. Animal fats are rich in saturated fatty acids, while plant oils are rich in unsaturated fatty acids.

Lipids are an important part of a healthy human diet and are needed for normal growth, blood clotting, and healthy skin. They also are essential to the proper hormonal functioning of many animals. People also have found many practical uses for lipids, and use them in the production of many industrial products such as cosmetics, cleaners, and lubricants. Much of the soap, detergents, and cosmetics we use are made from purified animal and plant sources.

Besides fats and oils, lipids also include waxes. Waxes are soft, slippery substances that are similar in their chemical structure to fats and oils. Waxes usually resist attack by other chemicals and are produced by plants and animals. Plants use waxy lipids to coat their leaves and fruit and to prevent moisture loss. Animal skin is covered with a waxy lipid, and lamb's wool is protected by a very soft wax called lanolin. The wax made

by bees is also considered to be a lipid. While lipids are generally considered a type of biological fuel since they can be converted into energy, wax can be digested by very few animals.

Although some portion of a healthy diet should include lipids, diets high in animal fats are known to cause serious health problems, such as arteriosclerosis (the abnormal thickening and hardening of the arterial walls), heart disease, and cancer. Generally, animal foods are rich in both saturated fats and cholesterol, while plant foods contain unsaturated fats and no cholesterol. Cholesterol is a kind of lipid called a steroid, and although it is essential to the body, too much cholesterol can accumulate in the arteries and cause heart disease. For this reason, it is important for humans to maintain a diet low in saturated fat and cholesterol.

[See alsoMetabolism; Nutrition ]