Oliver Bradley, age 13, of Haggerstown, Md., for his question:
WHICH FOODS ARE RICH IN LIPIDS?
A lipid is one of a large group of oily or fatty substances essential for good health. Foods rich in lipids include egg yolks, liver and embryos of grains and other cereals.
Lipids, carbohydrates and proteins are the classes of compounds present in all living things. All animal fats and plant oils are lipids. So are animal sex hormones and vitamins A, D, E and K.
Lipids are vital to animals and plants in many ways. They are a concentrated source of food energy and yield about twice as many calories as an equal weight of protein or carbohydrate.
Many kinds of organisms store food in lipid form. As an example, the seeds of many plants contain lipids as food reserves for their embryos. The bone marrow, tissues beneath the skin and intestines, and tissues surrounding body organs in animals consist mostly of stored lipids.
Certain lipids form an essential part of the membranes that enclose and protect every living cell. Similar membranes surround all bodies within the cell, so that each cell body can do its job without unwanted interference from other cell bodies.
Lipids are also valuable as solvents or dissolving substances for vitamins A, D, E and K, which do not dissolve in water.
Lipids are classified as simple lipids or complex lipids, according to their structure.
Simple lipids contain only carbon, hydrogen and oxygen. They consist of an alcohol in combination with certain organic acids containing a variable number of carbon atoms.
A molecule of fat, the most common type of simple lipid, contains one molecule of an alcohol called glycerol and three molecules of fatty cells, a kind of organic acid.
Complex lipids have a more complicated structure than simple lipids. They include phospholipids, steroids and other compounds.
Phospholipids are found in all bacteria and in the cells of all plants and animals. They are most plentiful in sperm, eggs, embryos and brain cells.
A molecule of phospholipid contains a molecule of glycerol, a phosphate ion and two molecules of fatty acid. Most phospholipids also contain a compound with nitrogen in it, and some contain the vitamin inositol.
Steroids make up an important part of living things. Many animal hormones, including the sex hormones and those produced by the cortex or outer part of the adrenal glands, are steroids.
Cholesterol, the substance believed responsible for narrowing the heart arteries, is a steroid. Yeasts and other fungi and the seeds of higher plants also contain steroids.
Fats, which are simple lipids, include butter, lard, tallow, blubber, castor oil, coconut oil and olive oil. Waxes, another common group of simple lipids, contain an alcohol molecule that is larger than the glycerol molecule.