Lipid, is one of a large group of oily or fatty substances essential for good health. Lipids, carbohydrates, and proteins are the classes of compounds in all living things. Animal fats and plant oils are lipids. So are animal sex hormones and vitamins A, D, E, and K. Egg yolks, liver, and embryos of grains are rich in lipids.

Importance of lipids. 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. For example, the seeds of many plants contain lipids as food reserves for their embryos. The bone marrow, tissues beneath the skin and in the intestines, and tissue 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 repel water, but they are valuable solvents (dissolving substances) for vitamins A, D, E, and K, which do not dissolve in water.