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One of a class of compounds which contains long-chain aliphatic hydrocarbons (cyclic or acyclic) and their derivatives, such as acids (fatty acids), alcohols, amines, amino alcohols, and aldehydes. The presence of the long aliphatic chain as the characteristic component of lipids confers distinct solubility properties on the simpler members of this class of naturally occurring compounds.
The lipids are generally classified into the following groups:
- A. Simple lipids
- 1. Triglycerides or fats and oils are fatty acid esters of glycerol. Examples are lard, corn oil, cottonseed oil, and butter.
- 2. Waxes are fatty acid esters of long-chain alcohols. Examples are beeswax, spermaceti, and carnauba wax.
- 3. Steroids are lipids derived from partially or completely hydrogenated phenanthrene. Examples are cholesterol and ergosterol.
- B. Complex lipids
- 1. Phosphatides or phospholipids are lipids which contain phosphorus and, in many instances, nitrogen. Examples are lecithin, cephalin, and phosphatidyl inositol.
- 2. Glycolipids are lipids which contain carbohydrate residues. Examples are sterol glycosides, cerebrosides, and plant phytoglycolipids.
- 3. Sphingolipids are lipids containing the long-chain amino alcohol sphingosine and its derivatives. Examples are sphingomyelins, ceramides, and cerebrosides.
Lipids are present in all living cells, but the proportion varies from tissue to tissue. The triglycerides accumulate in certain areas, such as adipose tissue in the human being and in the seeds of plants, where they represent a form of energy storage. The more complex lipids occur closely linked with protein in the membranes of cells and of subcellular particles. More active tissues generally have a higher complex lipid content; for example, the brain, liver, kidney, lung, and blood contain the highest concentration of phosphatides in the mammal. See Fat and oil, Fat and oil (food), Glycolipid, Sphingolipid, Steroid, Terpene, Triglyceride, Vitamin, Wax, animal and vegetable