Lipids are one of the major components of animal and plant cells and therefore of feedstuffs. Fats and oils contain almost exclusively lipids. Fatty acids are the basic building blocks for synthesis of the complex lipids, triacylglycerol, the major energy storage molecule, and phospholipids, major components of cellular membranes. Fatty acids may be obtained from the diet or synthesized by animal cells. Fatty acids may also be elongated and double bonds inserted to produce unsaturated fatty acids. Animal cells are limited in the position to insert double bonds so that some unsaturated fatty acids must be supplied in the diet. Fatty acids serve not only as building blocks for complex lipids, but also as a major oxidative substrate for many cells and can be precursors for regulatory molecules, e.g., eicosanoids. The other type of complex lipid is based on the steroid nucleus; in animal cells this is primarily cholesterol that is supplied in the diet and is synthesized de novo. Cholesterol is important in membrane structure and is the precursor for the sex hormones and the adrenal corticoid hormones.

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