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Cholesterol is doubtless the most publicized lipid, notorious because of the strong correlation between high levels of cholesterol in the blood and the incidence of human cardiovascular diseases. Less well advertised is cholesterol s crucial role as a component of cellular membranes and as a precursor of steroid hormones and bile acids. Cholesterol is an essential molecule in many animals, including humans, but is not required in the mammalian diet—all cells can synthesize it from simple precursors. 

Cholesterol is obtained from the diet and synthesized in most cells of the body. It is a component of cell membranes and the precursor of steroid hormones and of the bile salts used for fat absorption. High concentrations of cholesterol in the blood, particularly the cholesterol in lipoprotein particles called low density lipoproteins (LDL), contribute to the formation of atherosclerotic plaques. These plaques (fatty deposits on arterial walls) are associated with heart attacks and strokes. A high content of saturated fat in the diet tends to increase circulatory levels of LDL cholesterol and contributes to the development of atherosclerosis. 

Steroids and sterols are vital to all life forms. Sterols from plants are called phytosterols and sterols from animals are zoosterols. Cholesterol is the most important sterol in animals. It serves as an essential structural component of the cell membrane to establish membrane permeability and fluidity, and also acts as a precursor to the S5Tithesis of steroid hormones and fat-soluble vitamins. Cholesterol is produced in the body, and we will illustrate pol5mier chemistry using this example. Cholesterol is ingested from food however, diet contributes < 10% of the total blood cholesterol level. All steroids and sterols made in cells are derived from the cyclization of squalene (Figure 4.2 note the red... 

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