Absolutely Small - Michael D. Fayer [113]
CHOLESTEROL
When talking about foods and fat, you frequently hear that it is unwise to eat too much fat because it will increase your cholesterol level. Therefore, many people have the mistaken impression that cholesterol is a fat. They think that eating a lot of fat means eating a lot of cholesterol. However, cholesterol is not a fat. Rather, it is an alcohol, as indicated by the suffix, ol. The ol indicates that a molecule is an alcohol, as for example, in ethanol (see Figure 15.1). The structure of cholesterol is shown in Figure 16.7. The top is a diagram of cholesterol, the middle portion is a ball-and-stick model, and the bottom shows a space-filling model. The alcohol—OH group is on the left side of the diagram and is the bottom left group in the ball-and-stick and space-filling models. The molecule has four carbon rings labeled 1 to 4. In the figure, carbons are at all of the vertices, and each carbon will make four bounds. Hydrogens are not shown in the figure except where necessary to indicate if a hydrogen is pointing into or out of the plane of the page. A dashed triangle is pointing into the plane, and a solid triangle is pointing out of the plane. If there is no H at the end of the triangle, then the group is a methyl,—CH3. The top portion of the figure makes it easy to see which atoms are connected to each other. The ball-and-stick model provides a more detailed three-dimensional illustration of the molecular structure. The space-filling model gives a more representative picture of the molecule’s three-dimensional structure. The space-filling model reflects the regions of space where most of the electron probability is located. It is important to keep in mind that molecules are not balls and sticks but rather delocalized electron clouds surrounding the atomic centers, the positively charged atomic nuclei.
FIGURE 16.6. Ball-and-stick model of capric acid triglyceride, which is composed of three capric acid chains. Each chain is a saturated fatty acid with 10 carbon atoms labeled 1 to 10. These are attached to three carbons labeled A, B, and C.
Comparing the structure of cholesterol in Figure 16.7 to any of the models of fats (fatty acids) displayed above shows that cholesterol is a very different type of molecule from fats. For example, the space-filling model of stearic acid (Figure 16.1) is very different from the space-filling model of cholesterol in Figure 16.7. Clearly, at the molecular level, cholesterol has little relationship to fatty acids. Yet it is frequently linked to discussions of fat in food, and the cholesterol molecule has a very negative aura associated with it.
FIGURE 16.7. Cholesterol. Top: Diagram of cholesterol. Middle: Ball-and-stick model. Bottom: Space-filling model. Cholesterol is an alcohol (—OH group) composed of four carbon rings, labeled 1 to 4, and a hydrocarbon chain.
Cholesterol Is Good, Contrary to Public Perception
Well, cholesterol is getting a bad rap. Cholesterol is a fundamentally important biological molecule. Cells are surrounded by membranes. Inside the cell are all of the complex molecular machines necessary to perform the chemistry responsible for life. Outside the cell are a large number of other chemicals, including oxygen, salts, and large biological molecules. The cell membrane that separates