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Unformatted text preview: usually negative. Cholesterol, illustrated in Figure 17, has many biological uses, it occurs in cell membranes, and its forms the sheath of some types of nerve cells. However, excess cholesterol in the blood has been linked to atherosclerosis, hardening of the arteries. Recent studies suggest a link between arterial plaque deposits of cholesterol, antibodies to the pneumonia-causing form of Chlamydia , and heart attacks. The plaque increases blood pressure, much the way blockages in plumbing cause burst pipes in old houses. Figure 17. Structure of four steroids. Image from Purves et al., Life: The Science of Biology , 4th Edition, by Sinauer Associates and WH Freeman used with permission....
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