CoachA triglyceride (triacylglycerol, a fat when solid, an oil when liquid) is glycerol — a three-carbon alcohol — esterified on its three hydroxyls by three fatty acids. It has no charge and no polar group left: it is entirely hydrophobic, and it is stored as anhydrous droplets in adipocytes, cells of the adipose tissue that are little more than a droplet of fat with a nucleus pushed to one side.
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Example 9.5 (Why not store everything as fat)
Fat can only be burnt with oxygen, slowly, and cannot be turned back into glucose in animals (Chapter 16); the brain and the red cells need glucose, and a sprinting muscle needs it faster than fat can supply. Glycogen is the fast, oxygen-free, glucose store for hours; fat, the dense store for weeks. A migrating bird, a hibernating dormouse and a camel are fat; a sprinter’s legs are glycogen.
Example 9.9 (Bile salts)
Dietary fat arrives in the intestine as oil; the lipase that digests it works only at the oil–water interface. Bile salts, amphiphilic derivatives of cholesterol, coat the fat into droplets a micrometre across, multiplying the interface a thousandfold, and then carry the fatty acids released, in micelles, to the absorbing cells (Chapter 22). The same trick is used by soap.
Example 9.13 (Butter, olive oil, fish oil)
Butter ( saturated) is solid in the refrigerator and soft at room temperature; olive oil ( oleic acid) is liquid at room temperature and clouds in the refrigerator; fish oil, rich in omega-3 acids with five and six double bonds, stays liquid at , which is what a cod’s membranes need in the North Atlantic.