Each carbon is numbered as shown in the above graphic. The primary steroid, i.e., that one compound from which all steroids are derived is cholesterol, Figure 17 . Carbons 18 and 19 are called bridging carbons. The hydrocarbon tail off carbon 17 is enumerated for you, above. In addition, the hydrocarbon chain will be modified by the body as necessary for the synthesis of specific steroids.
Cholesterol also plays an important role in digestion.
Detergents emulsify fat in the small bowel as small particles so that the enzymes in our small bowels may begin digesting the lipid. The two big bile salts, as these detergents are called, are glycocholic acid and taurocholic acid, Figure 18 . Glycocholic acid is formed by the reaction of gly with cholic acid. Taurocholic acid is formed by reacting the oxidized product of cys, taurine, with cholic acid. Note that there is a polar, charged end and a nonpolar, uncharged, lipid-like end.