Mammals cannot synthesize unsaturated fatty acids having double bonds further than 9 carbons from the carboxyl group. Thus, mammals can synthesize oleic acid and palmitoleic acid (double bonds at carbon 9), but not linoleic acid (double bons at carbons 9 and 12) or linolenic acid (double bonds at carbons 9,12, and 15). As a result, fatty acids such as linoleic acid and linolenic acid are called 'essential' fatty acids because they must be present in the diet. Figure 18.33 shows how linoleic acid is converted to arachidonic acid, an important precursor to the prostaglandins and thromboxanes.