Worked Example 24.1 Digesting and Transporting Fats Describe how the fat in an ice cream cone gets from the ice cream to a liver cell. Analysis Dietary fat from animal sources (such as the whole milk often found in ice cream) is primarily triacylglycerols with a small amount of cholesterol present. Cholesterol is not degraded in the digestive system. Fat-digesting enzymes are secreted by the pancreas and delivered via the common duct to the small intestine, along with bile acids. As discussed above, only free fatty acids and mono- and diacylglycerols can cross the intestinal cell wall before being passed on to the blood stream. Smaller molecules such as some free fatty acids and glycerol diffuse across the cell membrane to enter the bloodstream; larger molecules must be delivered there in special packaging, called lipoproteins. Solution As the ice cream cone is eaten, it passes through the mouth to the stomach, where mixing occurs. This mixing action promotes the formation of triacylglycerols into small droplets. No enzymatic digestion of lipids occurs in the stomach. When the stomach contents move to the small intestine, bile acids and pancreatic lipases are secreted into the mixture. The bile acids help to emulsify the fat droplets into micelles. Once micelles have formed, lipases hydrolyze the triacylglycerols to mono- and diacylglycerols; the hydrolysis also produces fatty acids. These three hydrolysis products cross into the cells lining the small intestine, are resynthesized into triacylglycerides, and are secreted into the bloodstream in the form of chylomicrons. Chylomicrons travel to the liver and enter cells for processing. The small amount of cholesterol in the ice cream will be directly absorbed, packaged into chylomicrons as well, and sent to the liver.

Worked Example 24.2 Spiraling through β-Oxidation How many times does stearic acid (CH3(CH2)16COOH) spiral through the β-oxidation pathway to produce acetyl-CoA? Analysis Each turn of the β-oxidation spiral pathway produces 1 acetyl-CoA. To determine the number of turns, divide the total number of carbon atoms in the fatty acid, 18 in this case, by 2 since an acetyl group contains two carbon atoms and they come from the fatty acid. Subtract one turn, since the last turn produces two acetyl-CoA molecules. Solution Stearic acid contains 18 carbon atoms; the acetyl group contains two carbon atoms. Therefore eight β-oxidation turns occur, and nine molecules of acetyl-CoA are produced.