Digestion involves the breakdown of complex food molecules into smaller components that can be absorbed and utilized by cells. Digestion begins as soon as food is chewed, when the enzyme amylase begins to break down starches. As food reaches the stomach, the strong acidic (pH 1-3) environment and the enzyme pepsin begin the process of protein digestion. The low pH also helps dissolve the intercellular structures that bind the ingested tissues together.
The digestion of fats begins in the small intestine, where specialized enzymes from the pancreas, and bile salts from the liver play critical roles in fat digestion.
Fat digestion begins when chyme, an acidic fluid mixture of gastric juice and partly digested food, passes from the stomach into the initial section of the small intestine, the duodenum.
Despite its name, the inner surface area of the small intestine is enormous. This is due partly to its great length—over six meters in humans—but principally to the numerous folds and tiny villi and microvilli that line its inner surface.
Digestion requires many specialized enzymes, as well as other secretions. The liver synthesizes bile, which aids in digesting lipids. The bile is stored in the gallbladder, until it is needed to assist in fat digestion.
When the chyme enters the duodenum, a hormonal signal causes the walls of the gallbladder to contract rhythmically, squeezing the bile into the common bile duct and down into the duodenum.
The pancreas lies just beneath the stomach. It produces a host of digestive enzymes, including lipases, which are essential in fat digestion. The pancreas also secretes bicarbonate ions, which help to neutralize the pH of the chyme entering from the stomach. This is essential because intestinal enzymes, unlike stomach enzymes, function best at a neutral or slightly alkaline pH.
Let's examine the process of fat digestion in greater detail. Fats are hydrophobic molecules, that is, they are not soluble in water. However, most enzymes, lipase included, are water soluble and require an aqueous medium to function.
Left to themselves, fat molecules would aggregate to form large globules that would present only a small surface area where the water soluble lipases could act. Bile prevents this from happening by emulsifying the fat. The bile molecules have a hydrophobic end, which is absorbed into the fat droplet, and a hydrophilic end, which sticks out. This hydrophilic coating prevents the fat droplets from aggregating.
These stabilized tiny droplets of fat are called micelles.
Once stabilized in micelles, digestive enzymes from the pancreas can begin their work. Lipase breaks down the fats into fatty acids and monoglycerides. Because these products are lipid soluble, they can easily pass through the membrane in the intestinal mucosa and enter the epithelial cells.
Once inside the cells, the fatty acids and monoglycerides enter the endoplasmic reticulum, where they are resynthesized into triglycerides, combined with cholesterol and phospholipids, and coated with a protein to form chylomicrons. The protein coat serves to make the chylomicrons water soluble, and to facilitate exocytosis.
Chylomicrons serve as a shuttle to transport lipids through the body. The chylomicrons leave the mucosal cells via exocytosis and enter lymphatic vessels in the submucosa. From here, they are transported to the thoracic duct where they enter the bloodstream.
The digestion of fats begins in the first section of the small intestine, the duodenum. Globules of fat in the acidic chyme coming from the stomach are broken up by intestinal movements, and the smaller droplets become coated with bile molecules that are delivered to the duodenum through the common bile duct. The bile molecules emulsify the fat—they keep the small droplets from aggregating back into large globules. The small fat droplets called micelles offer a large surface area for the action of the fat digesting enzymes, or lipases, that are produced in the pancreas and delivered into the duodenum by the pancreatic duct. The pancreas also delivers a bicarbonate solution into the duodenum to neutralize the acidic chyme. Lipases and other digestive enzymes that work in the intestine require a neutral pH for maximum activity.
The action of pancreatic lipases breaks the triglycerides in the micelles down to free fatty acids, glycerol, and some monoglycerides. These products are fat soluble and easily pass into the epithelial cells lining the intestine. In the endoplasmic reticulum of the intestinal epithelial cells the free fatty acids and monoglycerides are reesterified into triglycerides. These newly formed triglycerides are packaged with cholesterol and coated with proteins to form chylomicrons. The chylomicrons are exported out of the basal ends of the epithelial cells where they are picked up by the tiny terminal vessels of the lymphatic system. The chylomicrons are passed through the lymphatic vessels and delivered into the blood circulation through junctions between the lymphatic system and the circulatory system at the base of the neck.