Most chemical digestion occurs in the small intestine

Natural Cholesterol Guide

Beat Cholesterol Ebook

Get Instant Access

In the small intestine, the digestion of carbohydrates and proteins continues, and the digestion of fats and the absorption of nutrients begin. The small intestine takes its name from its diameter; it is in fact very large organ, about 3 meters long in an adult. Because of its length, and because of the folds, villi, and microvilli of its lining, its inner surface area is enormous: about 550 m2, or roughly the size of a tennis court. Across this surface, the small intestine absorbs all the nutrient molecules derived from food.

The small intestine has three sections. The initial section (about 25 cm), called the duodenum—is the site of most digestion; the jejunum and the ileum (together about 270 cm) carry out 90 percent of the absorption of nutrients (see Figure 50.10).

Digestion in the small intestine requires many specialized enzymes, as well as several other secretions. Two accessory organs that are not part of the digestive tract—the liver and the pancreas—provide many of these enzymes and secretions.

liver. The liver synthesizes bile from cholesterol. Bile secreted from the liver flows through the hepatic duct to the i The liver produces bile.

i The liver produces bile.

Liver And Bile Secretion

50.15 The Ducts of the Gallbladder and Pancreas Bile produced in the liver leaves the liver via the hepatic duct. Branching off this duct is the gallbladder, which stores bile. Below the gallbladder, the hepatic duct is called the common bile duct and is joined by the pancreatic duct before entering the duodenum.

duodenum and through a side branch of the hepatic duct to the gallbladder (Figure 50.15), where it is stored until it is needed to assist in fat digestion. When fat enters the duodenum, a hormonal signal causes the walls of the gallbladder to contract rhythmically, squeezing bile out of the gallbladder and into the hepatic duct. Below the branch to the gallbladder, the hepatic duct is called the common bile duct. Bile from the gallbladder flows down the common bile duct to the duodenum.

To understand the role of bile in fat digestion, think of an oil and vinegar salad dressing. The oil, which is hydropho-bic, tends to aggregate in large globules. For that reason, many salad dressings include an emulsifier—something that prevents oil droplets from aggregating. Mayonnaise, for example, is oil and vinegar with egg yolk added as an emulsi-fier. Bile emulsifies fats in the chyme, and thereby greatly enlarges the surface area of the fats exposed to the lipases—the

(a) Digestion of fats lj Dietary fats are emulsified into tiny droplets called micelles through the action of bile salts in the intestinal lumen.

Large lipid droplet

Bile salts

2) Pancreatic lipase hydrolyzes fats in the micelles to produce fatty acids and monoglycerides.

Fatty acid

(b) Absorption of fats

| Fatty acids and monoglycerides are lipid-soluble and therefore readily dissolve in the plasma membrane and enter the cell, where they are resynthesized into triglycerides.

4 Triglycerides are packaged with cholesterol and phospholipids to form protein-coated chylomicrons.

| Chylomicrons are enclosed in vesicles and leave the cell by exocytosis.

Large lipid droplet

Bile salts

Fatty acid lj Dietary fats are emulsified into tiny droplets called micelles through the action of bile salts in the intestinal lumen.

2) Pancreatic lipase hydrolyzes fats in the micelles to produce fatty acids and monoglycerides.

| Fatty acids and monoglycerides are lipid-soluble and therefore readily dissolve in the plasma membrane and enter the cell, where they are resynthesized into triglycerides.

4 Triglycerides are packaged with cholesterol and phospholipids to form protein-coated chylomicrons.

| Chylomicrons are enclosed in vesicles and leave the cell by exocytosis.

Micelles Bile Salts
Lymphatic vessel

iV?Vfl 50-16 The Digestion and Absorption of Fats (a) Dietary [V "h fats are broken up by bile into small micelles that present a ^ J large surface area to lipase action. (b) The products of fat ' ' digestion are absorbed by intestinal mucosal cells, where they are resynthesized into triglycerides and exported to lymphatic vessels.

enzymes that digest fats. One end of each bile molecule is soluble in fat (it is lipophilic, or hydrophobic); the other end is soluble in water (it is hydrophilic, or lipophobic). Bile molecules bury their lipophilic ends in fat droplets, leaving their lipophobic ends sticking out. As a result, they prevent the fat droplets from sticking together. The very small fat particles that result are called micelles (Figure 50.16a).

pancreas. The pancreas is a large gland that lies just beneath the stomach (see Figures 50.10 and 50.15). It functions as both an endocrine gland (secreting hormones to the blood and tissue fluid; see Chapter 42) and an exocrine gland (secreting other substances through ducts to the outside of the body). Its exocrine products are delivered to the gut through the pancreatic duct, which joins the common bile duct.

The exocrine tissues of the pancreas produce a host of digestive enzymes, including lipases, amylases, and proteases (Table 50.3). As in the stomach, some of these enzymes— most notably the proteases—are released as zymogens; otherwise, they would digest the pancreas and its ducts before they ever reached the duodenum. Once in the duodenum, one of these zymogens, trypsinogen, is activated by an enzyme called enterokinase, which is produced by cells lining the duodenum. This process is similar to the activation of pepsinogen by low pH in the stomach: Active trypsin can cleave other trypsinogen molecules to release even more active trypsin. Similarly, trypsin activates other zymogens secreted by the pancreas.

The mixture of zymogens produced by the pancreas can be very dangerous if the pancreatic duct is blocked or if the pancreas is injured by an infection or a severe blow to the abdomen. A few trypsinogen molecules spontaneously converting to trypsin can initiate a chain reaction of enzyme activity that digests the pancreas in a short time, destroying both its endocrine and exocrine functions.

The pancreas also produces a secretion rich in bicarbonate ions (HCO3-). Bicarbonate ions are basic and neutralize the acidic pH of the chyme that enters the duodenum from the stomach. Intestinal enzymes function best at a neutral or slightly alkaline pH.

Was this article helpful?

+1 0
Get The Body Of Your Dreams

Get The Body Of Your Dreams

Everybody wants to lose weight. This is one fact that is supported by the countless weight loss programs on the market along with the numerous weight loss products, ranging from snack bars, powdered juices, shakes and even slimming soaps and lotions.

Get My Free Ebook


Responses

Post a comment