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zymogens

Chymotrypsin

Pancreas

Proteins ^ Peptides

Carboxypeptidase

Pancreas

Peptides ^ Peptides and amino acids

Aminopeptidase

Small intestine

Peptides ^ Peptides and amino acids

Dipeptidase

Small intestine

Dipeptides ^ Amino acids

Enterokinase

Small intestine

Trypsinogen ^ Trypsin

Nuclease

Small intestine

Nucleic acids ^ Nucleotides

Maltase

Small intestine

Maltose ^ Glucose

Lactase

Small intestine

Lactose ^ Galactose and glucose

Sucrase

Small intestine

Sucrose ^ Fructose and glucose

Many humans stop producing the enzyme lactase around the age of 4 years and thereafter have difficulty digesting lactose, which is the sugar in milk. Lactose is a disaccharide and cannot be absorbed without being cleaved into its constituent monosaccharides, glucose and galactose. If a substantial amount of lactose remains unabsorbed and passes into the large intestine, its metabolism by bacteria in the large intestine causes abdominal cramps, gas, and diarrhea.

the role of sodium. The mechanisms by which cells lining the intestine absorb nutrient molecules and inorganic ions are diverse and not completely understood. Many inorganic ions are actively transported by these cells. Transport proteins exist for sodium, calcium, and iron. Transporter proteins also exist for certain classes of amino acids and for glucose and galactose, but their activity is much reduced if active sodium transport is blocked.

Sodium diffuses from the gut contents into the mucosal cells and is then actively transported from the mucosal cells into the submucosa. To diffuse into a mucosal cell, a sodium ion binds to a symport in the mucosal cell plasma membrane. Symport also binds a nutrient molecule, such as glucose or an amino acid. The diffusion of the sodium ion, driven by a concentration gradient, thus drives the absorption of the nutrient molecule. This mechanism is called sodium cotransport.

fat absorption. The absorption of the products of fat digestion does not involve transporter proteins (see Figure 50.15^). Pancreatic lipases break down fats into diglycerides, mono-

glycerides, and fatty acids, all of which are lipid-soluble and thus able to pass through the plasma membranes of microvilli and diffuse into the intestinal mucosal cells. Once in the cells, the fatty acids and mono-glycerides are resynthesized into triglycerides, combined with cholesterol and phospholipids, and coated with protein to form water-soluble chylomicrons, which are little particles of fat. Rather than entering the blood directly, the chylomicrons pass into the lymphatic vessels in the sub-mucosa. They then flow through the lymphatic system and enter the bloodstream through the thoracic ducts at the base of the neck. After a meal rich in fats, the chylomi-crons can be so abundant in the blood that they give it a milky appearance.

The bile that emulsifies the fats is not absorbed along with the monoglycerides and the fatty acids, but shuttles back and forth between the gut contents and the microvilli. In the ileum, bile is actively resorbed and returned to the liver via the bloodstream. As noted earlier, bile is synthesized in the liver from cholesterol. Cholesterol comes from food, but it is also synthesized by liver cells and intestinal cells. The body has no way of breaking down excess cholesterol, so high dietary intake or high levels of synthesis can result in problems such as arterial plaque formation and cardiovascular disease (see Figure 49.14). One major way in which cholesterol leaves the body is through the elimination of unresorbed bile in the feces. Certain kinds of fiber bind bile, decreasing its resorption in the ileum and thus helping to lower cholesterol levels.

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