Enzymes break polysaccharides down to simple sugars, or monosaccharides. Monosaccharides are actively transported into brush border cells, then out into interstitial fluid. Proteins are broken into polypeptides, then amino acids. Amino acids are actively transported into brush border cells, then out into interstitial fluid.
1. In the small intestine, carbohydrates are broken down into monosaccharides, or simple sugars .
2. This process began in the mouth, where salivary amylase broke polysaccharides into disaccharides (two-unit sugars). A pancreatic amylase carries out the same reaction in the small intestine. Disaccharides are substrates for enzymes embedded in the plasma membrane of the brush border cells. The enzymes split disaccharides into monosaccharides. For example, sucrase breaks sucrose into glucose and fructose subunits. Lactase splits lactose into glucose and galactose. Monosaccharides are actively transported into a brush border cell, then out into the interstitial fluid inside a villus. From here they enter the blood.
Protein Digestion and Absorption
3. Protein digestion began in the stomach, where pepsin broke proteins into polypeptides. It is completed in the small intestine.
4. The pancreas secretes proteases such as trypsin and chymotrypsin that break polypeptides into peptide fragments. Enzymes at the surface of the brush border cell break these fragments into amino acids. Like monosaccharides, amino acids are actively transported into brush border cells, then out into the interstitial fluid. From here they enter the blood.
5. Movements of the intestinal wall break up fat globules into small droplets. Bile salts coat the droplets, so that globules cannot form again.
6. Pancreatic enzymes digest the droplets to fatty acids and monoglycerides.
7. Monoglycerides and fatty acids diffuse across the plasma membrane's lipid bilayer, into brush border cells.
8. A In a brush border cell, the products of fat digestion form triglycerides, which associate with proteins. I he resulting lipoproteins are then expelled by exocytosis into the interstitial fluid inside the villus.