Digestive System. Part 3

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Erin Mosley
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1 Digestive System Part 3

2 Digestion Ingested materials must be broken down for absorption Majority of absorption in small intestine Water and alcohol in stomach mucosa Some salts and vitamins in large intestine Non-absorbable materials removed by defecation

3 Fate of Digested Materials Carbohydrates Glucose, fructose, and galactose are directly absorbed Cellulose metabolized by bacteria Vitamins K and B 12 are byproducts

Carbohydrate digestion Foodstuff Enzyme(s) and source Site of action Path of absorption Starch and disaccharides Oligosaccharides and disaccharides Lactose Maltose Sucrose Galactose Glucose Fructose

4 Carbohydrate digestion Foodstuff Enzyme(s) and source Site of action Path of absorption Starch and disaccharides Oligosaccharides and disaccharides Lactose Maltose Sucrose Galactose Glucose Fructose Salivary amylase Pancreatic amylase Brush border enzymes in small intestine (dextrinase, glucoamylase, lactase, maltase, and sucrase) Mouth Small intestine Small intestine Glucose and galactose are absorbed via cotransport with sodium ions. Fructose passes via facilitated diffusion. All monosaccharides leave the epithelial cells via facilitated diffusion, enter the capillary blood in the villi, and are transported to the liver via the hepatic portal vein. Figure (1 of 4)

5 Fate of Digested Materials Proteins Amino acids, dipeptides and some tripeptides Absorbed by active transport Further metabolism in cells free amino acids bloodstream

Protein digestion Foodstuff Enzyme(s) and source Site of action Path of absorption Protein Large polypeptides Small polypeptides, small peptides Amino acids (some dipeptides and tripeptides) Pepsin

6 Protein digestion Foodstuff Enzyme(s) and source Site of action Path of absorption Protein Large polypeptides Small polypeptides, small peptides Amino acids (some dipeptides and tripeptides) Pepsin (stomach glands) in presence of HCl Pancreatic enzymes (trypsin, chymotrypsin, carboxypeptidase) Brush border enzymes (aminopeptidase, carboxypeptidase, and dipeptidase) Stomach Small intestine Small intestine Amino acids are absorbed by cotransport with sodium ions. Some dipeptides and tripeptides are absorbed via cotransport with H + and hydrolyzed to amino acids within the cells. Amino acids leave the epithelial cells by facilitated diffusion, enter the capillary blood in the villi, and are transported to the liver via the hepatic portal vein. Figure (2 of 4)

Lumen of intestine Amino acids of protein fragments Brush border enzymes Pancreatic proteases Na + Na + Absorptive epithelial cell Apical membrane (microvilli) 1 Proteins and protein fragments are

7 Lumen of intestine Amino acids of protein fragments Brush border enzymes Pancreatic proteases Na + Na + Absorptive epithelial cell Apical membrane (microvilli) 1 Proteins and protein fragments are digested to amino acids by pancreatic proteases (trypsin, chymotrypsin, and carboxypeptidase), and by brush border enzymes (carboxypeptidase, aminopeptidase, and dipeptidase) of mucosal cells. Amino acid carrier 2 The amino acids are then absorbed by active transport into the absorptive cells, and move to their opposite side (transcytosis). Active transport Passive transport Capillary 3 The amino acids leave the villus epithelial cell by facilitated diffusion and enter the capillary via intercellular clefts. Figure 23.33

8 Fate of Digested Materials Lipids Emuslification by bile salts and lipase micelles move between microvilli lipids diffuse into intestinal epithelium (bile salts later reabsorbed in ileum)

Fat globule Bile salts Fat droplets coated with bile salts 1 Large fat globules are emulsified (physically broken up into smaller fat droplets) by bile salts in the duodenum.

9 Fat globule Bile salts Fat droplets coated with bile salts 1 Large fat globules are emulsified (physically broken up into smaller fat droplets) by bile salts in the duodenum. 2 Digestion of fat by the pancreatic enzyme lipase yields free fatty acids and monoglycerides. These then associate with bile salts to form micelles which ferry them to the intestinal mucosa. Micelles made up of fatty acids, monoglycerides, and bile salts 3 Fatty acids and monoglycerides leave micelles and diffuse into epithelial cells. There they are recombined and packaged with other lipoid substances and proteins to form chylomicrons. Epithelial cells of small intestine Lacteal 4 Chylomicrons are extruded from the epithelial cells by exocytosis. The chylomicrons enter lacteals. They are carried away from the intestine by lymph. Figure 23.34

10 Fate of Digested Materials Lipids Within intestinal cells Triglycerides formed proteins and cholesterol added chylomicrons enter lymphatics

11 Fate of Digested Materials Lipids Plasma enzymes generate FFA s and glycerol pass capillary wall protein cholesterol combo returns to liver additional proteins added HDL and LDL created LDL accumulates in plasma or on artery walls Difficult for cells to recognize

12 Fat digestion Foodstuff Enzyme(s) and source Site of action Path of absorption Unemulsified fats Monoglycerides and fatty acids Glycerol and fatty acids Emulsification by the detergent action of bile salts ducted in from the liver Pancreatic lipases Small intestine Small intestine Fatty acids and monoglycerides enter the intestinal cells via diffusion. Fatty acids and monoglycerides are recombined to form triglycerides and then combined with other lipids and proteins within the cells, and the resulting chylomicrons are extruded by exocytosis. The chylomicrons enter the lacteals of the villi and are transported to the systemic circulation via the lymph in the thoracic duct. Some short-chain fatty acids are absorbed, move into the capillary blood in the villi by diffusion, and are transported to the liver via the hepatic portal vein. Figure (3 of 4)

13 Fate of Digested Materials Water After digested nutrients removed, large volumes of salt and water remain in LI Active Na + uptake passive Cl - and water uptake Undigested materials (cellulose) cause water to be retained in LI Antibiotics may kill bacteria digestion impaired

14 Disorders of the Digestive System Colon cancer Second most common cause of cancer death in U.S. in men 98,000 new cases annually 48,000 deaths/year Diagnosis Colonoscopy Fiberoptic endoscope Polyps often occur before tumor

15 Disorders of the Digestive System Colon cancer Contributing factors Contact time with carcinogenic material in colon Diets high in animal material = slowed motility P53 (tumor suppressor gene) mutation Hereditary component

16 Disorders of the Digestive System Gallstones A.K.A Cholelithiasis Bile salts precipitate Block bile ducts Jaundice Treatment Lithotripsy Medications Cholecystectomy

17 Questions?

2013 Pearson Education, Inc. THE DIGESTION PROCESS: PART II

2013 Pearson Education, Inc. THE DIGESTION PROCESS: PART II THE DIGESTION PROCESS: PART II Digestion in the Small Intestine Chyme from stomach contains Partially digested carbohydrates and proteins Undigested fats 2-3 hours in small intestine (up to 6 if sluggish)