Lecture series Gastrointestinal tract

1. Lecture series Gastrointestinal tract Dr Pradeep Kumar, Professor department of physiology, KGMU, Lucknow

2. What is food?

3. We need a range of different types of food

4. Food Composition Chemical substances found in the largest amounts in food: Water (found inside cells in plants and animals) Carbohydrates Fats Protein Miniral vitamins

5. Carbohydrates The important constituents Of Food

6. Learning Objectives: • Understand the different classes of carbohydrates • Understand the enzymes responsible for digestion of carbohydrates • What are sites for absorptin Molecular basis of glucose transpotation • Learn about the function of carbohydrates

7. Carbohydrates : provide a substantial energy substrate for metabolism (4 kcals/g). Approximately 45% to 60% of dietary carbohydrate is in the form of starch, which is a polysaccharide Simple sugars Complex (starch and fiber) Monosaccharide – One unit Disaccharides – Two units Oligosaccharides – 10 or fewer units Polysaccharides – Up to 1000 units

8. Monosaccharides Glucose Galactose Fructose

9. Disaccharides Sucrose Glucose + Fructose Lactose Glucose + Galactose Maltose Glucose + Glucose

10. Polysaccharides Starch Amylose Amylopectin Dextrins Produced when starch molecules are partially broken down by enzymes, acid, or heat. Less thickening power than starch Glycogen Plant Fiber Components

11. Amylose • The straight-chain polymers are called amylose contains multiple glucose residues, connected by α-1,4 linkages. • molecules consist typically of 200 to 20,000 glucose units

12. Amylopectin • the branched-chain molecules are called amylopectin • In addition to the α-1,4 linkages, amylopectin has frequent α- 1,6 linkages at the branch points • Amylopectins are usually present in much greater quantities than amylose.

13. Starch digestion in the mouth initiates • Salivary amylase in the mouth initiates starch digestion; • In healthy adults, this step is of relatively limited importance. • Salivary amylase is inactivated by gastric acid, but it can be partially protected by complexing with oligosaccharides

14. Starch digestion in the lumen of intestine • Pancreatic α-Amylase is an hydrolyzes internal α-1,4 linkages • α-Amylase does not cleave terminal α-1,4 linkages, α-1,6 linkages (i.e., branch points), or α-1,4 linkages that are immediately adjacent to α-1,6 linkages. • As a result, starch hydrolysis products are maltose, maltotriose, and α-limit dextrins.

15. Digestion in the lumen of intestine

16. Digestion at the brush border in the lumen of intestine • The intestine cannot absorb these products of amylase digestion of starch, and thus further digestion is required to produce monosaccharides • The human small intestine has three brush border oligosaccharidases: lactase, glucoamylase (most often called maltase), and sucrase-isomaltase.

17. Digestion at the brush border of intestine • The intestine cannot absorb these products of amylase digestion of starch, and thus further digestion is required to produce monosaccharides • The human small intestine has three brush border oligosaccharidases: lactase, glucoamylase and sucrase-isomaltase.

18. Lactose and maltotriose at brush border

19. α –limit Dextrins and maltose at brush border

20. Absorption of carbohydrate

21. Absorption of carbohydrate • There are three monosaccharide products of carbohydrate digestion—glucose, galactose, and fructose • They are absorbed by the small intestine in a two-step process: • Their uptake across the apical membrane into the epithelial cell • Their coordinated exit across the basolateral membrane • The Na/glucose transporter 1 (SGLT1) is the membrane protein responsible for glucose and galactose uptake at the apical membrane • the apical step of fructose absorption occurs by the facilitated diffusion of fructose through GLUT5 • a single transporter (GLUT2) is responsible for the movement of both monosaccharides across the basolateral membrane.

22. Absorption of glucose ,galactose and sucrose • The uptake of glucose across the apical membrane through SGLT1 (secondary active transport,) because the glucose influx occurs against the glucose concentration gradien • Glucose uptake across the apical membrane is energized by the electrochemical Na+ gradient, which, in turn, is maintained by the extrusion of Na+ across the basolateral membrane by the Na-K pump • The apical step of fructose absorption occurs by the facilitated diffusion of fructose through GLUT5

23. Absorption of glucose ,galactose and sucrose

24. Lactose Intolerance • In most mammals and in many races of humans, intestinal lactase activity is high at birth, then declines to low levels during childhood and adulthood. • The low lactase levels are associated with intolerance to milk ( lactose intolerance). • When such individuals ingest dairy products, they are unable to digest lactose sufficiently, and so symptoms--such as bloating, pain, gas, and diarrhea are produced • The simplest treatment for lactose intolerance is to avoid dairy products in the diet, • Commercial lactase preparations, Yogurt is better to administration

25. References • Lippincott’s Illustrated Reviews: Physiology (2013) • Medical Physiology, UPDATED SECOND EDITION (Walter F. Boron, MD, PhD) • BERNE & LEVY, PHYSIOLOGY, SIXTH EDITION, UPDATED EDITION • Ganong’s Review of Medical Physiology, T W E N T Y -F O U R T H E D I T I O N

26. MCQs

27. The concept that active transport is the mechanism for absorption of monosaccharides in the gut is supported by all of the following statements EXCEPT a. Transport can be blocked by metabolic inhibitors b. Transport is selective for different sugars c. A maximum rate of transport exists for individual monosaccharides d. Competition exists among sugars for the carrier mechanism e. Transport can occur bidirectionally

28. A 25 year old man visits his GP complaining of abdominal cramps and diarrhoea after drinking milk. What is the most likely cause of his problem? • a) Bacterial and yeast overgrowth in the large intestine • b) Infection with the intestinal parasite Giardia lamblia • c) Lack of pancreatic amylase • d) Lack of small intestinal lactase • e) Lack of small intestinal sucrase-isomaltase

29. Man is unable to digest dietary: (A)GLYCOGEN. (B)Dextrin. (C)Saccharose. (D)Cellulose. (E)glucose.

30. Which of the following statements is correct?: a. The gastric juice contains pepsins,gelatinase,lipase and α-amylase enzymes. b. The only essential constituent of the gastric juice is the intrinsic factor. c. The hunger contractions disappear after bilateral vagotomy or damage of the feeding center in the hypothalamus. d. HCL is essential for both carbohydrate and fat digestion.

31. The intestinal brush border help digestion by the followings except: (A) Facilitating movements of intestinal content. (B) Supplying digestive enzymes and special transport system. (C) Increasing surface area of intestinal mucosa

32. Lactase enzyme splits lactose into: (A) Glucose. (B) Glucose and galactose. (C) Glucose and fructose.

33. The followings are true statements about absorption from small intestine except: (A) Fructose is absorbed by facilitated diffusion. (B) Glucose is absorbed by Na+ dependent 2ry active transport. (C) Vit. B12 is absorbed by diffusion

34. Which of the following statements about the digestion of starch is correct? a) The digestion of starch by -amylase is an endergonic process. b) The digestion of starch by -amylase is an exergonic process. c) The Gibbs free energy for the digestion of starch by -amylase is positive. d) Starch digestion releases glucose which is actively transported from the epithelial cells into the blood.

35. Deficiency of sucrase leads to bloating, flatulence and diarrhoea after ingestion of sucrose. Why is that? a) Galactose accumulates in the blood and is fermented by bacteria causing gastrointestinal problems. b) The presence of sucrose leads to synthesis of fatty acids which give a feeling of nausea and lead to other gastrointestinal upsets. c) Fructose accumulates in the intestine and is fermented by bacteria causing gastrointestinal problems. d) Sucrose accumulates in the intestine and is fermented by bacteria causing gastrointestinal problems.

36. Which of the following statements about lactose digestion is correct? a) Lactose is hydrolysed by -amylase. b) Lactase deficiency is of little consequence as lactose is only a disaccharide. c) Lactose is hydrolysed by -galactosidase. d) The symptoms of lactose intolerance can be alleviated by drinking milk.

37. The transport of glucose across intestinal mucosa is directly dependent Upon the following except: (A)Na concentration in intestinal lumen. (B)Plasma insulin. (C)Inhibitors of energy metabolism

38. Absorption of Na ions from small intestine occur by the following except: (A)Special Na channel. (B)Cotransported with Cl. (C)Cotrannsported with glucose and amino acids. (D)None of the above.

39. Fructose is absorped from small intestine by : (A)Simple diffusion. (B)Facilitated diffusion. (C)Na+ dependent 2ry active transport.

40. Thank you