DIGESTION AND ABSORPTION.powerpoint presentation

Digestion

1. DIGESTION AND ABSORPTION

2. Digestion of carbohydrates Dietary Carbohydrates Polysaccharides like glycogen, starch and cellulose Disaccharides are sucrose, lactose and maltose Monosaccharides are mostly glucose & fructose. Other carbohydrates in diet include Alcohol, Lactic acid, pyruvic acid, pectins, dextrins The end products of carbohydrates is monosaccharides.

3. Digestion of carbohydrates In the mouth Salivary amylase → digests cooked starch to maltose In the stomach Salivary amylase act here for a longer time till pH becomes less than 4 (optimal pH - 6-7) In the intestine Pancreatic amylase can act both on boiled & unboiled starch Acts on polysaccharides and convert them into disaccharides

4. Digestion of carbohydrates Brush border enzymes α – limiting dextrinase → act on dextrin and converts into glucose Maltase → converts maltose to 2 molecules of glucose Sucrase → converts sucrose to glucose and fructose Lactase → converts lactose to glucose and galactose

5. Absorption of Carbohydrates Mostly absorbed from mucosal surfaces of jejunum and ileum Glucose and galactose are absorbed from lumen of S.I. into epithelial cells by means of sodium co-transport Carrier protein involved is Sodium dependent Glucose Transporter-1 ( SGLT – 1) The electrochemical gradient is created by Na – K ATPase at the basolateral membrane Fructose is absorbed into the epithelial cells by facilitated diffusion From epithelial cell, glucose is absorbed into blood capillaries by facilitated diffusion

6. Fate of glucose in body Storage as glycogen – about 5% of absorbed glucose Catabolism to produce energy – 50 -60% Conversion into fat – 30 – 40%

7. Digestion of Proteins Proteins: Food containing high protein are- meat, fish, egg, milk & pulses Digestion of proteins: (by proteolytic enzymes) Mouth - No protein digestion occurs Stomach - By pepsin Small intestine By pancreatic enzymes like trypsin , chymotrypsin in duodenum & jejunum Succus entericus which contains dipeptidases , tripeptidases & aminopeptidases The final products of protein digestion are amino acids absorbed from intestine.

8. Digestion of Proteins Area Juice Enzyme Substrate Endproduct Mouth Saliva NO proteolytic enzyme present Stomach Gastric juice Pepsin Proteins Proteoses , Peptones Small intestine Pancreatic juice Trypsin Proteins Proteins Proteoses peptones Small polypeptides Chymotrypsin Succus entericus Dipeptidases Dipeptides Amino acids Tripeptidases Tripeptides

9. Absorption of Proteins Absorption of amino acids from small intestine levo amino acid is actively absorbed by sodium co-transport dextro amino acid by facilitated diffusion Some of the dipeptidases and tripeptidases are absorbed directly into the enterocytes They are converted into amino acids by Intracellular peptidases Amino acids are absorbed into the blood capillaries by simple or facilitated diffusion

10. Protein & Amino Acid Transport

11. Digestion of Fats Lipids are consumed in form of neutral fats (triglycerides) Small amounts of phospholipids, cholesterol, FFA, cholesterol esters and lecithin The various sources are milk, cheese, butter, oils, fish, meats, nuts etc. Digestion: a) Mouth- By lingual lipase - Insignificant b) Stomach- By Gastric lipase - Insignificant c) Intestine- By bile salts, pancreatic enzymes, Intestinal lipase FINAL PRODUCTS OF FAT DIGESTION are fatty acids, cholesterol & monoglycerides.

12. Digestion of Fats Emulsification of fats Breaking of large fat droplets into small droplets Is necessary for the action of pancreatic lipase (water soluble, acts only on oil – water interface of fat) Emulsification also increases the surface area available for the action of lipase Emulsification is done by bile salts because of their property of lowering surface tension Bile salts surround fine fat droplets in such a way that their lipophilic non polar ends are toward fat and their hydrophilic polar ends separate the fat droplets from the aqueous phase of small intestine

13. Hydrolysis of fat droplets by pancreatic and intestinal lipases Pancreatic lipase – hydrolyses trigycerides (neutral fat) into fatty acids and monoglycerides Cholesterol ester hydrolase – hydrolyses cholesterol ester into cholesterol Phospholipase A2 – act on lysolecithin and lysocephalin and convert them into phospharyl choline Intestinal Lipase - Acts on triglycerides & converts them into fatty acids.

14. Absorption of Lipids Fatty acids, Monoglycerides, cholesterol etc form micelles and enter the enterocytes by simple diffusion Micelles are small water soluble cylindrical disc shaped particle Each micelle is composed of central fat globule surrounded by about 30 molecules of bile salts Lipid soluble non polar ends of bile salts are in the central fat globule water soluble polar ends form the outer covering

15. Absorption of Lipids Micelle act as a transport vehicle for the products of fat digestion Water soluble polar end diffuse through aqueous medium of brush border Once the micelle comes in contact with cell membrane the products are released Products then diffuse passively through the cell membrane to the interior of cell Bile salts are left behind into intestinal lumen

16. Absorption of Lipids In mucosal cells, most of monoglycerides and FFA are converted into triglycerides These are coated with a layer of protein, cholesterol & phospholipids to form particles called chylomicrons Chylomicrons enter the interstitial space by exocytosis These chylomicrons being larger in size cannot pass through membrane of blood capillaries Hence these are transported through lymph vessels and finally into blood

17. Transport of Lipids Across Intestinal Epithelium

18. Absorption of Water GIT receives about 9 L of water per day 2L – Ingested, 7L – contained in salivary, gastric, biliary, pancreatic and intestinal secretions Out of 9 liters, only 100-200 ml of water is excreted out Osmotic difference is the driving force for water absorption Water can move in either direction across wall of small intestine depending on osmotic gradients created by active absorption of electrolytes and nutrients

19. Absorption of Electrolytes, Minerals and Vitamins Absorption of sodium GIT receives about 40g of sodium per day 10g – ingested, 30g- from GI secretions All of it is absorbed Absorbed in entire length of intestine (maximum – jejunum) Mechanism of absorption In small intestine Na – Glucose co transport Na – Amino acid co transport Na – H exchange mechanisms In large intestine Passive diffusion via Na channels (stimulated by aldosterone) Transport of Na from enterocytes into interstitium By Na – K ATPase against electro chemical gradient

20. Absorption of calcium Absorbed from the upper small intestine Active transport mechanism Regulation of calcium absorption Factors promoting calcium absorption Vitamin D, Para thyroid hormone, Low pH, Lactose, Amino acids Factors inhibiting calcium absorption Phytates and oxalates – form insoluble calcium salts Phosphates – form insoluble calcium phosphates Free fatty acids – form calcium soaps High pH, dietary fibres

21. Absorption of iron Mainly in the duodenum and jejunum Total ingestion – 15 – 20mg 10% of total ingested iron is absorbed Mechanism of iron absorption Transport of iron across the brush border of enterocyte Fate of iron in the enterocyte Transport of iron in the plasma

22. Transport of iron across the brush border of enterocyte 1. Absorption of haem iron Transported across brush border by a haem transport protein In the enterocyte ferrous iron is released from haem by haemoxygenase

23. 2. Absorption of non- haem iron Most of the dietary non haem iron is present in ferric iron Iron is absorbed more efficiently in ferrous form HCl, Ascorbic acid and other reducing agents help in absorption by reducing ferric iron to ferrous form Ferrous iron is transported to brush border by the Iron transport protein Inside the enterocyte, fate of non haem iron is same as haem iron

24. Fate of iron in the enterocyte Depending on the body’s requirement, Part of ferrous iron is actively transported across basolateral membrane to interstitial space from where it enters blood Part is bound to apoferritin in the enterocyte to form ferritin. Ferritin stays in the cell till it is shed off Transport of iron in the blood Iron in the blood binds to apotransferrin to form transferrin which is carried to the tissues

25. Absorption of iron

26. Factors affecting absorption of iron Form of dietary iron Haem iron is absorbed directly Ferrous form of non haem iron is absorbed better Ferric form has to be converted to ferrous form by reducing agents (vit c increases iron absorption) Meat and fish → increases iron absorption Human breast milk → increases iron absorption The acid gastric juice HCl favours absorption of non haem iron by causing its solubilization and reduction

27. Factors inhibiting non haem iron absorption Phytates in food – by forming insoluble iron salts Phosphates , calcium, egg white and bovine milk proteins Phenols present in legumes, tea, coffee, wine Effect of iron stores on iron absorption Decrease in iron stores ( ex.. In iron deficiency anemia or increased erythropoiesis ) → increases iron absorption Increase in iron stores → decreases iron absorption

28. Mucosal Block theory of iron absorption States that Iron absorption is increased when body iron stores are depleted or when Erythropoiesis is increased and decreased under the reverse conditions. In iron deficiency states, more amount of dietary iron is absorbed and less amount forms ferritin in enterocytes In iron overload, more ferritin is formed in the intestine and less amount is absorbed

29. Absorption of vitamins Fat soluble vitamins are absorbed along with other lipids by becoming part of micelle Absorption of water soluble vitamins Is rapid compared to fat soluble vitamins Most vitamins are absorbed from jejunum except vit B12 (absorbed from terminal ileum) Most water soluble vitamins are absorbed by facilitated transport or by a Na + dependent active transport mechanism

30. Absorption of vitamin B12 Vitamin B12 binds with Intrinsic factor secreted from stomach and form a complex IF – Vit B12 complex enter the terminal ileum In the brush border epithelium, IF - vit B12 complex binds to the specific receptors, vit B12 is released from the complex and enters the enterocyte by endocytosis leaving behind IF From the enterocyte, vit B12 enters the portal circulation by binding to transcobalamine II In the liver, vit B12 is stored by binding to transcobalamine I ( Liver can store vit B12 up to 3 yrs)

31. Applied physiology Malabsorption syndrome Group of disorders with multiple nutritional deficiency Causes - a} Resection of small intestine b} Coeliac disease c} Sprue d} Gastro-colic fistula e) Blind loop syndrome f) Chronic pancreatitis

32. Coeliac disease: characterised by congenital absence of enzyme gluten hydrolase from intestinal mucosal cells. This results in formation of a toxic polypeptide ‘ Gliadin ’ from Gluten a protein present in barley, rye, wheat, etc. This Gliadin causes intestinal T cells to produce an inflammatory allergic response that flattens & disrupts the formation of microvilli . Tropical sprue : failure of absorption of folate with or without associated malabsorption of vit B12

33. BLIND LOOP SYNDROME Blind loop - portion of the gut with stagnation of intestinal contents. Allows bacteria to multiply and grow heavily → multiple problems Anemia due to Vitamin B 12 deficiency, steatorrhoea , loose motion & malnutrition Due to multiple diverticula , areas of disordered peristalsis in SI, Post surgery