2. Digestion & Absorption

Transcription

1 2. Digestion & Absorption How are water and ions absorbed? How are the main nutrient groups - carbohydrates, proteins, fats digested and absorbed? What happens in disorders of digestion/absorption? lactose intolerance and coeliac disease Reading: Stanfield, pp Digestion & Absorption of Nutrients GENERAL PRINCIPLES Breakdown of complex molecules Enzymes (ph) Absorption into gut cells Mechanisms of absorption: Passive -not energy-dependent Diffusion Facilitated diffusion carrier mediated Active - requires energy Carrier-mediated Primary active transport transporter protein directly hydrolyses ATP for energy Secondary active transport couples energy released from movement of one substance to co-transport another 2 1

2 Input Bulk flow of liquid in gut Ingestion ~ 2 litres per day Secretion (gut) ~ 7 litres/day Output Faeces ~100 ml/day Conclude ~ 9 litres/day absorbed 3 Absorption of Water Passive Driven by an osmotic gradient across mucosal epithelium Due to Na + [actively pumped from gut lumen into lateral spaces] Water can move either through the epithelial cells (transcellular) or through the tight junctions (paracellular) H 2 O ~95% absorption in small intestine Absorptive cell Na + Na + 4 2

3 Carbohydrate (CHO) Digestion Amylase digestion of carbohydrates produces the disaccharide maltose or short polysaccharides known as limit dextrins Starch (polysaccharide) 5 Carbohydrates are initially digested by amylases Further digestion occurs at the brush border / microvilli on intestinal epithelial (absorptive) cells Carbohydrate digestion and Absorption 6 3

4 Carbohydrate (CHO) Digestion of microvilli Disaccharides are further digested to monosaccharides by brush border enzymes 7 Digestion of Carbohydrates to Disaccharides is done by brush border enzymes CHOs must be monosaccharides to be absorbed Brush border enzymes hydrolyse disaccharides to monosaccharides Dextrins Sucrose Lactose Dextrinase Sucrase Lactase Glucose Glucose + Fructose Glucose + Galactose Maltase Maltose Glucose X 2 8 4

5 CHO Digestion Summary Polysaccharides to disaccharides (gut amylases) Disaccharides to monosaccharides (brush border) Glucose, galactose enter gut cells by energy-dependent secondary active transport i.e. piggy-back on Na + gradient (active) Fructose enters gut cells by facilitated diffusion (passive) 9 CHO Absorption into GI Epithelial Cells Glucose + galactose are co-transported with Na + across apical membrane Fructose absorbed by facilitated diffusion Glucose molecules cross basolateral membrane by facilitated diffusion and diffuse into capillaries 10 5

6 CHO Absorption into Bloodstream Glucose, galactose enter capillaries down concentration gradient Fructose enters capillaries via passive carrier-mediated transport 11 Lactose Intolerance Lactose intolerance is due to deficient production of lactase Lactase normally hydrolyses lactose (sugar present in milk) to glucose and galactose Insufficient lactase means lactose is not digested and absorbed in the small intestine Digestion of lactose by bacteria in colon results in production of gas and other bi-products leading to bloating and diarrhoea Incidence ~5 % in northern Europe, up to 90 % in Asian populations 12 6

7 Protein Digestion Essentially similar to CHO Proteins converted to peptides Protein digestion begins in stomach with gastric pepsinogen Pepsinogen is activated by HCl to form active pepsin 13 Protein Digestion Protein digestion is continued in small intestine by pancreatic enzymes Proenzymes (zymogens) secreted by the pancreas are converted to active forms following activation of trypsin by a brush border enzyme, enterokinase Precursor Trypsinogen Active form Trypsin Chymotrypsinogen Procarboxypeptidase Chymotrypsin Carboxypeptidase 14 7

8 Peptidase activation Pancreatic trypsinogen Activated by enterokinase Trypsin then activates other zymogens 15 Protein absorption similar to CHO Peptides broken down to amino acids (brush border) Amino acids are actively transported by carriers into gut epithelial cells Amino acids then transported across basolateral membrane by facilitated diffusion and diffuse into capillaries 16 8

9 Digestion and Absorption of Fat Most ingested fats are triglycerides They are broken down to monoglycerides and free fatty acids for absorption Digestion of fat is done by lipases secreted in the mouth and stomach but mostly by pancreatic lipases in small intestine 17 Digestion of Fat Hydrophobic fat globules are broken up into smaller droplets or emulsified by bile salts Bile salts contain polar OH and COOH groups that are hydrophilic 18 9

10 Digestion of Fat Bile salts + lecithin aggregates called micelles As fats are absorbed by epithelial cells, micelles release more fatty acids and monoglycerides for absorption 19 Micelles deliver fats to the absorptive cells Bile salts, lecithin and cholesterol aggregate to form micelles with a hydrophilic outside and hydrophobic lipid inside Fats can be carried through the water based intestinal fluids dissolved in the lipid core 20 10

11 Absorption of Fat Monoglycerides and FFA enter cells by diffusion Once absorbed they are reassembled as triglycerides They are then complexed with proteins in Golgi apparatus to form lipoproteins known as chylomicrons Chylomicrons are secreted by exocytosis into the interstitial fluid and from there enter the lymphatic vessels 21 Summary of Fat Digestion & Absorption Fat to triglycerides (pancreatic lipase) Bile salts emulsify (surface area) Bile salts micelles containing monoglycerides and free fatty acids (FFA) Enter passively (fat-soluble) Triglyceride synthesis chylomicrons Exocytosis and entry into lacteals 22 11

12 Coeliac Disease An autoimmune disease, high incidence in Ireland, up to 1 in 100 Intolerance to gluten a protein present in wheat and other grains such as rye, barley, oats Ingestion of gluten triggers an immune response, resulting in inflammation of the intestinal mucosa From: Autoimmune response results in damage to intestinal villi causing malabsorption Endoscopy Coeliac Reduced absorption leads to bloating, diarrhoea and vitamin deficiencies Disease is managed by following a glutenfree diet 23 12