Digestion and Absorption of Carbohydrates PDF

Definitions Digestion: the process of breaking down food into individual molecules small enough to be absorbed through the intestinal wall. Digestion and absorption Absorption: the pr...

Definitions Digestion: the process of breaking down food into individual molecules small enough to be absorbed through the intestinal wall. Digestion and absorption Absorption: the process of moving nutrients from the gastrointestinal (GI) tract into the bloodstream. Transport: the process of moving absorbed nutrients throughout the body through the circulatory and lymph systems Dr. Salwa Soliman Salman Lecturer of medical biochemistry Elimination: the excretion of undigested and unabsorbed food through the feces. and molecular biology FOMSCU Digestion and absorption of carbohydrates Digestion of carbohydrates Dietary carbohydrate principally consist of. Polysaccharides :- Starch, glycogen & cellulose.. Disaccharides :- Sucrose Maltose and lactose.. Monosaccharides :- glucose and fructose. Monosaccharides does not need digestion. Digestion of Carbohydrates  Digestion in mouth Digestion of carbohydrates starts at mouth. In mouth, food undergoes mastication.So, food comes in contact with saliva(secreted by salivary gland). Saliva contain salivary amylase (ptyalin). The enzyme hydrolyzes -(1-4) glycosidic bonds at random deep inside polysaccharide (starch, glycogen) Producing dextrins, maltose. Action of salivary amylase- It requires Cl- ion for activation and PH 6.7. N.B Due to Shorter duration of food in mouth , So it is incomplete digestion of starch or glycogen in the mouth Digestion in stomach Digestion in Intestine Digestion of carbohydrate temporarily stops in the stomach. The action of salivary amylase stops in stomach because of high There are two phase of intestinal digestion acidity of stomach.. Digestion due to pancreatic amylase in pancreatic Juice. No carbohydrate splitting enzymes available in gastric juice.. Digestion due to intestinal brush border enzyme.  Digestion in Intestine Why cellulose is not digested by humans? Action of pancreatic amylase Cellulose is polysaccharide found in plants. It hydrolyzes the dextrin to mixture of maltose, isomaltose, It contain - (1-4) glycosidic bond in its structure. limit dextrin. Humans cannot synthesize the enzyme which can break glycosidic Require Cl ion for its activity with Optimum bond. So, cellulose is not digested by humans but provide bulk of pH=7.1 fiber in the diet which helps in intestinal motility & as a stool Action of intestinal brush softer. border enzyme These enzymes are responsible for final digestion of carbohydrate. The enzymes & their reactions are as follows: Absorption of carbohydrates Absorption of carbohydrates 3 mechanisms The end product of carbohydrate digestion are Glucose Fructose Facilitated Active transport Passive diffusion diffusion/Carrier galactose mediated Three mechanism are responsible for the absorption of these sugars. Passive diffusion. Facilitated transport. Active transport Carbohydrates absorption Carbohydrates absorption 1) Passive transport(simple and facilitated diffusion) 2) active transport (glucose and galactose) This depends on the conc. of sugar across the intestinal lumen and mucosal cells. All the monosaccharide are absorbed o some extent by facilitated diffusion.. Carbohydrates absorption 2) active transport (glucose and galactose) It is an energy requiring process that requires transport protein and presence of sodium ions. A sodium dependent glucose transporter (SGLT- 1) binds both glucose and sodium at separate sites and transports them into the cell. The sodium transport down conc. gradient & glucose transported against conc. gradient. This process is called cotransport or symport. The energy for this reaction is provided by ATP linked to sodium pump. Uptake of glucose in peripheral cells Glucose transporters Mechanism: facilitated diffusion where GLUT2 Several glucose transporter (GLUT-1 to 7) have been identified in acts as a glucose carrier. various tissues. N.B There are 7 important glucose transporter for uptake of glucose into special cells. They have been numbered from 1 to 7 (GLUT 1 to GLUT 7).  Location of GLUT 2 and GLUT 5 in intestine Disorders of Digestion & Absorption of Carbohydrates: Lactose intolerance This is condition, in which there is deficiency of the enzyme Lactase. The enzyme which hydrolyze lactose to glucose and galactose. Due this, lactose gets accumulates in the gut where it is a substrate for bacterial fermentation in the large intestine with production of H2 & CO2 gases and low molecular weight acids, like acetic acid which are osmotically active. Abdominal cramps & flatulence results from the accumulation of gases and osmotically active products draw water from intestinal cells into lumen resulting in diarrhea and dehydration. Causes 1. Congenital 2. Acquired (primary and secondary) Digestion and absorption of lipids Digestion of lipids in stomach The heat of the stomach liquidizes the lipids. Emulsification of lipids starts in the stomach by peristaltic contraction. Both lingual lipase and gastric lipase acts on short chain triglycerides (SCT). Lingual lipase can attack 3- ester link only producing 1,2 diacylglycerol SCTs are present in milk, butter. So, the action of these lipase is significant in newborns. Digestion of lipids in Intestine Digestion of lipids in Intestine Emulsification of lipids. Digestion of lipids by pancreatic enzymes : Three enzymes which digest the lipids are : Enzymatic (pancreatic) action on lipids Pancreatic lipase (for TGL) What is emulsification ? Is the process of dispersion of lipids into Cholesteryl esterase (Cholesteryl esters) small droplets by reducing surface tension. 3-Phospholipase-A2 (Phospholipids). Emulsification is essential for the digestion of lipids Emulsification is done by (Bile salts and Peristalsis). Digestion of lipids in Intestine Digestion of dietary triacylglycerols. Pancreatic lipase hydrolyze fatty acids in the 1 & 3 positions of TGL, producing 2 monoacylglycerol (2- MAG) & 2 free fatty acids. These are the major end products of TGL digestion. An isomerase shift the ester bond from position 2 to 1 This is then hydrolyzed to glycerol & free fatty acid. major end products of TGL digestion are [2-MAG (78%) ,1-MAG (6%) &Glycerol & fatty acids (14%)] Orlistat, an anti- obesity drug, inhibits gastric and pancreatic lipases, thereby decreasing fat absorption, resulting in weight loss. Digestion of lipids in Intestine Digestion of lipids in Intestine Digestionof dietary Cholesterolester: Cholesteryl esters are Digestion of dietary phospholipids. hydrolyzed by pancreatic Cholesterol ester hydrolase. Phospholipase-A2 hydrolyze fatty acids at the 2- position of glycerophospholipid, producing -(C. esterase) to produce cholesterol and free fatty acids. lysophospholipids. lysophospholipids being detergents, helps in emulsification and digestion of lipids. Absorption of lipids in Intestine Absorption of lipids in intestine Products of Lipid Digestion. Mixed micelles are spherical particles with hydrophilic exterior and 1- Free fatty acids. hydrophobic interior core. 2-MAG All the digested products passively diffuse into mucosal cell. Glycerol Short &medium chain fatty acids do not require bile salt for there Free Cholesterol absorption. Lysophospholipids They absorbed directly into intestinal cells, and they enter portal blood rather than lymph & transport to the liver bound to serum albumin. -These together with bile salt form mixed micelles. Fat soluble vitamins (A,D,E & K) are also packaged in these micelles and are absorbed. Absorption of lipids in intestine Absorption of lipids in intestine Synthesis of Lipids in the intestinal cells oLong chain fatty acids are activated by Thiokinase to fatty acyl coA. oThis acyl coA combine with 2-MAG to give TAG. oThe absorbed cholesterol & lysophospholipids are reacyled to regenerate cholesteryl-ester and phospholipids. oFree glycerol released in the lumen of the intestine is not reutilized but passes directly to portal vein. oHowever, the glycerol 3-phosphate, formed within intestinal cells by glucose, can reutilized for TAG synthesis. Abnormalities in lipid digestion & Digestion and absorption of proteins absorption Lipid malabsorption Loss of lipid in the faeces results Lipid malabsorption. (loss may be as much as 30 gm/day ) This includes fat soluble vitamin & essential fatty acids. Steatorrhea It is a condition characterized by the loss of lipids in the feces. It may be due to : 1-Bile salt deficiency occurs in liver disease or due to obstruction in the bile -duct. 2-Pancreatic enzyme deficiency occurs in pancreatitis or cystic fibrosis. 3-defective chylomicron synthesis Digestion of proteins Digestion of proteins Whole proteins are not absorbed. Too large to pass through cell membranes intact. Digestive enzymes. Hydrolases :Break peptide bonds hydrolases are secreted as inactive pre-enzymes to prevents self-digestion. Digestion of proteins Digestion of proteins: stomach Digestion of proteins: stomach Digestion of proteins: intestine when acidic content reach duodenum , it stimulate the release of 1. Secretin stimulate the pancreatic secretion of HCO3 raising pH from 1.5 to 7 or 8 2. Cholecystokinin that stimulates pancreatic secretion of target protein digestive enzymes Digestion of proteins: pancreas Digestion of proteins: pancrease Digestion of proteins: Digestion of proteins: Intestinal enzymes Absorption of proteins: Absorption of proteins: