Digestion of Dietary Carbohydrates

Main Carbohydrates of Diet 1- Monosaccharides: mainly glucose & fructose ABORBED with NO DIGESTION 2- Disaccharides: Sucrose, lactose & maltose DIGESTED into monosaccharides 3-Polysaccharides: Starch (plant source e.g. rice, potato, flour) & glycogen (animal source) DIGESTED into monosaccharaides Cellulose (fibers of vegetables & fruits) NOT NOT DIGESTED

Digestion of Carbohydrates of Diet 1- In the Mouth The major dietary polysaccharides are of plant (starch, composed of amylose & amylopectin) & animal (glycogen) origin. salivary  -amylase During mastication, the enzyme salivary  -amylase acts on dietary starch and glycogen hydrolysing some  (1-4) bonds to give dexrtin N.B. humans do not have  amylase, so they can not digest cellulose, which is a carbohydate of plant origin containing  (1-4) glycosidic bonds between glucose molecules. Salivary  -amylase action on carbohydrates stops in the stomach as the media is strongly acidic.

2- In the Lumen of small intestine Pancreatic bicarbonate Pancreatic bicarbonate neutralizes the acidic juice of stomach. pancreatic  -amylase pancreatic  -amylase continues the process of starch and glycogen digestion Dextrin is cleaved into oligosachharides and disaccharides. Digestion of Carbohydrates of Diet

3- At the Mucosal Lining of Small Intestine Mucosa of small intestine secretes two types of enzymes: 1- Intestinal oligosaccharidases: 1- Intestinal oligosaccharidases: digest oligosaccharides into disaccharides & monosaccharaides. 2- intestinal disaccharidases : 2- intestinal disaccharidases : digest disaccharides into monosaccharides. Disaccharidases Disaccharidases are: Sucrase Sucrase: cleaves sucrose into glucose and fructose Maltase Maltase: cleaves maltose into glucose and glucose Lactase Lactase: cleaves lactose into glucose and galactose Finally, most carbohydrates of diet (polysaccharides & disaccharides) are hydrolysed (digested) into monosaccarides (mainly glucose, fructose & galactose). Only monosaccharides are absorbed ---- to blood Cellulosenot Cellulose of diet (in fibers of vegetables & fruits) are not digested. Digestion of Carbohydrates of Diet

Overview of Digestion of Carbohydrates of Diet

1- From lumen to inside cells: Only monosaccharaides are absorbed in the small intestine - The duodenum and upper jejunum absorb the bulk of the dietary sugars not - Insulin hormone is not required for uptake of glucose by intestinal cells. Galactose glucose i- Galactose & glucose are transported into the mucosal cells by an active energy requiring process that requires the concurrent uptake of sodium ions. The transport protein is the sodium dependent glucose cotransporter-1 Frucose not Ii- Frucose absorption requires a sodium-independent transporter (GLUG-5) for its absorption (energy is not required) 2- From inside cells into blood: Glucose, galactose & fructose Glucose, galactose & fructose are transported from intestinal mucosal cells into the portal circulation (blood) by GLUT-2 transporter. not (energy is not required) Absorption of Monosaccharaides

Disaccharidase Deficiencies Deficiency of a disaccharidase of the intestinal mucosa causes: not 1- The disaccharide is not digested to monosaccharaide. 2- The undigested disaccharide passes into the large intestine 3- In large intestine, disaccharides which are osmotically active draw water from the mucosa into the large intestine lumen osmotic diarrhea causing osmotic diarrhea. 4- Diarrhea is increased by the fermentation of the remaining carbohydrates to two- and three-carbon compounds which are also, osmotically active. 5- Large volumes of CO2 and H2 gases cause abdominal cramps flatulence abdominal cramps and flatulence.

Disaccharides Deficiencies Disaccharides Deficiencies cont. Causes of deficiency of enzymes: 1- Hereditary deficiencies of a disaccharidase deficiency For example: Lactose intolerance: Inability to digest lactose of milk due to deficiency of lactase enzyme So, diarrhea will occur on ingestion of milk or milk products lactose-free milk For infants (up to two years old): are treated by lactose-free milk 2- Intestinal diseases or drugs that injure the mucosa of the small intestine

Digestion of Dietary Proteins Proteins must be digested to yield amino acids which can be absorbed 1- In the Stomach Digestion by gastric secretion The gastric juice contains hydrochloric acid (HCL) & pepsinogen Hydrochloric acid: Hydrochloric acid: - Denatures proteins to make them more susceptible to hydrolysis by enzymes - kills some bacteria. Pepsin Pepsin: inactivepepsinogen - An acid-stable endopeptidase secreted by stomach cells as inactive pepsinogen - Pepsinogen is activated to pepsin either by HCL or by other pepsin molecules - Action of pepsin: digests polypeptides into smaller polypeptides.

Pepsin HCL Secreted inactive (Zymogen) = pepsinogen & needs HCL for starting activation Attacks peptide bonds formed by pH 2

In the Lumen of Small Intestine 2- In the Lumen of Small Intestine Digestion by pancreatic enzymes On entering the small intestine, pancreatic proteases Polypeptides produced in the stomach by the action of pepsin are further cleaved to oligopeptides by pancreatic proteases : Trypsin, chymotrypsin, elastase & carboxypeptidase A&B zymogens These proteases are released from the pancreas as zymogens (inactive forms). Release of zymogenscholecystokininsecretin Release of zymogens is mediated by cholecystokinin & secretin (hormones of GIT) Activation of zymogens enteropeptidase Activation of zymogens is mediated by the enzyme enteropeptidase i.e. Trypsinogen (zymogen) is converted to trypsin (active enzyme). Digestion of Dietary Proteins Digestion of Dietary Proteins cont.

cholecystokinin & Secretin (Hormones from intestinal Mucosa) cholecystokinin & Secretin (Hormones from intestinal Mucosa) Stimulate Pancreatic Secretions of Zymogens of Zymogens Stimulate Pancreatic Secretions of Zymogens of Zymogens Activation of Zymogens by enteropeptidase secreted from intestinal mucosa Activation of Zymogens by enteropeptidase secreted from intestinal mucosa

TrypsinTrypsin Secreted inactive (zymogen) & needs enterokinase for activation Attacks peptide bonds formed by PH 7-8 Enterokinase

ChemotrypsinChemotrypsin Secreted as inactive (zymogen) & needs trypsin for activation Attacks peptide bonds formed by PH 7-8

Elastase Elastase endopeptidase Is an endopeptidase which acts on elastic fibers. It is secreted as an inactive form (proelastase) & activated by trypsin. Elastase Elastase endopeptidase Is an endopeptidase which acts on elastic fibers. It is secreted as an inactive form (proelastase) & activated by trypsin. Carboxypeptidase exopeptidase It is an exopeptidase attacking the peptide chain at its carboxylic end liberating amino acids. secreted as an inactive procarboxypeptidase activated by trypsin. Carboxypeptidase exopeptidase It is an exopeptidase attacking the peptide chain at its carboxylic end liberating amino acids. secreted as an inactive procarboxypeptidase activated by trypsin.

3- On the mucosa of Small Intestine digestion by intestinal aminopeptidases Aminopeptidase Aminopeptidase is available on the luminal surface of the intestine cleaves the N-terminal amino acids from oligopeptides to produce smaller oligopeptide & free amino acids Digestion of Dietary Proteins Digestion of Dietary Proteins cont.

Absorption of amino acids sodium-dependent transport system.Free amino acids are taken up into the small intestinal cells by sodium-dependent transport system. Amino acids are taken via blood to the liver

Abnormalities in Protein Digestion In individuals with a deficiency in pancreatic secretion (for example, due to chronic pancreatitis, cystic fibrosis or surgical removal of the pancreas abnormal increase of lipids (steatorrhea) and undigested protein in the feces Incomplete digestion & absorption of fat & protein