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D igestion of Dietary Carbohydrates

D igestion of Dietary Carbohydrates. M ain C arbohydrates of Diet. 1- M onosaccharides : mainly glucose & fructose ABORBED with NO DIGESTION 2- D isaccharides : S ucrose , lactose & maltose DIGESTED into monosaccharides 3- P olysaccharides :

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D igestion of Dietary Carbohydrates

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  1. Digestion of Dietary Carbohydrates

  2. 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) NOTDIGESTED

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

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

  5. 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: digest oligosaccharides into disaccharides & monosaccharaides. 2- intestinal disaccharidases :digest disaccharides into monosaccharides. Disaccharidasesare: Sucrase: cleaves sucrose into glucose and fructose Maltase: cleaves maltose into glucose and glucose 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 Cellulose of diet (in fibers of vegetables & fruits) are notdigested.

  6. Overview of Digestion of Carbohydrates of Diet

  7. Absorption of Monosaccharaides 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 - Insulin hormone is not required for uptake of glucose by intestinal cells. i- Galactose& glucoseare 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 Ii- Frucoseabsorption requires a sodium-independent transporter (GLUG-5) for its absorption (energy is not required) 2- From inside cells into blood: Glucose, galactose & fructose are transported from intestinal mucosal cells into the portal circulation (blood) by GLUT-2 transporter. (energy is not required)

  8. DisaccharidaseDeficiencies Deficiency of a disaccharidase of the intestinal mucosa causes: 1- The disaccharide is not digested to monosaccharaide. 2- The undigested disaccharide passesinto the large intestine 3- In large intestine, disaccharides which are osmoticallyactive draw water from the mucosa into the large intestine lumen causing osmotic diarrhea. 4- Diarrheais 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 and flatulence.

  9. 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 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

  10. 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: - Denatures proteins to make them more susceptible to hydrolysis by enzymes - kills some bacteria. • Pepsin: - An acid-stable endopeptidase secreted by stomach cells as inactivepepsinogen - Pepsinogen is activated to pepsin either by HCL or by other pepsin molecules - Action of pepsin: digests polypeptides into smaller polypeptides.

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

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

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

  14. Trypsin Secreted inactive (zymogen) & needs enterokinasefor activation PH 7-8 Enterokinase Attacks peptide bonds formed by

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

  16. Elastase Is an endopeptidasewhich acts on elastic fibers. It is secreted as an inactive form (proelastase) & activated by trypsin. Carboxypeptidase It is an exopeptidaseattacking the peptide chain at its carboxylic end liberating amino acids. secreted as an inactive procarboxypeptidaseactivated by trypsin.

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

  18. Absorption of amino acids • 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

  19. 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 Incomplete digestion & absorption of fat & protein abnormal increase of lipids (steatorrhea) and undigested protein in the feces

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