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) NOTDIGESTED
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.
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.
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.
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)
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.
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
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.
Pepsin Secreted inactive (Zymogen) = pepsinogen & needs HCL for starting activation HCL pH 2 Attacks peptide bonds formed by
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).
cholecystokinin & Secretin (Hormones from intestinal Mucosa) Stimulate Pancreatic Secretions of Zymogens Activation of Zymogens by enteropeptidase secreted from intestinal mucosa
Trypsin Secreted inactive (zymogen) & needs enterokinasefor activation PH 7-8 Enterokinase Attacks peptide bonds formed by
Chemotrypsin Secreted as inactive (zymogen) & needs trypsin for activation PH 7-8 Attacks peptide bonds formed by
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.
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
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
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