Published on February 27, 2014
When I prayed for success, I forgot to ask for sound sleep and good digestion. -Mason Cooley
Happiness: a good bank account, a good cook, and a good digestion. #Jean-Jacques Rousseau
Digestion and Absorption of Food -Dr. Ganesh
Digestion and Absorption of Food Contents: Biological Importance Medical Importance Digestion and Absorption – General Aspects Digestion and Absorption of Carbohydrates Digestion and Absorption of Proteins Digestion and Absorption of Lipids
Biological Importance Digestion a chemical process – large molecules in the food are broken down to smaller molecules so that they can be absorbed
Biological Importance Food large molecules Digestion small molecules Absorption BLOOD small molecules vitamins, minerals, monosaccharides and free amino acids
Biological Importance • Absorption is transport of molecules, either digested products or other small molecules which do not require digestion, from the intestinal lumen into blood across the intestinal mucosal cells.
Medical Importance • Lactose intolerance deficiency of lactase intake of milk causes diarrhea • Hartnup’s disease genetic defect in the absorption of neutral amino acids, especially tryptophan.
Medical Importance • Steatorrhea excess fat is excreted in feces and is seen in diseases of pancreas, biliary obstruction, etc. • Chronic diarrhea can cause malabsorption as seen in celiac disease, Sprue, Crohn’s disease, etc.
Digestion – General Aspects major foodstuffs that require digestion macromolecules digestion smaller molecules carbohydrates, monosaccharides proteins Amino acids fats and oils (triacylglycerol) glycerol and fatty acids
Digestion – General Aspects Digestion takes place in the aqueous medium of various digestive juices – Site of Digestion Digestive Juices Mouth Saliva Stomach Gastric juice Small intestinal lumen Pancreatic juice, Bile Intestinal juice
Digestion – General Aspects • Digestion involves action of enzymes that are present in different digestive juices. • All digestive enzymes are hydrolases that hydrolyze the anhydride linkages –
Digestion – General Aspects anhydride linkages carbohydrates, Glycosidic linkage proteins Peptide linkage fats and oils (triacylglycerol) Ester linkage
Digestion – General Aspects • Bile, synthesized in liver and entering into duodenum, • helps in fat digestion and • neutralizes acidic stomach contents when it enters the duodenum.
Digestion – General Aspects • Cooking hydration of polysaccharides and denaturation of proteins helps digestion of these molecules. • Mastication helps in breaking down of food particles increases solubility and surface area for enzyme action. • Peristalsis also important in breaking down of food particles and mixing them with enzymes.
Absorption– General Aspects • small intestine main absorptive organ. About 90% of the ingested foodstuffs absorbed through the small intestine • Considerably more water is absorbed in the large intestine, so that the contents, gradually become more solid in the colon.
Absorption – General Aspects Absorption of substances into mucosal cells involves the passage across the plasma membrane simple diffusion carrier-mediated transports passive facilitated transport (no energy expenditure) (passive) No carrier protein active transport (requiring energy expenditure)
Absorption – General Aspects two pathways for the transport of nutrients absorbed by the intestine water-soluble nutrients hepatic portal blood liver lipid-soluble nutrients lymphatic vessels thoracic duct blood blood
Digestion and Absorption of Carbohydrates Contents: • Introduction • Digestion of Starch • Digestion of Disaccharides • Absorption of Carbohydrates • Clinical Significance
Digestion and Absorption of Carbohydrates Introduction major carbohydrates in the diet Monosaccharides fructose (present in fruits) Disaccharides sucrose lactose maltose Small amounts (present in malt, beer) Polysaccharides starch dietary fibers Starch - more than 50% of carbohydrates
Digestion and Absorption of Carbohydrates - Introduction • Dietary fibers – cellulose, hemicellulose, pectin, lignins, etc. are indigestible. For ‘Dietary fibers’, see Chapters – ‘Chemistry of Carbohydrates’ and ‘Nutrition’)
Digestion and Absorption of Carbohydrates - Introduction Human food also contains small amounts of • pentoses, • glucose, • trehalose (disaccharide present in mushroom), and • glycogen (present in liver in animal foods).
Digestion and Absorption of Carbohydrates - Introduction • All enzymes of carbohydrate digestion cleave glycosidic bonds by hydrolysis. • Disaccharides and polysaccharides are digested to their respective constituent monosaccharides units and absorbed in the small intestines along with free monosaccharides present in the food
Digestion and Absorption of Carbohydrates - Introduction • The absorbed monosaccharides hepatic portal circulation liver cells systemic circulation.
Digestion and Absorption of Carbohydrates Introduction Fructose DIGESTION ABSORPTION (Small Intestine) Fructose Sucrose FOOD Starch Glucose Lactose Galactose
Digestion of Starch1, 2 • Starch on complete digestion yields glucose • Digestion of starch takes place in • mouth, • small intestinal lumen and • small intestinal brush border (luminal surface of intestinal mucosal cells).
Digestion of Starch1, 2 • The enzymes and the steps of digestion of glycogen are same as those of amylopectin component of starch as both have similar structure. • Cooking hydrates the starch granules making it more susceptible to digestion.
Digestion of Starch1, 2 Enzymes required for complete digestion of starch – • amylase (salivary and pancreatic amylase), • maltase and • isomaltase. • Amylase and maltase cleave -1,4 glycosidic linkages and isomaltase cleaves -1,6 glycosidic linkages of starch. (Amylase hydrolyzes internal -1,4 glycosidic linkages.)
Digestion of Starch • Starch is mainly digested by pancreatic amylase. Contribution by salivary amylase is very little since the food remains in the mouth for a very short period of time and the enzyme is inactivated by gastric HCl as it enters stomach. • Maltase and isomaltase are present on the luminal surface of small intestinal epithelial cells (brush border cells).
Digestion of Starch Starch (amylose and amylopectin) Salivary amylase (in mouth)3 Or Pancreatic amylase (in small intestinal lumen Small unbranched oligosaccharides (e.g., Maltose, Maltotriose, etc) Isomaltase Maltase (brush border cells) (brush border cells) Limit dextrins4 Maltase Isomaltose Glucose Isomaltase (brush border cells)
Digestion of Starch • Cl- is an activator of Salivary amylase (ptyalin) • Limit dextrins derived from amylopectin component by the action of amylase and contain eight glucosyl units with one or two branches with α-1,6-glycosidic bonds.
Digestion of Disaccharides Major disaccharides present in the human diet are – • sucrose and • lactose Small amounts of • free maltose (present in malt, beer) and • trehalose (disaccharide present in mushroom)
Digestion of Disaccharides • However, quantitatively the major disaccharide digested in the gut is maltose • Most of the maltose in the gut is derived from digestion of starch. • Isomaltose is another disaccharide derived from starch and is digested by isomaltase. (See ‘Digestion of Starch’).
Digestion of Disaccharides • Disaccharidases are attached to the surface of the small intestinal brush border cells. • (For ‘digestion of maltose’, see ‘Digestion of Starch’.)
Digestion of Disaccharides Site of digestion of disaccharides – attached to the small intestinal brush border surface of the small intestinal brush border cells disaccharides disaccharidases sucrose sucrase (also called invertase) lactose lactase trehalose trehalase maltose maltase
Digestion of Disaccharides sucrase sucrose glucose + fructose lactase lactose glucose + galactose trehalase trehalose 2 glucose maltase 2 glucose maltose isomaltase Isomaltose 2 glucose
Absorption of Monosaccharides The major monosaccharides resulting from carbohydrate digestion are – • D-glucose, • D-galactose and • D-fructose. Absorption is carrier mediated. • Pentoses are absorbed by simple diffusion. • Monosaccharides are first transported from the lumen to the small intestinal epithelial cells and then into capillaries of portal venous system.
Absorption of Glucose from the small intestinal lumen into the intestinal epithelial cells by carrier mediated mechanism involving transporter proteins situated on the luminal surface of intestinal epithelial cells. • Glucose is absorbed mainly by • 1) Na+-dependent transporter by secondary active transport and to a less extent by • 2) Na+-independent transporter by passive transport.
Absorption of Glucose from the small intestinal lumen by carrier mediated mechanism involving transporter proteins 1) Na+-dependent transporter by secondary active transport and to a less extent by 2) Na+-independent transporter by passive transport into the intestinal epithelial cells
1. Na+-dependent transporter (SGLT) • This carrier protein carries glucose or galactose along with sodium ion from the lumen. The driving force for the Na+-dependent transport is derived from the maintenance of low intracellular levels of Na+ by the action of the Na+-K+ATPase (secondary active transport).
2) Na+-independent transporter Small amounts of glucose, transported by facilitated transport utilizing glucose transporter-5 (GLUT-5).
Transport of glucose from cells to portal venous capillaries: Glucose is transported from the intestinal epithelial cells into portal venous capillaries by glucose transporter-2 (GLUT-2). • It is a uniport facilitated transport system, which is sodium independent.
Absorption of Glucose facilitated transport Intestinal Epithelial Cell Glucose GLUT-5 Glucose secondary active transport Na+ Intestinal Lumen Na+-dependent transporter (SGLT) Na+ K+ ATP ADP + Pi GLUT-2 Glucose Na+ Portal Capillary Blood K+ Na+–K+ ATPase
Absorption of Glucose Fructose Galactose GLUT-5 Fructose Galactose Na+ Na+-dependent transporter (SGLT) Na+ K+ ATP ADP + Pi GLUT-2 Galactose Fructose Na+ K+ Na+–K+ ATPase
Absorption of Other Monosaccharides • Fructose facilitated trasporter GLUT-5, sharing with glucose. • Galactose Na+-dependant trasporter (SGLT) secodary active transport sharing with glucose • Any pentose present in food is absorbed by simple diffusion.
Absorption of Other Monosaccharides • Both fructose and galactose transported from the intestinal epithelial cells into portal venous capillaries by glucose transporter-2 (GLUT-2), sharing with glucose.
Clinical Significance Lactose Intolerance • This is a common condition gastrointestinal symptoms like diarrhea, abdominal cramps and flatulence after ingestion of milk or milk-based foods
Clinical Significance Lactose Intolerance • Cause deficiency of lactase1. Deficiency may be due to • genetic (primary/inherited) or • acquired (secondaryto other causes) The reason for acquired lactose intolerance may be damage to intestinal epithelial cells due to colitis, gastroenteritis, alcohol consumption or sudden change into a milk-based diet.
Clinical Significance Lactose Intolerance • Cause lactase enzyme defective at birth early onset lactose intolerance (inherited lactase deficiency). A significant number of adults exhibit late onset lactase deficiency (primary low lactase activity) especially Asian-, Native- and African-Americans.
Biochemical basis of Clinical Manifestation Lactase deficiency Accumulation of lactose, organic acids and gases (CO2 and H2) (Produced by action of bacteria on lactose in the gut) Osmotic movement of water from the intestines to the lumen flatulence Abdominal cramps diarrhea
Lactose Intolerance Treatment • avoiding milk • Curd is an effective treatment, because Lactobacilli in curd contains the enzyme Lactase
Digestion and Absorption of Proteins Contents: • Digestion of protein – general aspects • Reactions of protein digestion • Absorption of amino acids • Clinical significance
Digestion of Proteins – General Aspects • • • • • • Contents: Introduction Sites of protein digestion, GIT juices and proteases Endopetidases and exopetidases Specificity of proteases Proteases - zymogen form and activation Role of HCl in protein digestion
Digestion of Proteins – General Aspects Introduction • Digestion of dietary proteins – hydrolysis of peptide bonds catalyzed by a group of enzymes called proteases or peptidases in the gastrointestinal tract • • Complete digestion of proteins yields amino acids.
Digestion of Proteins – General Aspects Introduction • Dietary proteins denatured on cooking and therefore, cooked proteins more easily digested1. During the process of denaturation unfolding of protein molecule takes place and thus peptide bonds become more accessible for enzyme action.
Digestion of Proteins – General Aspects Sites of Protein Digestion, GIT Juices and Proteases Protein digestion takes place in • stomach and • intestinal lumen. Enzymes of protein digestion are secreted in • gastric juice, • pancreatic juice and • intestinal juice.
Digestion of Proteins – General Aspects GIT Juices and Proteases GIT Juices Proteases Present Gastric juice Pepsin (chief cells of stomach), Rennin2 Pancreatic Juice Trypsin Chymotrypsin Elastase Carboxypeptidases Intestinal Juice Aminopeptidases Dipeptidases Tripeptidases
Rennin • Rennin, a protease, active in infants and involved in curdling of milk. Rennin denatures casein of milk to paracasein irreversibly, which then is acted upon by pepsin.
Endopeptidases and Exopeptidases Pepsin Trypsin Carboxypeptidases carboxy terminal Chymotrypsin Elastase Aminopeptidases amino terminal hydrolyze peptide bonds in the interior of the protein chain to cleave the protein molecule into more than one smaller polypeptides and peptides. hydrolyze terminal bond releasing one amino acid at a time.
Digestion of Proteins – General Aspects Specificity of Proteases • Endopeptidases hydrolyze specific peptide bonds in protein molecules. Specificity differs from one protease to another3.
Specificity of Proteases Enzyme Hydrolysis of petide bonds formed by carboxyl groups of Pepsin Phe, Tyr, Trp, Met Trypsin Arg, Lys (basic amino acids) Chymotrypsin Phe, Tyr, Trp, Val, Leu (Aromatic, uncharged amino acids) Elastase Ala, Gly, Ser (small amino acid residues)
Digestion of Proteins – General Aspects Proteases – Zymogen Form and Its Activation Proteolytic enzymes are secreted as inactive zymogens/proenzymes, which are converted to their active form in the intestinal lumen4.
Digestion of Proteins – General Aspects Proteases – Zymogen Form and Its Activation Activation of enzymes involves cleavage of small peptides so that active sites are exposed. prevents auto-digestion of the secretory acini.
Digestion of Proteins – General Aspects Proteases – Zymogen Form and Its Activation Active Enzyme Inactive Zymogen Pepsin Pepsinogen Trypsin Trypsinogen Chymotrypsin Carboxypeptidase Elastase Chymotrypsinogen Procarboxypeptidase Proelastase
Digestion of Proteins – General Aspects Proteases – Zymogen Form and Its Activation (Parietal cells of stomach) Pepsinogen HCl Pepsin Autoactivation (a protease present on the intestinal Enterokinase mucosal membranes) Trypsinogen Trypsin Autoactivation
Digestion of Proteins – General Aspects Proteases – Zymogen Form and Its Activation Trypsin Chymotrypsinogen Proelastase Procarboxypeptidase Chymotrypsin Elastase Carboxypeptidase
Digestion of Proteins – General Aspects Role of HCl in Protein Digestion 1) Activation of pepsinogen 2) Denaturation of dietary proteins and 3) Providing optimum pH (pH 2 to 3) for the action of pepsin. • (HCl also kills microorganisms present in food).
Reactions of Protein Digestion (In Stomach and Small Intestinal Lumen) Dietary Proteins In Stomach Pepsin HCl Gastric Juice (polypeptides and Proteoses + Peptones smaller polypeptides) In Small Intestinal Lumen Trypsin Chymotrypsin Elastase Small Polypeptides + Peptides Pancreatic Juice
Dietary Proteins In Stomach Pepsin HCl Gastric Juice (polypeptides and Proteoses + Peptones smaller polypeptides) In Small Intestinal Lumen Trypsin Chymotrypsin Elastase Small Polypeptides+ Peptides Pancreatic Juice
HCl (polypeptides and Proteoses + Peptones smaller polypeptides) In Small Intestinal Lumen Trypsin Chymotrypsin Pancreatic Juice Elastase Small Polypeptides+ Peptides In Small Intestinal Lumen Carboxypeptidases Pancreatic Juice Aminopeptidases Dipeptidases Amino acids Intestinal Juice
Absorption of Amino Acids absorbed from the intestine into portal blood. • transported by a number of carriers many by secondary active transport – Na+-dependent carriers similar to glucose transporter system.
Absorption of Amino Acids Different Na+-dependent carriers are: • Neutral amino acid carrier • Phenylalanine and methionine carrier • Carrier specific for imino acids (proline and hydroxy proline) There are also Na+-independent carriers specializing in the transport of • Neutral and lipophilic amino acids (e.g. Phe, Leu) • Cationic amino acids (e.g. Lys)
Clinical Significance • Allergy to certain food proteins (milk, fish) believed to result from absorption of partially digested proteins.
Clinical Significance • Defect in non-tropical sprue is located within the mucosal cells of the intestine and permits the polypeptides (resulting from the peptic and tryptic digestion of gluten, the principal protein of wheat) to be absorbed into the circulation and thus elicit the production of antibodies.
Clinical Significance • Defect in the intestinal amino acid transport systems seen in -- Hartnup’s disease, [defect in intestinal neutral amino acid (Trp) carrier], I -- iminoglycinuria, cystinuria etc.
Clinical Significance • Acute pancreatitis (acute inflammation of pancreas) caused by autodigestion of pancreas by its proteolytic enzymes a life threatening disorder. Autodigestion is due to unusual conversion of proenzymes into active enzymes by trypsin.
Digestion of Fat (Triacylglycerols) Contents • Digestion of fat • Digestion of other lipids • Absorption of lipids • Clinical significance
Digestion of Fat (Triacylglycerols) main site Small intestinal lumen Hydrolysis of ester bonds Fat Lipases Fatty acids Glycerol Monoacyl glycerols (MAGs) main enzyme for digestion of most of the fat - Pancreatic lipase other lipases lingual lipase, gastric their contribution is negligible lipase and intestinal lipase Colipase cofactor a proteinsalts Bile secreted by pancreas digestion by emulsifying fat help fat
Role of Bile Salts in Fat Digestion Bile salts present in the bile lower the surface tension – emulsify fat in the intestine. – Intestinal peristalsis also helps in this. Emulsification increases the surface area of fat droplets enabling more enzyme (lipase) molecules to act and thus speeding up digestion
Reactions of Digestion of Fat (Triacylglycerol)1 Triacylglycerol (Fat) Lipase Pancreatic Juice Colipase Fatty Acid Diacylglycerol (DAG) Lipase Colipase Fatty Acid Monoacylglycerol (MAG) Lipase Colipase Fatty Acid Glycerol
Reactions of Digestion of Fat (Triacylglycerol)1 Digestion of fat requires another enzyme also an isomerase which isomerzes 2-MAG into 1-MAG, as lipase cannot hydrolyze 2-MAG.
Reactions of Digestion of Fat (Triacylglycerol)1 The major end products of digestion of fat are – • monoacylglycerols (MAGs), • glycerol and • fatty acids
Digestion of Other Lipids Pancreatic secretion also contains cholesterol esterase cholesterol ester cholesterol Fatty acid phospholipase A2 phospholipid lysophospholipid Fatty acid
Absorption of Digested Products of Lipids Digested Products of Lipids Water-insoluble Water-soluble Monoacylglycerols Glycerol Long-chain fatty acids Micelles Short- and Medium-chain Fat-soluble fatty acids(SMCFA) Cholesterol vitamins Mixed micelles SMALL INTESTINAL LUMEN Intestinal Epithelial Cell Monoacylglycerols Long-chain fatty acids Cholesterol Fat-soluble Triacylglycerol vitamins Phospholipids Apolipoproteins Chylomicron LYMPHATIC VESSELS Chylomicron Glycerol (SMCFA) PORTAL BLOOD
Absorption of Digested Products of Lipids • Normally over 98% of the dietary lipid is absorbed.
Clinical Significance • Steatorrhea • Chyluria and Chylothorax When daily excretion of fat in feces more than 6g per day • may be due to defective digestion or defective absorption of fat. • - Defective digestion may be due to absence of or deficiency of pancreatic lipase as in chronic diseases of pancreas or surgical removal of pancreas. bile salt
Clinical Significance • Steatorrhea • Defective absorption of fat occurs when bile salts do not enter the intestine as in biliary obstruction (e.g. due to biliary stone) • - Defective absorption may also be due to malabsorptive diseases e.g. celiac disease, Sprue, Crohn’s disease, etc or surgical removal of large lengths of the intestine.
MCQS on Digestion and Absorption 1. All of the below are true about digestion, EXCEPT, A. All digestive enzymes are hydrolases. B. Digestion ensures the absorption of nutrients. C. Sites of digestion in the GIT are mouth, stomach, and the lumen of small and the large intestines. D. Vitamins, minerals, monosaccharides and free amino acids do not need digestion.
2. The anhydride linkages that are broken during digestion are: A.glycosidic, peptide and ester linkages carbohydrates, proteins and fats, respectively. of B.ester, peptide and glycosidic linkages carbohydrates, proteins and fats, respectively. of C.glycosidic, peptide and ester linkages carbohydrates, fats and proteins, respectively. of D.peptide, glycosidic and ester linkages carbohydrates, proteins and fats, respectively. of
3. Factors, other than enzymes that help in digestion are the following, EXCEPT, A.Cooking B.Exercise C.Mastication D.Peristalsis
4. All the following are true about absorption, EXCEPT, A.Considerably more water is absorbed in the large intestine than in the small intestines. B.Absorption of all substances require carriermediated transports. C.Water-soluble nutrients are absorbed through hepatic portal system. D.Lipid-soluble nutrients are absorbed through the lymphatic vessels.
5. The full set of enzymes required for complete digestion of starch are – A.amylase, maltase and isomaltase. B.amylase, maltase and sucrase. C.amylase, maltase, isomaltase and sucrase. D.amylase, sucrase and isomaltase. •
6. Contribution by salivary amylase in digestion of starch is very little because: A.activity of the enzyme is very low. B.the food remains in the mouth for a very short period of time. C.the food remains in the mouth for a very short period of time and the enzyme is inactivated by gastric HCl. D.the enzyme is inactivated by gastric HCl.
7. Which of the following is FALSE about digestion of starch? A.The enzymes and the steps of digestion of glycogen are same as those of amylopectin component of starch. B.Cooking hydrates the starch granules making it more susceptible to digestion. C.Cl- is an activator of salivary amylase. D.Limit dextrins are derived from amylose component of starch by the action of amylase.
8. Quantitatively the major disaccharide digested in the gut is A.sucrose B.maltose C.lactose D.isomaltose
9. The major monosaccharides resulting from carbohydrate digestion are A.glucose, galactose and ribose. B.glucose, mannose and fructose. C.glucose, ribose and fructose. D.glucose, galactose and fructose.
10. Glucose is absorbed mainly by A.simple diffusion B.Na+-dependent transporter C.Na+-independent transporter D.passive transport •
11. Enzymes of protein digestion are secreted in A.gastric juice, pancreatic juice and intestinal juice. B.saliva, gastric juice and pancreatic juice. C.saliva, gastric juice and intestinal juice. D.saliva, gastric juice and intestinal juice.
12. Proteases secreted in the pancreatic juice are A.pepsin, trypsin, chymotrypsin carboxypeptidases. B.trypsin, chymotrypsin, elastase carboxypeptidases. C.trypsin, chymotrypsin, elastase aminopeptidases. D.trypsin, chymotrypsin, aminopeptidases carboxypeptidases. and and and and
13. HCl has following roles in protein digestion, EXCEPT. A.kills microorganisms present in food B.activation of pepsinogen C.denaturation of food proteins D.providing optimum pH for the action of pepsin
14. Intestinal absorption of amino acids involves all of the following, EXCEPT, A.simple diffusion B.Na+-dependent transporter C.Na+-independent transporter D.passive transport
15. Which of the following is false about action of bile salts? A.They help both in digestion of fat and absorption of digested products of lipids. B.They lower the surface tension and emulsify fat in the intestine. C.They stimulate pancreatic secretion. D.Emulsification increases the surface area of the fat droplets.
16. Digestion of fat produces: A.fatty acids, glycerol and diacyl glycerols. B.fatty acids and glycerol. C.glycerol and monoacyl glycerols. D.fatty acids, glycerol and monoacyl glycerols.
17. Lipases other than pancreatic lipase present in the human GIT are: A.salivary lipase, gastric lipase and intestinal lipase B.lingual lipase, gastric lipase and intestinal lipase C.lingual lipase, gastric lipase and hepatic lipase D.gastric lipase, biliary lipase and intestinal lipase
18. Steatorrhea is a condition when daily excretion of fat in feces is more than: A.6g B.8g C.10g D.12g
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