Digestive and absorptive functions

of the gastrointestinal

system

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ObjectivesObjectives

Describe the mechanical and chemical digestion Describe the mechanical and chemical digestion of the stomach.of the stomach.

Describe the digestion and absorption of Describe the digestion and absorption of carbohydrates.carbohydrates.

Describe the digestion and absorption of protein.Describe the digestion and absorption of protein.

Describe the digestion and absorption of lipids.Describe the digestion and absorption of lipids.

Describe the absorption of electrolytes, and Describe the absorption of electrolytes, and water-soluble vitamins, including the role of water-soluble vitamins, including the role of intrinsic factor in the absorption of vitamin B12.intrinsic factor in the absorption of vitamin B12.

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CarbohydratesFatsProteins+ vitamins, minerals

Primary foods on which the body Primary foods on which the body lives:lives:

Cannot be absorbed in their natural forms—must be digested

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Digestion of CarbohydratesDigestion of Carbohydrates

3 major sources of carbohydrate in food3 major sources of carbohydrate in food Sucrose----- Sucrose----- disaccharidedisaccharide, cane sugar, cane sugar Lactose------Lactose------disaccharidedisaccharide, milk, milk Starch--------Starch--------polysaccharidepolysaccharide, non-animal food eg. Potato and grains, non-animal food eg. Potato and grainsCelluloseCellulose

Essentially all the carbohydrates in the food are absorbed as monosaccharide and very small amount as disaccharide

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Digestion of CHO in mouth and stomachSalivary amylasehydrolyses starch into maltose

(trisaccharide) and glucose polymersIn the stomach continue to digest CHO

before mixing with gastric juiceAccounts for 20-40% of CHO digestion

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Digestion of CHO in small intestineDigestion by pancreatic amylase Very powerful amylase, virtually all the CHO will be digested into

maltose and very small glucose polymers

Digestion by Intestinal Epithelial enzymes Intestinal microvilli brush border contain enzymes that split

disaccharides and glucose polymers into monosaccharide. Glucose represents more than 80% of the final products of CHO

digestion Lactose splits into glucose and galactose by lactase enzyme Sucrose splits into fructose and glucose by sucrase enzyme Maltose and glucose polymers splits into glucose by maltase

and dextrinase enzymes

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Dietary proteins are chemically long chains of amino acids bound by peptide linkages

Protein digestion in the stomach Pepsin (Recall from previous

lecture) Pepsin important in digesting

collagen. why? 10-20% of total protein

digestion

Digestion of ProteinsDigestion of ProteinsProtein digestion in small intestineProtein digestion by pancreatic enzymesMost of the protein digestion results from action of pancreatic enzymes Trypsin, chemotrypsin, carboxylpolypeptide, elastaseProtein digestion by small intestine enzymesDigestion at the brush border of the enterocytesAminoplpypeptidase and dipeptidaseDigestion inside the enterocytesContain multiple peptidases77

The most abundant fat in the food is The most abundant fat in the food is triglyceridestriglycerides

Triglycerides composed of Triglycerides composed of glycerol and 3 glycerol and 3 fatty acid moleculesfatty acid molecules

Digestion by Digestion by salivary lipasesalivary lipase and and stomach stomach lipaselipase is limited (<10%)and generally is limited (<10%)and generally

unimportantunimportantEssentially all fat digestion occur in small Essentially all fat digestion occur in small

intestineintestine

Digestion of lipids (fat)Digestion of lipids (fat)

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First step in fat digestion is emulsification of fat with bile acids and First step in fat digestion is emulsification of fat with bile acids and lecithin in the duodenum.lecithin in the duodenum.

Lipase enzyme is water soluble and can attack fat globules on their Lipase enzyme is water soluble and can attack fat globules on their surfaces.surfaces.

Digestion of lipids (fat)Digestion of lipids (fat)

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Digestion of lipids (triglycerides) by pancreatic Digestion of lipids (triglycerides) by pancreatic lipaselipase

The most important enzyme in triglycerides The most important enzyme in triglycerides digestion is the digestion is the pancreatic lipase enzyme.pancreatic lipase enzyme.

Triglycerides will be digested into free fatty acids Triglycerides will be digested into free fatty acids (long and short chain) and 2 monglycerides(long and short chain) and 2 monglycerides

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Basic principles of gastrointestinal absorption

Small intestine is the major area of Small intestine is the major area of absorption (90%)absorption (90%)

Stomach is a poor absorptive area (highly Stomach is a poor absorptive area (highly lipid soluble substance and aspirin)lipid soluble substance and aspirin)

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Essentially all CHO are absorbed in the form of Essentially all CHO are absorbed in the form of monosaccharidesmonosaccharides

80% glucose80% glucose20% fructose and galactose20% fructose and galactose

All monosaccharides absorbed by All monosaccharides absorbed by active transport.active transport.Glucose transported by sodium co-transport mechanismGlucose transported by sodium co-transport mechanismGalactose similar to glucoseGalactose similar to glucoseFructose transported by facilitated diffusionFructose transported by facilitated diffusion

Absorption of CarbohydratesAbsorption of Carbohydrates

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Most proteins are absorbed through the Most proteins are absorbed through the luminal membrane of the intestinal epithelial luminal membrane of the intestinal epithelial cells in the form of dipeptide, tripeptide and cells in the form of dipeptide, tripeptide and amino acidsamino acids

Transported by Transported by active transport.active transport.

Absorption of ProteinsAbsorption of Proteins

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Absorption of lipids All dietary lipids absorbed by simple diffusionShort chain free fatty acids

Hydrophobic and very small sizePass through enterocytes by simple diffusion then to

bloodLong chain free fatty acids and monoglycerides

Hydrophobic, large size Form micelles with bile salts (Micelles are composed of central fat globule (containing monoglycerides and long chain free fatty acids) with molecules of bile salts projecting outward to cover the surface of micelle)

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Micelle carry monoglycerides and free fatty acids to enterocytes brush border.Monoglycerides and fatty acids diffuse out of micelle into the enterocyte.Inside enterocyte, monoglyceride and fatty acids are recombined to form triglycerides along with protein, phospholipids and cholesterol to form chylomicrons.Chylomicrones leave the enterocytes via exocytosis.Chylomicrones enter lymph then to the blood

Absorption of lipids cont..

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Blood

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Absorption of waterAbsorption of water

Water absorption is entirely by simple diffusion (osmosis)

Water can move across intestinal membrane in both directions to maintain osmotic equilibrium with blood

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Absorption of Absorption of ElectrolytesElectrolytes

Sodium is actively transported Sodium is actively transported

Negative charged ions chloride, iodide, Negative charged ions chloride, iodide, bicarbonate and nitrate passively follow bicarbonate and nitrate passively follow sodium absorption.sodium absorption.

Calcium actively transported in a process Calcium actively transported in a process stimulated by vitamin D.stimulated by vitamin D.

Iron is actively transported.Iron is actively transported.

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Absorption of VitaminsAbsorption of Vitamins

Fat soluble vitamins (A, D, E, K)Fat soluble vitamins (A, D, E, K)Included with ingested dietary lipids in the Included with ingested dietary lipids in the

micelles and absorbed via simple diffusionmicelles and absorbed via simple diffusion

Water soluble vitamins and vitamin CWater soluble vitamins and vitamin CAbsorbed via simple diffusionAbsorbed via simple diffusion

Vitamin B12 combined with intrinsic factor Vitamin B12 combined with intrinsic factor secreted by stomach and then absorbed in secreted by stomach and then absorbed in the ileum via active transport mechanismthe ileum via active transport mechanism

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Large IntestineLarge Intestine

Absorption of water and

electrolytes

Storage of fecal matter

2020

• Contains no Villi: all the work of digestion and absorption is already accomplished in the small intestine.

• Secretes mucus: to protect epithelial cells from excoriation and to provide adherent media for holding feces together.

• Secrets HCO3-: to neutralize the acid secreted by colonic

bacteria.

• Its bacteria synthesize Vit K: what we eat is never enough.

• Absorbs fluid: The proximal part. The longer feces remain the harder they become.

• Stores feces: in the distal part until the conditions are right to defecate

Large IntestineLarge Intestine

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Constituents of FecesConstituents of Feces

Normally 75% water and 25% solid

Indigestible material

Some inorganic material (e.g. calcium and phosphate)

Bacteria

Stercobilin and urobilin → give the color of feces

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