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THE GASTROINTESTINAL SYSTEM: DIGESTION AND ABSORPTION

Irawan Yusuf

Department of Physiology

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INTRODUCTION

The gastrointestinal system is the portal through which nutrition and fluids enter the body.

Proteins, fats, and complex carbohydrate are broken own into absorbable units (digested), principally in the small intestine.

The digested product cross the mucosa and enter the blood or the lymph (absorption)

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INTRODUCTION

Digestion of major nutrient is an orderly process involving the action of large number of digestive enzymes.

Most substances must pass from intestinal lumen into mucosal cells (enterocytes) and then to intestinal fluid and then to the blood and lymph.

The transport of all molecules by diffusion, facilitated diffusion, active transport or others mechanisms.

Mucosal of Small Intestine

DIGESTION AND ABSORPTION OF CARBOHYDRATE

Principal source of calories in the human diet

Types of carbohydrates typically ingested

– Polysaccharides (starches, cellulose)

– disaccharides (lactose, sucrose)

– Monosaccharides (fructose, glucose)

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Digestion of Carbohydrate Salivary and pancreatic enzymes

– Digestion by -amylase begins in the mouth into a variety of oligosaccharides such as maltose, maltotriose, malto-oligosaccharides and -dextrins

Intestinal Brush Border enzymes – isomaltase/-dextrinase debranches -limit dextrin

– glucoamylase cleaves malto-oligosaccharides to glucose

– lactase cleaves lactose into glucose and galactose

– sucrase cleaves sucrose into fructose and glucose

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Absorption of Carbohydrate

Small intestine brush border– Glucose and galactose are absorbed by Na+-

glucose transporter (SGLT)– Fructose is absorbed by glucose transporter

5 (GLUT5)

Basolateral membrane– Glucose, galactose and fructose absorbed by

glucose transporter 2 (GLUT2)

DIGESTION AND ABSORPTION

OF PROTEINS Proteins digested include:

– ingested protein 0.5 - 0.7 gm/day/kg required

– proteins in GI secretions 10-30 gm/day

– proteins from shed GI epithelial cells 10-20 gm/day

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Digestion of Proteins

Enzymes Involved in Protein DigestionsSecreted as zymogens

– gastric – pancreatic

Activated after secretion Exhibit pH optimal

– acidic for gastric – alkaline for pancreatic

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Digestion of Proteins

PROTEIN DIGESTION ENZYMES Gastric

– Pepsin • secreted as pepsinogen

• activated by low gastric pH

• cleaves proteins to peptides and amino acids

Pancreatic – Variety of enzymes

• cleave proteins to peptides

– 85% of protein digestion occurs in small intestine

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Digestion of Proteins

PROTEIN DIGESTION ENZYMES Gastric Pancreatic

– trypsinogen trypsin – chymotrypsinogen chymotrypsin – procarboxypeptidase A carboxypeptidase A – procarboxypeptidase B carboxypeptidase B – proelastase elastase

Brush border peptidases

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Digestion of Proteins

BRUSH BORDER PEPTIDASES Reduce peptides to oligopeptides and amino

acids Present at highest levels in jejunum Three types

– Dipeptidases – Aminopeptidases – Dipeptidyl Aminopeptidases

Principal digestion products – Amino acids and di-, tri- and tetra-peptides

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Absorption of Protein

Brush border transport proteins Amino acid transporters

– Na-dependent – Na-independent

Di- and tri-peptide transporters – H+/dipeptide cotransporter

Basolateral transport proteins – Amino acid transporters

• Na-dependent • Na-independent

DIGESTION AND ABSORPTION

OF LIPIDS Lipids in the diet

– triglycerides (greatest contribution)

– sterols – sterol esters – phospholipids

Water insolubility overcome by – emulsification – formation of micelles

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Digestion of Lipids

Gastric lipase – provides small amount of hydrolysis

Pancreatic lipolytic enzymes – glycerol ester hydrolase (pancreatic lipase) – co-lipase – cholesterol ester hydrolase – phospholipase A

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Absorption of Lipids

Most occurs in duodenum and jejunum Fat digestion products aggregate with bile acids

to form micelles Micelles greatly increase the lipid/aqueous

interface Small size of micelle allows diffusion among

brush border microvilli A halo of lipid digestion products in aqueous

solution surrounds the micelle

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Absorption of Lipids

Most lipid digestion products readily diffuse across the brush border membrane – Fatty acids – 2-monoglycerides – Cholesterol – Lysophoosphatide

Microvillus membrane fatty acid-binding protein – A Na-dependent cotransporter – Transports long chain fatty acids and cholesterol

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Absorption of Lipids

Cytosolic proteins prevent lipids from coalescing into droplets – Fatty acid binding proteins

• I-FABP – long chain fatty acids

• L-FABP – cholesterol, lysophosphatides and monoglycerides

• Sterol carrier proteins – SCP-1 and SCP-2

• bind cholesterol and other sterols

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Absorption of Lipids

Smooth ER reprocesses lipid digestion products and reforms: – triglycerides – phospholipids – cholesterol esters

Prechylomicrons – form in SER as resynthesized lipids accumulate

Mature chylomicrons – form in Golgi and are exocytosed basolaterally – taken up by central lacteal