Digestion and Absorption in the GI Tract

The major foods can be classified into carbohydrates, fats and proteins

Digestion: The enzymatic breakdown of food to components that can be absorbed

Absorption: The movement of nutrients, salts and water across the GI epithelium into blood or lymph

Digestion and Absorption in the GI Tract

Most digestion and absorption occurs in the small intestine

The pancreas supplies the digestive enzymes for most foodstuffs

Human lacks of digestive enzymes for plant polysaccharides cellulose

The indigestible polysaccharides contain in vegetables and fruits make up dietary fibre

Digestion of various foods by hydrolysis

Monosaccharides, fatty acids and amino acids are bound to one another by condensation

The process of hydrolysis is involved in the digestion of carbohydrates, fats and proteins

Digestion and absorption of Carbohydrates

The most abundant dietary carbohydrates are starch and the three disaccharides sucrose, lactose and maltose

Carbohydrates are built from monosaccharides and must be digested to their component monosaccharides so they can be absorbed

Starch are digested to glucose Sucrose is digested to glucose and fructose Lactose is digested to glucose and galactose Maltose is digested to glucose

Digestion and absorption of Carbohydrates

Plant starch and glycogen are long polymers of glucose

Digestion of carbohydrates begins in the mouth with salivary amylase

Amylase break starch down to disaccharides maltose; fragments of three glucose molecules (Maltotriose); and to small branched fragments (limit dextrin)

Acid in the stomach denatures amylase

Digestion and Absorption of Carbohydrate

The carbohydrate that enter the small intestine for digestion include

1. Starch2. Breakdown products from starch

from salivary amylase digestion3. Dietary disaccharides

Digestion and Absorption of Carbohydrate

Pancreatic amylase continues the breakdown of starch

Brush border enzyme called maltase breaks down maltose and Maltotriose

Brush border enzyme called dextrinase breaks down small branched segments of starch

Brush border enzymes also break down sucrose and lactose through sucrase and lactase enzyme

Steps in Carbohydrate Digestion

StarchesAmylase (saliva)Pancreatic amylase

Maltose, Maltotriose Lactose SucroseDextrin

Maltase & Dextrinase Lactase Sucrase(Intestine) (Intestine)

(Intestine)Glucose Galactose Fructose

Digestion and Absorption of Carbohydrate

Most nutrients are absorbed transepithelial transport moving first to the intestinal epithelial cell at the luminal surface then out at the basal surface

Glucose and galactose enter intestinal cells cotransported with sodium by secondary active process

Fructose enters intestinal cells by facilitated diffusion

All monosaccharides leave epithelial cells on a common transporter by the process of facilitated diffusion

Carbohydrates: Hydrolyzed into Monosaccharides

Digestion and absorption of Proteins

Proteins are digested to amino acids and small peptide chains of two or three amino acids

In the stomach, pepsin initiates the process of protein digestion

Pepsin convert protein to proteoses, peptones and peptide fragment

In the small intestine, pancreatic secretions of proteolytic enzyme: trypsin, chymotrypsin and carboxypeptidase continue protein digestion

Trypsin and chymotrypsin cleave proteins into smaller peptides

Carboxypeptidase cleave one amino acids at a time from the carboxyl end of a protein

Most proteins remain as dipeptides and tripeptides

Digestion and absorption of Proteins

Enterocyte line the villi of the small intestine continue protein digestion

In the membrane of the microvilli are enzymes peptidase

Two types of peptidase enzymes, aminopeptidase and dipeptidase

They split the remaining larger polypeptides into tripeptides, dipeptides and amino acids

Inside the cytosol of the enterocyte are peptidases

Dipeptides and tripeptides are digested to form single amino acid

Digestion and absorption of Proteins

Proteins Pepsin ProteosesPeptonesPolypeptides

Trypsin, chymotrypsin & carboxypeptidase

Polypeptides & PeptidasesAmino acids Amino acids

Digestion and absorption of Proteins

Many single amino acids enter intestinal cells cotransported with Na by secondary active transport

Most proteins absorbed as dipeptides and tripeptides use secondary active transport, some driven by Na, others by H

Some amino acids leave epithelial cells by diffusion

Other amino acids leave cells by facilitated diffusion or cotransport with Na

Digestion and absorption of Proteins

Amino Acid Transport

Digestion and Absorption of Fats

Triglyceride are made from glycerol molecule to which three fatty acids are attached

Fatty acids may be short, medium or long chain

Almost all fat digestion occurs in the small intestine

Pancreatic lipase break down triglyceride

Agitation in the stomach breaks the large drops of fats into small drops and disperses them throughout the chyme, this process called emulsification

Most of the emulsification occurs in the duodenum under influence of bile

Bile salts and lecithin are important for emulsification of fat

The structure of bile salt and lecithin consist of hydrophobic in one side and hydrophilic on the other side

Digestion and Absorption of Fats