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  1. 1. Digestion and Absorption in the GI Tract<br />The major foods can be classified into carbohydrates, fats and proteins<br />Digestion: The enzymatic breakdown of food to components that can be absorbed<br />Absorption: The movement of nutrients, salts and water across the GI epithelium into blood or lymph<br />
  2. 2. Digestion and Absorption in the GI Tract<br />Most digestion and absorption occurs in the small intestine<br />The pancreas supplies the digestive enzymes for most foodstuffs<br />Human lacks of digestive enzymes for plant polysaccharides cellulose <br />The indigestible polysaccharides contain in vegetables and fruits make up dietary fibre <br />
  3. 3. Digestion of various foods by hydrolysis<br />Monosaccharides, fatty acids and amino acids are bound to one another by condensation<br />The process of hydrolysis is involved in the digestion of carbohydrates, fats and proteins<br />
  4. 4. Digestion and absorption of Carbohydrates<br />The most abundant dietary carbohydrates are starch and the three disaccharides sucrose, lactose and maltose<br />Carbohydrates are built from monosaccharides and must be digested to their component monosaccharides so they can be absorbed<br />Starch are digested to glucose<br />Sucrose is digested to glucose and fructose<br />Lactose is digested to glucose and galactose<br />Maltose is digested to glucose<br />
  5. 5. Digestion and absorption of Carbohydrates<br />Plant starch and glycogen are long polymers of glucose<br />Digestion of carbohydrates begins in the mouth with salivary amylase<br />Amylase break starch down to disaccharides maltose; fragments of three glucose molecules (Maltotriose); and to small branched fragments (limit dextrin)<br />Acid in the stomach denatures amylase<br />
  6. 6. Digestion and Absorption of Carbohydrate<br />The carbohydrate that enter the small intestine for digestion include <br />Starch<br />Breakdown products from starch from salivary amylase digestion<br />Dietary disaccharides<br />
  7. 7. Digestion and Absorption of Carbohydrate<br />Pancreatic amylase continues the breakdown of starch <br />Brush border enzyme called maltase breaks down maltose and Maltotriose<br />Brush border enzyme called dextrinase breaks down small branched segments of starch<br />Brush border enzymes also break down sucrose and lactose through sucrase and lactase enzyme<br />
  8. 8. Steps in Carbohydrate Digestion<br />Starches<br /> Amylase (saliva)<br /> Pancreatic amylase<br />Maltose, Maltotriose Lactose Sucrose<br />Dextrin<br />Maltase & Dextrinase Lactase Sucrase <br />(Intestine) (Intestine) (Intestine)<br />Glucose Galactose Fructose<br />
  9. 9. Digestion and Absorption of Carbohydrate<br />Most nutrients are absorbed transepithelial transport moving first to the intestinal epithelial cell at the luminal surface then out at the basal surface<br />Glucose and galactose enter intestinal cells cotransported with sodium by secondary active process <br />Fructose enters intestinal cells by facilitated diffusion<br />All monosaccharides leave epithelial cells on a common transporter by the process of facilitated diffusion<br />
  10. 10. Carbohydrates: Hydrolyzed into Monosaccharides<br />
  11. 11. Digestion and absorption of Proteins<br />Proteins are digested to amino acids and small peptide chains of two or three amino acids<br />In the stomach, pepsin initiates the process of protein digestion<br />Pepsin convert protein to proteoses, peptones and peptide fragment <br />
  12. 12. In the small intestine, pancreatic secretions of proteolytic enzyme: trypsin, chymotrypsin and carboxypeptidase continue proteindigestion<br />Trypsin and chymotrypsin cleave proteins into smaller peptides <br />Carboxypeptidase cleave one amino acids at a time from the carboxyl end of a protein<br />Most proteins remain as dipeptides and tripeptides<br />Digestion and absorption of Proteins<br />
  13. 13. Enterocyte line the villi of the small intestine continue protein digestion<br />In the membrane of the microvilli are enzymes peptidase <br />Two types of peptidase enzymes, aminopeptidase and dipeptidase<br />They split the remaining larger polypeptides into tripeptides, dipeptides and amino acids<br />Inside the cytosol of the enterocyte are peptidases<br />Dipeptides and tripeptides are digested to form single amino acid<br />Digestion and absorption of Proteins<br />
  14. 14. Proteins Pepsin Proteoses<br /> Peptones<br /> Polypeptides<br /> Trypsin, chymotrypsin & <br /> carboxypeptidase<br />Polypeptides & Peptidases<br />Amino acids Amino acids<br />Digestion and absorption of Proteins<br />
  15. 15. Many single amino acids enter intestinal cells cotransported with Na by secondary active transport<br />Most proteins absorbed as dipeptides and tripeptides use secondary active transport, some driven by Na, others by H <br />Some amino acids leave epithelial cells by diffusion<br />Other amino acids leave cells by facilitated diffusion or cotransport with Na<br />Digestion and absorption of Proteins<br />
  16. 16. Amino Acid Transport<br />
  17. 17. Digestion and Absorption of Fats<br />Triglyceride are made from glycerol molecule to which three fatty acids are attached<br />Fatty acids may be short, medium or long chain<br />Almost all fat digestion occurs in the small intestine<br />Pancreatic lipase break down triglyceride<br />
  18. 18. Agitation in the stomach breaks the large drops of fats into small drops and disperses them throughout the chyme, this process called emulsification<br />Most of the emulsification occurs in the duodenum under influence of bile<br />Bile salts and lecithin are important for emulsification of fat<br />The structure of bile salt and lecithin consist of hydrophobic in one side and hydrophilic on the other side<br />Digestion and Absorption of Fats<br />