Human Physiology L8: Chemical Digestion and Absorption PDF

L8: Chemical Digestion and Absorption Human Physiology MLS Dept-S5 Charmo University 2024-2025 Dr. Shelanah M. Salih  Chemical Digestion and Absorption § Digestion is the process in which complex food substances (carbohydrates, proteins, and lipids) are de...

L8: Chemical Digestion and Absorption Human Physiology MLS Dept-S5 Charmo University 2024-2025 Dr. Shelanah M. Salih  Chemical Digestion and Absorption § Digestion is the process in which complex food substances (carbohydrates, proteins, and lipids) are degraded to products that can be absorbed. § Absorption (or uptake) of these products occur when they are transported into absorptive cells in the small intestine. § Several major organs contribute to the digestion and absorption of food substances. They include the mouth, stomach, small intestine, liver, pancreas, and the gall bladder. § Each organ is responsible for degrading and/or absorbing food substances. They contain enzymes and other substances that allow this process to occur.  Digestion of Carbohydrates § Dietary Carbohydrates § Dietary intake of carbohydrates is 250-850 gm/day, which represents 50 to 60% of the diet. § Major carbohydrates in the human diet are present in following forms: 1. Polysaccharides (starch and glycogen) 2. Oligosaccharides (di, tri, tetra, and penta- saccharides) 3. Monosaccharides (glucose and fructose)  Digestion of Carbohydrates In the Mouth → Salivary a-amylase Starch (cocked) → Maltose In the Stomach → Salivary amylase → denature by HCl and carbohydrate digestion stop temporarily In the small Intestine * Pancreatic amylase → Starch (boiled and unboiled) → Maltose, Maltriose, and dextrin § Pancreatic amylase hydrolyses almost all the starch within 15 to 30 minutes of the entry of chyme into the duodenum. § Pancreatic amylase acts in an alkaline medium and its digestive activity is increased by the presence of bile salts.  Digestion of Carbohydrates Small Intestine Brush border enzymes Dextrin → Glucose Maltose → Glucose Sucrose → Glucose + Fructose Lactose → glucose + galactose (Monosaccharides)  End Products of Carbohydrate Digestion § The carbohydrate digestion is completed in the small intestine (mainly in jejunum and proximal ileum) § The end products of carbohydrates are monosaccharides such as glucose, fructose and galactose. § The glucose represents 80% and galactose and fructose combinedly represent only 20% of the end products. § A little amount of other monosaccharide called pentoses are the end products of digestion of nucleic acids and partial digestion of pentosans.  Absorption of Monosaccharides Monosaccharides are absorbed by enterocytes (intestinal cells) using different transport mechanisms. § Glucose and galactose are absorbed via active transport through sodium dependent glucose transporter (SGLT1) § Fructose is absorbed by facilitated diffusion through glucose transporter 5 (GLUT5). § Monosaccharides enter the bloodstream through the portal vein and are transported to the liver. Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings  Absorption of Monosaccharides Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings  Chemical Digestion: Proteins § Enzymes used: pepsin in the stomach § Enzymes acting in the small intestine § Pancreatic enzymes – trypsin, chymotrypsin, and carboxypeptidase § Brush border enzymes – aminopeptidases, carboxypeptidases, and dipeptidases  Chemical Digestion: Proteins Organ Substrate Enzyme End product(s) Stomach Polypeptides Pepsinogen +HCl Smaller peptides = pepsin Trypsinogen, chymotrypsinogen Smaller peptides (inactive enzymes released from the Polypeptides pancreas. intestine These enzymes are activated by enterokinase from small intestine to trypsin and chymotrypsin Smaller Amino acids polypeptides Aminopeptidase, carboxypeptidase Brush Amino acids border of small Dipeptides Dipeptidase intestine  Absorption of Amino Acids & Dipeptides § Absorption into epithelial cell § active transport with Na+ or H+ ions (symporters) § Movement out of epithelial cell into blood § diffusion Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings  Digestion of Fats Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings  Emulsification of Fats § Emulsification is the process by which the fat globules are broken down into minute droplets to form a milky fluid called emulsion in small intestine, by the action of bile salts. § Lipolytic enzymes of GI tract cannot digest the fats directly because the fats are insoluble in water due to the surface tension. § Bile salts emulsify the fats by reducing the surface tension due to their detergent action. § Now the fats can be easily digested by lipolytic enzymes. Chemical Digestion: Fats  Digestion of Lipids § Enzymatic fat digestion is carried out by lipases, enzymes that remove two fatty acids from each triglyceride molecule. § The result is one monoglyceride and two free fatty acids § because lipase is unable to penetrate the bile salts, fat digestion requires colipase, a protein cofactor secreted by the pancreas. § Colipase displaces some bile salts, allowing lipase access to fats inside the bile salt coating.  Digestion of Fats (a) Bile salts coat lipids to make emulsions. Micelle  Digestion of Fats § Phospholipids are digested by pancreatic phospholipase. Free cholesterol is not digested and is absorbed intact. § As enzymatic and mechanical digestion proceed, fatty acids, bile salts, mono- and diglycerides, phospholipids, and cholesterol coalesce to form small disk-shaped micelles § Micelles then enter the unstirred aqueous layer at the edge of the brush border. Digestion and Absorption of Fats  Absorption of Fats § Small fatty acids enter cells & then blood by simple diffusion § Larger lipids exist only within micelles (bile salts coating) § Lipids enter cells by simple diffusion leaving bile salts behind in gut § Bile salts reabsorbed into blood & reformed into bile in the liver § Fat-soluble vitamins are enter cells since were within micelles  Absorption of Fats (2) § Inside epithelial cells fats are rebuilt and coated with protein to form chylomicrons § Chylomicrons leave intestinal cells by exocytosis into a lacteal § travel in lymphatic system to reach veins near the heart § removed from the blood by the liver and fat tissue  Digestion of Nucleic Acids § Pancreatic juice contains 2 nucleases § ribonuclease which digests RNA § deoxyribonuclease which digests DNA § Nucleotides produced are further digested by brush border enzymes (nucleosidease and phosphatase) § pentose, phosphate & nitrogenous bases § Absorbed by active transport  Absorption of Electrolytes § Sources of electrolytes § GI secretions & ingested foods and liquids § Enter epithelial cells by diffusion & secondary active transport § sodium & potassium move = Na+/K+ pumps (active transport) § chloride, iodide and nitrate = passively follow § iron, magnesium & phosphate ions = active transport § Intestinal Ca+ absorption requires vitamin D & parathyroid hormone  Absorption of Vitamins § Fat-soluble vitamins § travel in micelles & are absorbed by simple diffusion § Water-soluble vitamins § absorbed by diffusion § B12 combines with intrinsic factor before it is transported into the cells § receptor mediated endocytosis  Absorption of Water 9 liters of fluid dumped into GI tract each day Small intestine reabsorbs 8 liters Large intestine reabsorbs 90% of that last liter Absorption is by osmosis through cell walls into vascular capillaries inside villi