Unit: II Energy – Units, Metabolisms, Energy expenditure, and Energy imbalance. Digestion, absorption and transport of Food Proteins and eye Attribution-NonCommercial-ShareAlike 4.0 International (CC BY-NC-SA 4.0) Unit: II Digestion, absorption and transport of Food Attribution-NonCommercial-ShareAlike 4.0 International (CC BY-NC-SA 4.0) Overview of Digestive System • Digestion: The process of breaking down food into individual molecules small enough to be absorbed through the intestinal wall • Absorption: The process of moving nutrients from the gastrointestinal (GI) tract into the bloodstream • Transport: The process of moving absorbed nutrients throughout the body through the circulatory and lymph systems • Elimination: The excretion of undigested and unabsorbed food through the feces Overview of Digestive System Overview of Digestive System Overview of Digestive System Overview of Digestive System Overview of Digestive System Small Intestine • Most digestion occurs in the small intestines • Extends from the pyloric sphincter to the ileocecal valve • Contains three sections • Duodenum • Jejunum • Ileum • Takes 3 to 10 hours for food to traverse the small intestine Overview of Digestive System Overview of Digestive System Overview of Digestive System Large Intestine • Site of water, sodium, potassium, and chloride absorption • Bacteria produces vitamin K, thiamin, riboflavin, biotin, and vitamin B12 • Only biotin and vitamin K can be absorbed • In the large intestine, 1 liter of fluid material is gradually reduced to 200 grams of brown fecal material • Brown color is due to unabsorbed iron mixed with yellowish-orange substance called bilirubin • Greater the iron content, the darker the feces Overview of Digestive System Overview of Digestive System The major functions of each region are: Mouth • Starch hydrolysis catalysed by amylase, secreted by the salivary glands; • Fat hydrolysis catalysed by lingual lipase, secreted by the tongue; • Absorption of small amounts of vitamin C and a variety of non-nutrients (including nicotine). Overview of Digestive System Stomach • Denaturation of dietary proteins and the release of vitamin B12, iron and other minerals from protein binding, for which gastric acid is important; • Protein hydrolysis catalysed by pepsin; • Fat hydrolysis catalysed by lipase. • Secretion of intrinsic factor, required for the absorption of vitamin B12 Overview of Digestive System Small intestine (duodenum, jejunum and ileum) • starch hydrolysis catalysed by amylase secreted by the pancreas; • hydrolysis of disaccharides within the brush border of the intestinal mucosa; • fat hydrolysis catalysed by lipase secreted by the pancreas; • protein hydrolysis catalysed by a variety of exo- and endopeptidases secreted by the pancreas and small intestinal mucosa; • hydrolysis of di- and tripeptides within the brush border of the intestinal mucosa; • absorption of the products of digestion; • absorption of water (failure of water absorption, as in diarrhoea, can lead to serious dehydration). Overview of Digestive System Large intestine (caecum and colon) – bacterial metabolism of undigested carbohydrates and shed intestinal mucosal cells – absorption of some of the products of bacterial metabolism – absorption of water. Rectum – storage of undigested gut contents prior to evacuation as faeces Digestion and absorption of carbohydrates Classification of carbohydrates Nutritionally, it is useful to consider sugars (both monosaccharides and disaccharides) in two groups: • Sugars contained within plant cell walls in foods. These are known as intrinsic sugars. • Sugars that are in free solution in foods, and therefore provide a substrate for oral bacteria, leading to the formation of dental plaque and caries. These are known as extrinsic sugars. Digestion and absorption of carbohydrates Classification of carbohydrates Digestion of carbohydrates Digestion of Carbohydrates in the Mouth and Stomach • When food is chewed, it is mixed with saliva, which contains the digestive enzyme ptyalin (an α -amylase) secreted mainly by the parotid glands. • This enzyme hydrolyzes starch into the disaccharide maltose and other small polymers of glucose that contain three to nine glucose molecule • Only about 5% of starch is digested because food remains in the mouth only for a short time Digestion of carbohydrates Digestion of Carbohydrates in the Mouth and Stomach • At stomach activity of the salivary amylase is blocked by acid of the gastric secretions ( amylase is nonactive at pH < 4.0) • 30 to 40 per cent of the starches will have been hydrolyzed mainly to form maltose at stomach Digestion of carbohydrates Digestion of Carbohydrates in the Small Intestine • Pancreatic secretion contains a large quantity of α-amylase • within 15 to 30 minutes after the chyme empties from the stomach into the duodenum and mixes with pancreatic juice, virtually all the carbohydrates will have become digested. • carbohydrates are almost totally converted into maltose and/or other very small glucose polymers before passing beyond the duodenum or upper jejunum. Digestion of carbohydrates Digestion of Carbohydrates in the Small Intestine • The enterocytes lining the villi of the small intestine contain four enzymes (lactase, sucrase, maltase, and α-dextrinase), which split the disaccharides lactose, sucrose, and maltose into their constituent monosaccharides. • These enzymes are located in the enterocytes covering the intestinal microvilli brush border Digestion of carbohydrates Digestion of Carbohydrates in the Small Intestine • Maltase catalyses the hydrolysis of maltose to two molecules of glucose. • Sucrase–isomaltase is a bifunctional enzyme that catalyses the hydrolysis of sucrose to glucose and fructose, and of isomaltose to two molecules of glucose. • Lactase catalyses the hydrolysis of lactose to glucose and galactose. • Trehalase catalyses the hydrolysis of trehalose to two molecules of glucose. Digestion of carbohydrates Digestion of Carbohydrates in the Small Intestine • Lactose splits into a molecule of galactose and a molecule of glucose. • Sucrose splits into a molecule of fructose and a molecule of glucose Digestion of Proteins 1. Endopeptidases cleave proteins by hydrolysing peptide bonds between specific amino acids throughout the molecule. 2. Exopeptidases remove amino acids one at a time from either the amino or carboxyl end of the molecule, again by the hydrolysis of the peptide bond. Digestion of Proteins 1. The first enzymes to act on dietary proteins are the endopeptidases: pepsin in the gastric juice 2. trypsin, chymotrypsin and elastase secreted by the pancreas into the small intestine. Digestion of Proteins • The result of the combined action of the endopeptidases is that the large protein molecules are broken down into a number of smaller polypeptides with a large number of amino and carboxy terminals for the exopeptidases to act on • There are two classes of exopeptidase: 1. Carboxypeptidases, secreted in the pancreatic juice, release amino acids from the free carboxyl terminal of peptides. 2. Aminopeptidases, secreted by the intestinal mucosal cells, release amino acids from the amino terminal of peptides. Digestion of Proteins Digestion of Proteins in the Stomach • Pepsin, the important peptic enzyme of the stomach (active at a pH • of 2.0 to 3.0) • Pepsin can digest collagen fiber. • only 10 to 20 per cent of the total protein digestion to convert the protein to proteoses, peptones, and a few polypeptides Digestion of Proteins Digestion of Proteins y pancreatic secretion • Most protein digestion occurs in the upper small intestine, in the duodenum and jejunum • major proteolytic pancreatic enzymes: trypsin, chymotrypsin, carboxypolypeptidase, and proelastase • trypsin and chymotrypsin split protein molecules into small polypeptides • Carboxypolypeptidase then cleaves individual amino acids from the carboxyl ends of the polypeptides. • Proelastase, in turn, is converted into elastase, which then digests elastin fibers that partially hold meats together. Digestion of Proteins Digestion of Proteins y pancreatic secretion • Only a small percentage of the proteins are digested all the way to their constituent amino acids by the pancreatic juices. Most remain as dipeptides and tripeptides Digestion of Proteins Digestion of Proteins by intestine secretions • Enterocytes have a brush border that consists of hundreds of microvilli projecting from the surface of each cell. • In the membrane of each of these microvilli are multiple peptidases that protrude through the membranes to the exterior • Two types of peptidase enzymes are especially important, aminopolypeptidase and several dipeptidases. • inside the cytosol of the enterocyte are multiple other peptidases Digestion of fat Digestion of Proteins by intestine secretions • A small amount of triglycerides is digested in the stomach by lingual lipase that is secreted by lingual glands in the mouth and swallowed with the saliva. • • The first step in fat digestion is physically to break the fat globules into very small sizes so that the water-soluble digestive enzymes can act on the globule surfaces. This process is called emulsification of the fat, • most of the emulsification occurs in the duodenum under the influence of bile Digestion of fat Digestion of Proteins by intestine secretions • bile salts and the phospholipid lecithin are extremely important for emulsification of the fat. • The lipase enzymes are water-soluble compounds and can attack the fat globules only on their surfaces Digestion of fat Digestion of Proteins by pancreatic secretions • The most important enzyme for digestion of the triglycerides is pancreatic lipase, present in enormous quantities in pancreatic juice • In addition, the enterocytes of the small intestine contain still more lipase, known as enteric lipase, but this is usually not needed Digestion Absorption • Majority of absorption takes place in the small intestine • Nutrients are absorbed via • Passive diffusion – nutrients move from high concentration to low concentration; no energy is required • Facilitated diffusion – nutrients move from high concentration to low concentration with the help of a carrier protein; no energy is required • Active transport – nutrients move from low concentration to high concentration with the help of a carrier protein, energy is required • Endocytosis – cell forms a vesicle to surround and engulf a nutrient Absorption Absorption Absorption Absorption of Carbohydrates Absorption Absorption of Carbohydrates Absorption Absorption Absorption Absorption Absorption • Nutrients are absorbed into the circulatory or lymphatic system • Water-soluble nutrients are absorbed into the circulatory system • Carbohydrates, amino acids, and water-soluble vitamins • Water-soluble nutrients GI tract Capillaries Hepatic Portal Vein • Watersoluble nutrients • Watersoluble nutrients Liver Absorption • Fat-soluble nutrients are absorbed into the lymphatic system • Fat-soluble vitamins, long-chain fatty acids, and proteins too large to be transported via the capillaries • Fatsoluble nutrients Lymph Capillaries Lymphatic vessels • Fatsoluble nutrients • Fatsoluble nutrients Thoracic Duct
