LO28-人体代谢.docx

X Hello and welcome! Ihn this online session,you will learn about the Extracellular Metabolism in the module:Metabolism. Page 1/21 School of Biological Sciences Distinguish between extracellular and intracellular metabolism. Explain why gastrointestinal tract is considered the main site of ex...

X Hello and welcome! Ihn this online session,you will learn about the Extracellular Metabolism in the module:Metabolism. Page 1/21 School of Biological Sciences Distinguish between extracellular and intracellular metabolism. Explain why gastrointestinal tract is considered the main site of extracellular metabolism. Describe the fates of various classes of macromolecules as it passes through the gastrointestinal tract. List the major activities occurring in various organs in the digestive system. At the end of the session,you should be able to: -Distinguish between extracellular and intracellular metabolism. -Explain why gastrointestinal tract is considered the main site of extracellular metabolism. -Describe the fates of various classes of macromolecules as it passes through the gastrointestinal tract. -List the major activities occurring in various organs in the digestive system. Page 2/21 School of Biological Sciences Living things carry out metabolism to process energy for work. We learnt in the previous session that ATP is the main biological currency for driving reactions,by releasing free energy to be coupled to endergonic reactions. However,ATP itself needs to be continually generated.The energy of this comes from either the sun (for plants that carry out photosynthesis)or the food/nutrients taken. Metabolism of food/nutrients to generate biological energy occurs in two distinct phases in multi-organ animals: Extracellular and Cellular (Intracellular). Page 3/21 In extracellular metabolism,ingested Extracellular Metabolism food stuff such as (lipids,carbohydrates and,proteins)are digested (broken down) to smaller molecules through a set of UPIDS CARBOHYDRATES PROTEINS Large Molecules reactions that occur in the digestive system. Once the molecules are made sufficiently small and simple to be internalised into a cell,cellular metabolism takes over. Broken down/Digested by Gl system to smaller molecules Enstgy Glucose and other sugars Cellular Metabolism Page 3/21 Ingestion,digestion and absorption of food stuff occur in the gastrointestinal (Gl) system,which consists of two parts: GI System =Gl Tract +Accessory Organs The Gl Tract components include mouth, esophagus,stomach,small and large intestines. Pancreas It is a continuous tract that "opens"from the mouth and the central hollow portion is termed the "lumen"of the Gl tract,where food substances pass through. Page 4/21 部 NANYANG TECHNOLOGICAL UNIVERSITY School of Biological Sciences Accessory organs include the salivary glands,the liver,the pancreas,and the gallbladder. These have portals that attach to some parts of the Gl tract,allowing secretion to be introduced into the lumen. It is important to note that the lumen of Gl tract is continuous with the outside environment,and is 'separated'from the 'inside'of the body which forms the walls of the Gl tract. This is why digestive activities in the lumen are referred to as "extracellular"metabolism. Sslivarygland Esophagus Liver Gallbladdar Stomaxh Pancreas Duodenum Jujenum uleum Colon Page 4/21 School of Biological Sciences Starch is the major carbohydrate in our food.Other carbohydrates that can be found in foodstuff are Starch digestion is started in the mouth by the enzyme amylase secreted by the sallvary glands and continues in the upper part of the stomach. Starch,sucrose,lactose and maltose are then fully digested to monosaccharides (glucose,galactose and fructose)in the small intestine by pancreatic amylase. The monosaccharides need to be absorbed "into the body across the epithelial cells lining the villus. wku WTESTNE Fructose enters the epithelial cells by facilitated diffusion, while glucose and galactose enter by active transport. They then move through the epithelial cells and cross the membrane by facilitated diffusion in order to enter the blood. They are then distributed to and taken up by cells,within which cellular metabolism occurs. For dietary proteins,extracellular metabolism begins in the stomach. In the acidic pH of the stomach,the dietary proteins are first unfolded (denatured). The enzyme pepsin then cleaves some peptide bonds in these unfolded proteins, thereby making small peptides. PEPSIN STOMACH PEPSIN School of Biological Sciences These small peptides are then carried to the small intestine where the pH is near neutral (but the peptides cannot refold since they are only fragments of the original proteins). In the small intestine,the peptides are further cut by other enzymes,such as trypsin,into amino acids and smaller peptides. SMALL INTESTINE Fats (or triglycerides)are processed and transported differently by a rather elaborate mechanism. The main reason is that fats are insoluble in the aqueous medium such as the cytosol or blood. Fats in our food first aggregate into large droplets through hydrophobic interaction in the upper part of the stomach and move to the intestine. 和 NANYANG TECHNOLOGICAL UNIVERSITY School of Biological Sciences Fats (or triglycerides)are processed and transported differently by a rather elaborate mechanism. The main reason is that fats are insoluble in the aqueous medium such as the cytosol or blood. Fats in our food first aggregate into large droplets through hydrophobic interaction in the upper part of the stomach and move to the intestine. SMALL INTESTINE School of Biological Sciences Fats (or triglycerides)are processed and transported differently by a rather elaborate mechanism. The main reason is that fats are insoluble in the aqueous medium such as the cytosol or blood. Fats in our food first aggregate into large droplets through hydrophobic interaction in the upper part of the stomach and move to the intestine. Here,these large lipid droplets are emulsified into smaller droplets by bile salt and phospholipids which have been secreted into the small intestine by the liver (stored in the gall bladder). Emulsified droplets of fat are then digested by the enzyme lipase,secreted by the pancreas,into fatty acids and monoglycerides. School of Biological Sciences The fatty acids and monoglycerides then diffuse into the intestinal epithelial cells,where they are recombined by enzymes into triglycerides again. These triglycerides aggregate and are released as chylomicrons through the other side of the epithelial cells via exocytosis. Chylomicrons find their way into the lymphatic system and are then delivered to the systemic veins to enter the circulation for eventual uptake by the liver,to be broken down into fatty acids again. 部 NANYANG TECHNOLOGICAL UNIVERSITY School of Biological Sciences For an average adult,over 8 litres of water enters the Gl tract to digest less than 1 kg of foodstuff. Of this 8 litres of water,99%is absorbed back into the blood at the end of the process. Why is so much water needed? To understand that,let's look at this table below and the figure. Notice that water is needed to hydrolyse just one monomeric or dimeric unit of each macromolecule. To hydrolyse all the macromolecules obtained from food to their monomeric units (monosaccharides,amino acids,nucleotides,and fatty acids),this is the amount of water that needs to participate in the chemical reactions, Enzyme DIzestion Sallvary armylase Scarch+H2O→maltose Pancreatic amylase Starch+HzO→maltose Maltase Maltose+H2O→glucose+glucose Pepsin Protein+HO→peptides Trypsin Protein+HzO→peptdes Peptidases Peptide+HaO→amino acids Nuclease RNA and DNA·HDO nudeotides Nucleosidases Nucleotide+HO→baset sugar * phosphate Lipase Fat droplet+HzO→glycerol+fatty acids Select the magnifying glass to have a closer look. i put *6i gavuy 1500 ml u 500 m w 2000 mi asrk 1500 ml P acreticas 1500 ml mtasaal- * 1 ml 8200 ml et hul eub4 back inte Moed- we ga huioo mi sa·sog se Page 12/21 Enzyme Digestion Salivary amylase Starch+H20 →maltose Pancreatic amylase Starch +H2O →maltose Maltase Maltose +H2O→glucose +glucose Pepsin Protein+H2O→peptides Trypsin Protein +H20→peptides Peptidases Peptide +H2O→amino acids Nuclease RNA and DNA+H2O →nucleotides Nucleosidases Nucleotide+H.O→base+sugar + phosphate Lipase Fat droplet+H20→glycerol +fatty acids Page 12/21 > input 1200 ml water/day 800 g solids/day 1500 ml saliva" 500 ml bile- 2000 ml gastric secretions 1500 ml pancreatic secretions 1500 ml intestinal secretions 6700 ml absorbed from intestine 1400 ml absorbed in colon 8100 ml of fluid is recycled back Into blood 100 ml water+50g solids Page 12/21 School of Biological Sciences In this section on Extracellular Metabolism,you should take home the following messages: 1.Extracellular metabolism of carbohydrates, proteins,and lipids takes place in the gastrointestinal system. 2.The digestive products of extracellular mechanism are then transported from the 'outside'of the body to the inside'and taken up by cells elsewhere for cellular metabolism. 3.Digestions of each macromolecule occur in specific organs in the digestive system. 4.A large quantity of water is turned over in the digestive system during a cycle of digestion.