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We eat a wide variety of foods, containing many different biochemicals. Most of them are useful to the body but cannot be absorbed or used in the form they are in when they are eaten– they have to be digested. Types of Nutrients Micronutrients- vitamins, minerals, & water Macronutrients- proteins, lipids, carbohydrates, etc… Important fact on digestion. 1- Proteins, polysaccharides and lipids are all digests. 2- Large molecules cannot be absorbed through the wall of the gut. 3- The membranes of the cell are impermeable to large molecules. 4- Digestion is a chemical process. Plants, fungi, ot other animals have produced them, to perform functions in those organisms. But they are unlike to be ideally suited to perform functions in the human body. By breaking down proteins and other macromolecules, the human body can obtain all the subunits that it needs to build op its own macromolecules. + There are 9 amino acids that humans must obtain by digesting proteins in food. Other amino acids can be made from these nine and all human proteins can be synthesized. Essential amino acids tryptophan methionine valine histidine theorine phenylalanine leucine Isoleucine lysine Digestion of macromolecules Most food molecules are large polymers and insoluble They must first be digested to smaller soluble molecules before they can be absorbed into the blood 6.1.2 Enzymes and digestion. Enzymes are biological catalysts that increase the rate of reaction. Digestive enzymes are secreted into the lumen of the gut. Digestive enzyme increase the rate of reaction of the hydrolysis of insoluble food molecules to soluble end products. Digestive enzymes increase the rate of reaction at body temperature. This image illustrates the reduction in activation energy that is achieved by the use of an enzyme. Notice that the normal reaction requires a higher activation energy which would correspond to a high body temperature. This is usually not possible in living organisms. The enzyme-catalysed reaction has a lower activation energy. This lower activation energy would correspond to body temperature but is only possible in the presence of an enzyme. 6.1.3 Types of digestive enzyme Example 1 Pancreatic amylase: Conditions: Source the Pancreas Optimal pH 7.5-7.8 Substrate is starch (amylose) End product is the disaccharide maltose Action: hydrolysis of 1-4 glycosidic bonds 16 Example 2: Pepsin is a protease produced in the stomach Conditions: Source is the stomach Optimal pH is 2 Substrate is a polypeptide chains of amino acids End product is small polypeptides Action is the hydrolysis of peptide bonds within the polypeptide chain (endopeptidase). 19 Example 3: Pancreatic lipases: Source is the pancreas The optimal pH is 7.2 The substrate is a triglyceride lipid The product is glycerol and fatty acid chains The action of pancreatic amylases also requires the presence of bile salts that emulsify the lipid. This emulsification has two effects: Increases the surface area of the lipid for the digestion of fat Exposes the glycerol 'head' structure to the enzyme Action: hydrolysis of ester bonds between the glycerol molecules and the fatty acid chains. 21 Enzymes and pH Enzyme Main secreting pH glands optimum of enzyme Region where enzyme works Amylase 1- salivary glands 2- pancreas Mouth Small intestine 7 7 Protease 1- stomach wall 2- pancreas 2 7 lipase pacreas 7 Stomach Small intestine Small intestine pH of region where enzyme works 6.0- 7.0 7.0- 8.3 1.0- 3.5 7.0- 8.3 7.0- 8.3 The digestive System It consists of the digestive tract, a tube extending from the mouth to the anus, and its associate accessory organs. The regions of the digestive tract include the following: Oral cavity or mouth Pharynx or throat Esophagus Stomach Small intestine Consists of the duodenum, jejunum and ileum, with liver, gallbladder, and pancreas Large Intestine Includes cecum, colon, rectun and anal canal Anus Structure of the digestive system The part of the human body used for digestion can be described in simple terms as a tube through which food passes from mouth to the anus. Human digestive system GI (gastrointestinal) tract = alimentary canal Ingestion Mouth mechanical digestion teeth breaking up food chemical digestion saliva amylase enzyme digests starch mucin slippery protein (mucus) protects soft lining of digestive system lubricates food for easier swallowing buffers neutralizes acid to prevent tooth decay anti-bacterial chemicals kill bacteria that enter mouth with food mouth break up food digest starch kill germs moisten food Mouth Chemical and mechanical digestion. Food is chewed (masticated) mechanically. A bolus (lump) is formed with saliva and the tongue. Swallowing (& not choking) Epiglottis flap of cartilage closes trachea (windpipe) when swallowing food travels down esophagus Peristalsis involuntary muscle contractions to move food along Which type of digestion is the following? 1. Chewing a saltine? - 2. Saliva breaking the saltine down into molecules of glucose? 3. Your tongue breaking pieces of a hamburger apart? 4. Pepsin (an enzyme) in your stomach breaking the hamburger into amino acids? Pharynx The back of the throat. Larynx- passage for air, closes when we swallow. Is approximately 15cm long. Esophagus The part of the digestive tract that extends between the pharynx and the stomach. It is about 25 cm long and lies in the mediastrinum, anterior to the vertebrae and posterior to the trachea. The esophagus transports food from the pharynx to the stomach. The wall of the tube is made of living tissues, which include: Muscle fibers, to push the food along Secretory cells, which produce mucus for lubrication and enzymes for digestion. A single layer of epithelium cells, which form the inner lining and absorb digested food. Peristalsis Series of involuntary wave-like muscle contractions which move food along the digestive tract Peristalsis propels material through most of the digestive tract. Stomach Food is temporarily stored here. Gastric juices are secreted. Has layers of muscle that line the inside. Mechanically and chemically breaks down food. Peristaltic waves Muscular contractions consisting of a wave of relaxation of the circular muscles in front of the bolus, followed by a wave of strong contraction of the circular muscles behind the bolus, which force the bolus along the digestive tube. An enlarged segment of the digestive tract that primarily function as a storage and mixing chamber. It is located in the left superior part of the abdomen. Function of the stomach 1- The wall of the stomach is glandular, secreting mucus, hydrochloric acid and protease. 2- Hydrochloric acid creates very acid conditions that kill bacteria in food that has been swallowed. Without this, we would often catch food poisoining or other diseases from the food that we eat. Function of the stomach 3- Acid conditions denature proteins and suit protein digestion by hydrolysis. Protease secrete in the stomach, called pepsin, begins the process by breaking up polypeptides into shorter chains of amino acids. Function of the stomach 4- The wall of the stomach is elastic, so large meals can be swallowed quickly, stored and then gradually released into the small intestine. 5- The wall of the stomach is muscular and contracts rhythmically to squeeze the food, mixing it and breaking it into smaller lumps. Stomach Functions food storage can stretch to fit ~2L food disinfect food HCl = pH 2 kills bacteria chemical digestion pepsin enzyme breaks down proteins But the stomach is made out of protein! What stops the stomach from digesting itself? mucus secreted by stomach cells protects stomach lining mouth break up food digest starch kill germs moisten food stomach kills germs break up food digest proteins store food sphincter sphincter Gastric Juices Secreted by the stomach. Acidic (pH 1.5-2.5) (HCl). Pepsin- an enzyme that breaks down large proteins into amino acids. Food is further broken down into a thin liquid called chyme. Accessory Organs Pancreas Gall Bladder Spleen The gallbladder is a saclike structure on the inferior surface of the liver. The liver continually secretes bile, which flows to the gallblader where it stores. Gall bladder Pouch structure located near the liver which concentrates and stores bile Bile duct – a long tube that carries BILE. The top half of the common bile duct is associated with the liver, while the bottom half of the common bile duct is associated with the pancreas, through which it passes on its way to the intestine. BILE Bile emulsifies lipids (physically breaks apart FATS) Bile is a bitter, greenish-yellow alkaline fluid, stored in the gallbladder between meals and upon eating is discharged into the duodenum where it aids the process of digestion. Pancreas A complex organ composed of both endocrine and exocrine tissue that perform several functions. The exocrine secretions of the pancreas, called pancreatic juice, have an aqueous component and an enzymatic component. Pancreatic juice is delivered to the small intestine through the pancreatic ducts, where it functions in digestion. Pancreas Pancreas An organ which secretes both digestive enzymes (exocrine) and hormones (endocrine) ** Pancreatic juice digests all major nutrient types. Nearly all digestion occurs in the small intestine & all digestion is completed in the SI. Pancreas Digestive enzymes digest proteins trypsin, chymotrypsin digest starch amylase Buffers neutralizes acid from stomach Liver It performs important digestive and excretory functions, stores and processes nutrients, detoxifies harmful chemicals, and synthesizes new molecules. Liver Function produces bile bile stored in gallbladder until needed breaks up fats act like detergents to breakup fats bile contains colors from old red blood cells collected in liver = iron in RBC rusts & makes feces brown mouth break up food digest starch kill germs moisten food liver produces bile - stored in gall bladder break up fats pancreas produces enzymes to digest proteins & starch stomach kills germs break up food digest proteins store food Small Intestine It consists of the three parts: the duodenum, the jejunum and the ileum. The entire small intestine is about 6 m long. Two major accessory glands, the liver and the pancreas, are associated with the duodenum. The small intestine is where the greatest amount of digestion and absorption occurs. Small Intestine a) Villus which increase the surface area for absorption of the products of digestion (b) Microvilli border of the epithelial cell increases the surface are for absorption. (c) Lacteals are connect to the lymphatic system for the transport of lipids. (d) In the wall of the small intestine are the blood vessels to transport absorbed products to the general circulation, There are also the muscle to maintain peristalsis Function of the small intestine 1- Pancreatic juice is secreted into the small intestine. It contains protease, amylase and lipase, so the three main groups of the macromolecules are all digested here. Function of the small intestine 2- The wall of the small intestine secretes a variety of other enzymes, which digest other substances. Nucleases, for example, are secred to digest DNA and RNA in food eaten. Because of the great length of the small intestine, food spends hours passing thorugh, giving time for digestion to be completed. Function of the small intestine 3- Digested foods are absorbed by the small intestine. 4- Areas in the wall of the small intestine called Peyer´s patches contain lymphocytes that detect pathogenic organisms in the food in the small intestine and help in the production of antibodies against them. Small Intestine Most chemical digestion takes place here. Simple sugars and proteins are absorbed into the inner lining. Fatty acids and glycerol go to lymphatic system. Lined with villi, which increase surface area for absorption, one cell thick. Small intestine Function chemical digestion major organ of digestion & absorption absorption through lining over 6 meters! small intestine has huge surface area = 300m2 (~size of tennis court) Structure 3 sections duodenum = most digestion jejunum = absorption of nutrients & water ileum = absorption of nutrients & water Duodenum 1st section of small intestines acid food from stomach mixes with digestive juices from: pancreas liver gall bladder mouth break up food digest starch kill germs moisten food pancreas produces enzymes to digest proteins & starch stomach kills germs break up food digest proteins store food Absorption in the SI Much absorption is thought to occur directly through the wall without the need for special adaptations Almost 90% of our daily fluid intake is absorbed in the small intestine. Villi - increase the surface area of the small intestines, thus providing better absorption of materials Absorption by Small Intestines Absorption through villi & microvilli finger-like projections increase surface area for absorption Structure and function of the villus The structure of the villus increases the surface are for the absorption of digested food molecules. (a) folds increase SA:VOL ration by X 3 (b) Villi project into the lumen of the gut increasing the surface area by X 10 (c) Microvilli are outward folds of the plasma membrane increasing the surface area another X10 This sequence of light microscope and electron micrograph images show the same sequence as the diagram above. This sequence of light microscope and electron micrograph images show the same sequence as the diagram above. Histological adaptations within the villus. •Blood supply in the villus which absorb the end products of digestion from the epithelial cells •The lacteals (green) that receive the lipoproteins before transporting them to the circulatory system. •Muscular walls that maintain the movement of chyme by peristalsis. 80 Inside the small intestine VILLI Large Intestine The movement of the large intestine are more sluggish than those of the small intestine. While in the colon, chyme is converted to feces. The formation of feces inclives the absorption of water and salts, secretion of mucus and extensive action of microorganisms. The colon stores the feces until they are eliminated by defecation. Large intestines (colon) Function re-absorb water use ~9 liters of water every day in digestive juices > 90% of water reabsorbed not enough water absorbed diarrhea too much water absorbed constipation Large Intestine Solid materials pass through the large intestine. These are undigestible solids (fibers). Water is absorbed. Vitamins K and B are reabsorbed with the water. Rectum- solid wastes exit the body. You’ve got company! Living in the large intestine is a community of helpful bacteria Escherichia coli (E. coli) produce vitamins vitamin K; B vitamins generate gases by-product of bacterial metabolism methane, hydrogen sulfide Appendix Vestigial organ Rectum Last section of colon (large intestines) eliminate feces undigested materials extracellular waste mainly cellulose from plants roughage or fiber masses of bacteria 88 Defecation Stretch receptors in rectal wall initiate spinal reflex Motor through parasympathetic fibers Longitudinal muscles contract Aided by voluntary contraction of diaphragm and abdominal muscles and relaxation of external anal sphincter 89 Insoluble food molecules are digested to soluble products in the lumen of the gut. Absorption: The soluble products are first taken up by various mechanisms into the epithelial cells that line the gut. These epithelial cells then load the various absorbed molecules into the blood stream. Assimilation: The soluble products of digestion are then transported to the various tissues by the circulatory system. The cells of the tissues then absorb the molecules for use within this tissues