Digestive System PDF - Biology 2

INU0119/INU0519 Biology 2 Digestive System Dr Rachelle Maddison INU0119/INU0519 Biology 2 Contents Learning outcomes......................................................

INU0119/INU0519 Biology 2 Digestive System Dr Rachelle Maddison INU0119/INU0519 Biology 2 Contents Learning outcomes.........................................................................3 Digestive System............................................................................4 Functions of the gut wall.................................................................4 Structure of the gut wall..................................................................4 Tunica adventitia........................................................................................5 Tunica serosa............................................................................................5 Tunica muscularis......................................................................................5 Tunica submucosa.....................................................................................5 Tunica mucosa..........................................................................................5 Anatomy of the digestive system....................................................6 Mouth............................................................................................................6 Saliva.........................................................................................................6 Oesophagus..................................................................................................7 Stomach........................................................................................................8 Small Intestine..............................................................................................9 Duodenum...............................................................................................10 Jejunum...................................................................................................11 Ileum........................................................................................................12 Large intestine............................................................................................12 Rectum and anus........................................................................................13 Digestion of food groups...............................................................13 Carbohydrates............................................................................................14 Carbohydrate digestive enzymes.............................................................14 Proteins.......................................................................................................15 Protein digestive enzymes.......................................................................15 Triglycerides................................................................................................17 2 INU0119/INU0519 Biology 2 Triglyceride digestive enzymes................................................................18 Other substances........................................................................................18 Food tests.....................................................................................18 Learning outcomes By the end of this topic, I will be able to:  Describe the structure of the alimentary canal and its role in the mechanical and chemical breakdown of food.  Describe the importance of hydrolytic enzymes in the digestion of carbohydrates, lipid and protein.  Describe the absorption of the products of digestion and their role in the body.  Explain the reagents needed to test food for its components – carbohydrate, lipid, or protein. 3 INU0119/INU0519 Biology 2 Digestive System Food must be:  Ingested - Large molecules are taken into the mouth  Masticated – Chewed and mixed with saliva. This increases the surface area of the food over which the enzymes can act and makes the food easier to swallow.  Digested – Begins in the mouth. It involves mechanical digestion (mixing and churning) and chemical digestion (using enzymes).  Absorbed – Products of digestion are passed across the gut lining.  Egested – The undigested waste is eliminated by defecation. Functions of the gut wall  Secretion: Delivery of enzymes, mucus and ions into the lumen, and hormones into blood.  Absorption: Transport of water, ions and nutrients from the lumen, across the epithelium and into blood.  Motility: Contractions of smooth muscle in the wall of the tube cause crushing, mixing and movement of its contents. Structure of the gut wall The structure of the gut wall is basically the same from the oesophagus to end of the colon. There is variation in the folding of the mucosa (e.g. less folding in the oesophagus, 4 INU0119/INU0519 Biology 2 more folding in the ileum) and the glands (e.g. salivary glands in the mouth, ducts connecting the liver and pancreas to the duodenum). Tunica adventitia The outer layer of connective tissue (oesophagus and rectum) for the digestive tube that is located outside of the abdominal cavity. Tunica serosa The outermost covering of the digestive tube. Most of the digestive tract (stomach and intestines) consists of a thin layer of loose connective tissue covered by mesothelium (a type of squamous epithelium that lines body cavities). Tunica muscularis This allows the digestive tube to be motile. In most of the digestive tube this is two thick layers of smooth muscle. Muscle fibres in the inner layer are arranged around the tube, whereas those in the outer layer are positioned along the tube. Tunica submucosa A layer of loose to dense connective tissue containing blood and lymphatic vessels that is beneath the mucosa. The submucosa also contains the submucous plexus that provides nervous control to the mucosa. Tunica mucosa The innermost layer of the digestive tube that lines the lumen. The epithelial cells that cover the mucosa are in direct contact with the lumen. This epithelial cell sheet is distinctly different in different regions of the tract. There are specialised cells in the epithelium that are responsible for the secretion, absorption or production of hormones. 5 INU0119/INU0519 Biology 2 Anatomy of the digestive system Mouth The sight, smell and thought of food is enough to stimulate the release of saliva into the mouth. This is by a conditional reflex (a learned response). When food enters the mouth the taste buds are stimulated. This leads to an unconditional reflex (an innate response), whereby the sensory neurons signal to the brain which then signals to the salivary glands (via motor neurons) to release saliva. Our teeth start to mechanically breakdown the food to provide a larger surface area for the enzymes to work on. Saliva We produce between 1 and 1.5 litres of saliva a day and it contains water, mucus, mineral salts, and enzymes. The water acts to dissolve soluble substances so that we can taste 6 INU0119/INU0519 Biology 2 food. The mucus acts to lubricate the food and the mineral salts act to activate the enzymes. Lysozyme kills the bacteria entering with the food and amylase starts to break down starch into shorter polysaccharides and then maltose. The food and saliva mix is pushed into a ball called a bolus and swallowed by an involuntary reflex through the pharynx. During swallowing the epiglottis blocks the trachea to stop food entering the lungs. Oesophagus The oesophagus is a muscular tube though the thorax which connects the mouth to the rest of the gut. No digestion occurs here. The oesophagus is lined with squamous epithelia and mucus glands, which produce mucus to lubricate the passage of the food. Unlike the trachea, the oesophagus is a soft tube that can be completely closed. The oesophagus has a thick muscular layer, which moves the food by peristalsis. Peristalsis is a wave of muscle contraction and is completely involuntary. During peristalsis the circular muscle contracts behind the food and the longitudinal muscle contracts in front of the food. This creates a constricted gut behind the food and a shortened gut in front of the food and therefore forces the food along. 7 INU0119/INU0519 Biology 2 Peristalsis moves the food down the oesophagus and when it reaches the bottom a sphincter relaxes and opens, allowing the food to enter the stomach. Without food, this sphincter remains closed so that the stomach acid doesn’t enter and burn the oesophagus. Stomach The stomach stores the food for a couple of hours. Once food enters the stomach the muscular walls contract and relax to churn and mix the food into a liquid called chyme. To increase the efficiency of this churning the stomach lining has an extra layer of oblique (diagonal) muscle on the lumen-side of the circular muscle. The stomach lining (mucosa) contains mucus-secreting goblet cells in the columnar epithelium layer. This mucus lubricates the food and provides a barrier to protect the stomach from autodigestion. The stomach contains gastric pits (about 10 4 every cm2) created by infolding of the epithelium. These pits lead to gastric glands which contain parietal cells to secrete hydrochloric acid (pH 1) and chief cells that release an inactive enzyme called pepsinogen. 8 INU0119/INU0519 Biology 2 The stomach acid acts to: 1. Create the right pH for the enzymes to work efficiently 2. Kills any bacteria that have survived the lysozyme in the mouth 3. Removes a portion of the inactive pepsinogen so it becomes active pepsin (digests protein) The production of hydrochloric acid and pepsinogen (gastric juice) is stimulated even before food enters the stomach, by the sight, smell and thought of food. Gastric juice production is also stimulated by the physical stretching of the stomach when food enters. This stretching stimulates the glands to produce the hormone gastrin. Gastrin stimulates gastric juice production and stimulates the pyloric sphincter to relax. The acidic, churned, partially digested chyme is gradually released through the pyloric sphincter into the small intestine. Small Intestine The small intestine consists of the duodenum (30 cm long), jejunum (2 m long) and ileum (4 m long). 9 INU0119/INU0519 Biology 2 Duodenum Almost all the chemical digestion by enzymes occurs in the duodenum. The mucosa is folded into millions of microscopic projections called villi. In between the villi are intestinal glands (crypts of Leiberkuhn) which secrete intestinal juice. The possession of the villi and the folds in the cell surface membranes of the epithelial cells lining the villi (microvilli, also called brush border) massively increase the surface area of the duodenum. The epithelial cells produce various enzymes that are bound to the surface of the microvilli. Other enzymes are found in the lumen of the duodenum and have been produced and released by the pancreas (pancreatic juice). Sodium bicarbonate solution is secreted into the duodenum to neutralise the stomach acid, so that the enzymes in the pancreatic juice can work. The liver produces bile which is released into the duodenum through the bile duct. Bile can also be stored in the gall bladder before release. Bile consists of bile salts that emulsify 10 INU0119/INU0519 Biology 2 fats (the large globules of fats are mechanically dispersed into smaller globules, so that lipases have a larger surface area to act on). The release of pancreatic juice, sodium bicarbonate and bile is controlled:  When chyme touches the mucosa of the duodenum impulses are sent to the brain, which then tells the gastric glands to slow down the release of gastric juice (food is no longer in the stomach).  Chyme also stimulates the release of the hormone secretin, which causes the release of sodium bicarbonate solution (from the pancreas) and production of bile (by the liver).  Food in the duodenum stimulates the glands to produce their enzymes and release another hormone (pancreozymin), which causes the release of bile and pancreatic juice. Jejunum Large molecules have been chemically digested and are ready to be absorbed. The absorption of the products of digestion (sugars, amino acids and fatty acids) occurs in the jejunum. This occurs by simple diffusion, facilitated diffusion and active transport. Sugars and amino acids pass into the blood for transport around the body. Fatty acids are transported through the lymphatic system. 11 INU0119/INU0519 Biology 2 Ileum The ileum absorbs any remaining products of digestion and vitamin B12 and bile salts. Large intestine The large intestine consists of the colon, cecum and anus. The main role of the large intestine is to absorb water and some water-soluble vitamins and form the faeces. Food can spend 36 hours in the large intestine having the water absorbed. The mucosa contains villi, but no microvilli. Water absorption happens because sodium is actively transported out of the lumen and water follows by osmosis. Many symbiotic bacteria live in the colon and produce vitamins. For example, vitamin K (needed for blood clotting) is produced by bacteria in the colon and then absorbed. 12 INU0119/INU0519 Biology 2 Rectum and anus The faeces solidify and are stored in the rectum until the rectum is distended. The anal sphincter then opens and the faeces leave through the anus. The anal sphincter is a rare example of an involuntary muscle that we can learn to control (during potty training). Faeces consist of plant material (cellulose), cholesterol, bile, mucus, mucosa cells (250g of cells are lost every day), bacteria and water. Digestion of food groups 13 INU0119/INU0519 Biology 2 Carbohydrates The most abundant carbohydrate in the human diet is starch, and we also eat a lot of sugar (mostly sucrose) and some glycogen (in meat). Starch starts to be digested in the mouth by the addition of salivary amylase; however, this amylase is quickly denatured once it reaches the stomach. Pancreatic amylase digests the remaining starch in the duodenum. Amylase breaks the starch into the disaccharide maltose. Glycogen is also digested in the duodenum. Enzymes to digest disaccharides (e.g. maltase, sucrase and lactase) are in the membrane of the ileum and complete the digestion to monosaccharides (glucose, fructose and galactose). These are then absorbed into the epithelial cells of the jejunum and ileum and then diffuse into the capillaries of the villi and are used to release energy in the cells. Carbohydrates in plant fibres (cellulose, lignin, etc) cannot be digested, so pass through the digestive system as fibre. Carbohydrate digestive enzymes 14 INU0119/INU0519 Biology 2 Proteins Proteins are digested into amino acids that are required for cell growth and repair. Once digested the amino acids are absorbed by active transport into the epithelial cells of the jejunum and ileum, before diffusing into the capillaries. Protein digestive enzymes Pepsin Peptides are 6-12 amino acids long. Proteins Peptides Optimum pH = 2. Rennin Caesin Is insoluble, so keeps casein in Caesin salt stomach longer so pepsin can digest it. Pancreatic endopeptidases Different endopeptidases target Proteins Peptides different amino acids sequences. Exopeptidases: remove amino acids 15 INU0119/INU0519 Biology 2 Peptides Exopeptidases Amino from the ends of peptide chains. acids Carboxypeptidases: work form C- terminus. Aminopeptidases: work from N- terminus. These proteases are potentially very dangerous because they can break down other enzymes and proteins in our body. To prevent this they are made is inactive forms, called zymogens, and are only activated in the lumen of the intestines when they are needed. Active enzyme Inactive form Activated by: Pepsin Pepsinogen Acid in stomach Rennin Prorenin Pepsin in stomach Pancreatic Enzymes in duodenum exopeptidases 16 INU0119/INU0519 Biology 2 Some proteases are membrane-bound, so cannot come into contact with cell proteins. The mucus lining of the stomach also acts to protect the cells from the proteases. Triglycerides Fats and oils are converted into fatty acids and glycerol that are required to make cell membranes and to insulate the body. Triglycerides are emulsified by bile salts to form small oil droplets called micelles, which have a large surface area for lipases to work on. Fatty acids and glycerol are lipid soluble and diffuse across the epithelial cells of the villi in the ileum. In the epithelial cells the triglycerides are re-synthesised and combine with proteins to form tiny lipoprotein particles called chylomicrons. The chylomicrons diffuse into the lacteal (lymph vessel in the villus) and are then carried through the lymphatic system to enter the bloodstream at the vena cava. Triglycerides are stored in adipose tissue until needed and are only properly broken down when they are used for respiration in liver and muscle cells. 17 INU0119/INU0519 Biology 2 Triglyceride digestive enzymes Pancreatic lipase Triglycerides Fatty acids + Glycerol Other substances Many substances we eat are small enough to enter out blood and do not have to be digested. These include monosaccharides, mineral ions, vitamins and water. They are absorbed by different transport mechanisms.  Cholesterol and the fat-soluble vitamins (A, D, E, K) are absorbed into the epithelial cells of the ileum by lipid diffusion  Mineral ions and water-soluble vitamins are absorbed by passive transport in the ileum  Dietary monosaccharides are absorbed by active transport in the ileum  Water is absorbed by osmosis in the ileum and colon. Food tests These are simple tests to determine what a food consists of. These tests will be conducted in a practical class. Food type Test Positive test Starch Iodine solution Blue-black colour Sugars Benedict’s reagent Brick-red Lipid Emulsion test Cloudy-white emulsion Protein Biuret test Lilac colour 18