The Digestive System

Transcription

1 The Digestive System Composed of 4 sections: 1. Mouth 2. Stomach 3. Small intestine 4. Large intestine Digestion is the breakdown of large molecules into smaller molecules so that they can be absorbed by the blood. Alimentary canal is the tube from the mouth to the anus. mouth tongue salivary gland epiglottis trachea (wind pipe) oesophagus (gullet) right lung gall bladder bile duct liver pancreas diaphragm stomach pyloric sphincter colon duodenum ileum caecum rectum appendix anus Duodenum + Ileum = Small intestine Colon + Rectum = Large intestine

2 The Mouth Digestion begins in the mouth. Saliva is released from the salivary gland and mixes the food, softening it, dissolving it and making it easier to swallow. Saliva also contains the enzyme amylase, which begins to break down starch into maltose. The physical action of chewing (mastication) breaks food into smaller pieces. This makes it easier to swallow, and increases the surface area for enzymes to work on. The food (bolus) passes into the throat and the epiglottis should close, preventing the food entering the trachea. Eustachian tube base of skull food bolus nasal cavity salivary glands windpipe soft palate epiglottis glottis gullet tongue squeezes food against palate larynx cartilage raised soft palate seals off nasal cavity back of tongue and epiglottis direct food over opening of windpipe circular muscle partly closes glottis The bolus travels down the oesophagus to the stomach by a process called peristalsis. This is how it travels through the rest of the alimentary canal. muscular wall of gullet circular muscle contracting food

3 Stomach It is a large muscular bag, which churns and mixes food. It stores the food temporarily and releases it gradually into the duodenum. It produces gastric juices containing Hydrochloric Acid and enzymes. Mucus protects the lining of the stomach from damage by the acid. Small Intestine (a) Duodenum where most digestion takes place Liver gall bladder bile bile duct pancreatic juice pancreas Bile is not an enzyme: it emulsifies fats making them easier to digest by enzymes. large fat droplet BILE small droplets, so larger surface area for enzymes to work on. (b) Ileum final digestion but mainly absorption of small nutrient molecules Food Carbohydrates Proteins Lipids Final product to be absorbed Glucose/Fructose Amino Acids Fatty acids + glycerol Large intestine (a) Colon: removes water from residual indigestible material. Little water lost in faeces. Minerals and some vitamins are also absorbed by the colon. (b) Rectum: stores waste material until it is egested.

4 Where it is produced Digestive Enzymes Secretion Enzyme(s) What it does Salivary Gland Saliva Salivary amylase Walls of stomach (gastric pits) Pancreas Gastric juice (pepsin and Hydrochloric Acid) Pancreatic juices Pepsin (works best in acidic conditions of stomach) Lipase Pancreatic Amylase Trypsin Walls of the Ileum Intestinal juice Maltase Peptidase Sucrase Breaks down Starch into Maltose Protein broken down into Polypeptides Breaks down fats to Glycerol and Fatty acids Breaks down starch into Maltose Breaks down Proteins into Polypeptides Breaks down Maltose into Glucose Breaks down Polypeptides into Amino Acids Breaks down Sucrose into Fructose and Glucose Protease is the general name for enzymes, which digest protein: pepsin, trypsin and peptidase. Absorption This takes place in the ileum through small finger-like projections called villi. Muscle layers to allow peristalsis Bolus Blood vessels to remove digested food molecules Folded inner wall of ileum. Increases surface area for absorption Lumen hollow part of gut

5 On the surface of the inner folded wall are millions of villi, which greatly increase the surface area for absorption. Molecules move into villus by diffusion Lacteal Digested food Wall of villus only one cell thick Network of capillaries Low food molecule concentration Lymph Vessel - Amino acids and glucose diffuse into the capillaries and are taken away in the blood. - Fatty acids and glycerol diffuse into the lacteal and are taken away into the lymph vessel they are added to the blood again later. Advantages - Thin membrane means a short diffusion pathway for materials. Therefore absorbs food molecules quickly. - Vast network of blood capillaries take food material quickly. - Substances taken away quickly so that it maintains a steep concentration gradient. Advantages of the Small Intestine - It is long so that there is time for digestion to take place. - It is folded which makes a greater surface area. - It has a good blood supply so the digested food can be absorbed quickly by the blood. - It has thin walls so that diffusion can take place short diffusion pathway. - It has lots of villi, which give more surface area. - It has various enzymes, which break down various foods.

6 Assimilation Amino acids Glucose etc. Blood capillaries Fatty acids Glycerol Lacteals/ lymphatic system (returned to blood later) All blood from the intestines is taken to the liver by the hepatic portal vein. The blood needs to have a constant concentration of various substances (e.g. glucose). The liver regulates blood from the alimentary canal ensuring these substances remain constant. Examples in the liver: 1. Excess glucose is stored as glycogen (using insulin). Glycogen can then be turned back into glucose (using glucagon). 2. Further glucose is stored as fat. Fat can be turned back into glucose. Enzymes Metabolism Anabolic reactions Catabolic reaction all chemical reactions in an organism building up smaller molecules into larger molecules (e.g. amino acids to proteins) Breaking down large molecules into smaller molecules (e.g. proteins into amino acids) Some enzymes work inside cells intracellular enzymes. Others (like digestive enzymes) work outside cells: extra cellular enzymes. Enzymes are biological catalysts. They speed up the rate of chemical reactions and control these reactions within living organisms. The chemical an enzyme works on is called the substrate. The chemical(s) produced at the end is the product. e.g. Starch substrate Amylase Maltose product

7 Properties of enzymes Enzymes are proteins Temperature sensitive ph sensitive Enzymes are specific They are not consumed. this is one reason why we need to eat proteins to make enzymes. a rise in temperature will increase enzyme activity (kinetic theory). However, if the temperature is too high (normally above 45ºC) will destroy enzymes this is permanent denaturation. all enzymes work within a narrow ph. The ph they work best in is called their optimum ph. they only work on one substrate or one group of substrates. E.g. amylase only breaks down starch nothing else. Enzymes are not consumed during the reaction and so can be re-used over and over. However, eventually they wear out Lock and key hypothesis Enzyme molecule Substrate molecule active site Substrate binds to active site forming an enzyme-substrate complex. The enzyme breaks down the substrate into the product. Enzyme is free to break another substrate molecule down. It is re-usable. product

8 Proteins (enzymes) are altered by changes in ph and in temperature. Optimum ph is 2.5 Optimum ph is 7 Rate of reaction Pepsin Trypsin ph Enzymes work within narrow ph limits. If they are placed in a ph from their optimum, they rapidly become less active. Most enzymes work best in a neutral ph. A change in ph changes the shape of the active site and it no longer fits the substrate. B C D A Temperature (ºC) Optimum temperature for enzymes vary, but many are around 40ºC. A B C D Very slow rate of reaction. Enzymes have little kinetic energy and so they for enzyme substrate complexes very slowly. Rapid increase in rate of reaction. As temperature increases, the enzyme/substrate molecules have more kinetic energy so form complex molecules more quickly. Optimum temperature enzymes and substrates work at fastest rate. Enzymes begin to be denatured. This happens relatively quickly over about 10ºC. Bonds in the protein structure of enzyme are broken. Active site loses its shape and is unable to fit the substrate. Denaturation is permenant.