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| Mechanism of Digestion |
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| Once the food is ingested into the mouth the action of the upper and the lower jaw crushes and grinds the food. This also stimulates the production of saliva. Saliva is thoroughly mixed with the food with the action of the jaws, teeth, inner walls of cheeks and tongue. The food is formed into bolus (a soft ball). Due to salivary amylase, partial digestion of starch takes place here. |
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| The bolus is then passed into the pharynx by an involuntary swallowing action. It is made easier by the presence of mucus along the walls of the alimentary canal. |
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| Bolus next moves into the oesophagus. It moves along the oesophagus by contraction of the portion of the oesophagus above the bolus and expansion of the portion below. Thus it is pushed forward and then the next region is contracted and expanded. This alternate wave of contraction and expansion is called peristalsis. It is by this process that the food moves entirely through the alimentary canal. No digestion takes place in the oesophagus. |
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| The food then moves into the stomach. The opening is controlled by the cardiac sphincter which is a muscular ring. The stomach opens only in response to swallowing. Here the proteins are broken down into proteoses and peptones by the action of pepsin and the soluble milk protein caesinogen is degraded into insoluble casein in children. The bolus thus partially digested is now called the chyme. Chyme is then passed into the small intestine by the opening of the pyloric sphincter due to the muscular contraction of the lower region of the stomach. |
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| The first portion of the small intestine, the duodenum gets the pancreatic and the bile juice. The chyme is thus acted upon by the enzymes and salts present in these two secretions. The starch is converted into maltose by the pancreatic amylase and the remaining proteins, proteoses and peptones into peptides and amino acids by trypsin. The bile juice emulsifies the fats and then converts them into fatty acids and glycerol by the action of lipase. |
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| In jejunum, there is no digestion. In ileum, the food is completely broken down into the simplest of forms - proteins into amino acids and carbohydrates into monosaccharides. This digested mass is now called the chyle and it is in a liquid form. |
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| Since the food is now broken down into the soluble form, the intestinal walls of ileum are able to absorb the nutrients and also water. Due to its large surface area, thin wall lining the cavity and large number of finger-like projections, the villi, maximum absorption takes place in this portion of the alimentary canal. The food that is absorbed passes through the epithelial cells of the villi and then into the blood of the capillaries in the villi. From the capillaries, the fat soluble substances pass into the lymph. These substances are then transported through-out the body by the lymphatic system which drains them into the blood near the heart. |
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| The undigested food then passes into the large intestine where it is temporarily stored. More water is absorbed from the waste matter to form solid faeces. |
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| The faeces are egested out of the anus by the movements of the colon. |
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| The food that is digested is absorbed into the blood and lymphatic systems. The lymphatic system carries the digested fats as fatty acids and glycerol into the heart. The digested food in the blood stream reaches the liver. Here, the glucose is stored as glycogen to be used when the need arises. Cholesterol is manufactured from some fatty acids. The amino acids are used to form the required proteins. This process produces ammonia as a waste. This ammonia is converted into harmful urea. The latter is then transported through the blood stream to the kidney from where it is excreted as urine. |
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| Boil egg white in distilled water till a cloudy suspension is obtained. Cool it and divide the mixture into five equal portions in separate test tubes. Label them A to E. |
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| Add different substances to each of the five tubes as shown in the table below and explain the observations- |
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| Rinse your mouth 2 to 3 times to remove any food particles and collect saliva in three test tubes labelled A, B and C. To A and B each add half the quantity of 2% starch solution. To C add boiled saliva. Leave them for 5 minutes. |
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| At the end of five minutes, |
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| a. Addition of iodine to A gives no blue colour. Why? |
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| The action of salivary enzymes on starch converts it to sugar and hence no starch in the test-tube. Thus no blue colour. |
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| b. Addition of Benedict's reagent to B gives red precipitate. Why? |
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| Benedict's reagent gives red colour precipitate with sugar. In B also the salivary enzyme has converted starch to sugar and the latter gives the red precipitate. |
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| c. Additon of iodine to one portion of C gives blue colour and addition of Benedict's reagent to the other portion of C does not produce any red precipitate. Why? |
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| C consists of contained boiled saliva. Heating denatures the enzyme present in saliva. Hence there is no action on the starch. The solution therefore gives blue-black colour with iodine. Since sugar has not been formed, there is no red precipitate with Benedict's solution. |
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