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Describe the following processes in our body: 

(i) Coagulation of milk in alimentary canal 

(ii) Digestion of starch in alimentary canal.

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(i) Coagulation of milk in the alimentary canal: 

(a) When the food or milk reaches stomach, the protein digestion starts. 

(b) Pepsin stimulates the digestion of proteins in milk (casein). HCl activates pepsinogen into pepsin. 

(c) It hydrolyses soluble casein into paracasein which gets precipitated as calcium paracaseinate to make solid curd i.e., coagulation of milk. 

(d) There is a milk-coagulating enzyme called rennin which is found in calf gastric juice.

(e) Rennin is secreted as pro-rennin (inactive state) but in presence of HCl it is hydrolysed into active rennin. 

(f) Rennin hydrolyses casein into paracasein leading to milk coagulation. 

(ii) Digestion of starch in the alimentary canal: 

(a) In man and most of the other mammals, digestion starts in mouth or buccal cavity. 

(b) There are three pairs of salivary glands present in the mouth. These glands secrete a alkaline fluid called saliva. 

(c) The crushed food is mixed with saliva and is converted into a pulpy mass called bolus. 

(d) Saliva contains enzyme ptyalin (salivary amylase) which acts on cooked starches (polysaccharides) changing them to the disaccharide maltose. 

(e) Optimum pH for this reaction is 6.8. As soon as the food reaches the stomach, the ptyalin stops because of the acidic, reaction of gastric juice, pH 1.5 to 1.8. The stomach contents entering the duodenum are now called as chyme. 

(f) When acid chyme passes into the small intestine it is mixed first with the pancreatic juice and bile and then with the intestinal juice. 

(g) Pancreatic juice contains enzyme amylase which converts all digestible polysaccharides (starches) not affected by ptyalin to disaccharides (sugars). 

(h) Intestinal juice also contains amylase, sucrase, maltase and lactase. 

(i) These enzymes convert disaccharide into monosaccharides and hence digestion starch is completed.

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