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| Other Digestive Juices |
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| Gastric juice |
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| It is secreted by the secretory glands of the stomach wall. They are called the gastric glands. |
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| The gastric glands are formed by the infoldings of the epithelium and open into the lumen through the gastric pits. There are two types of cells that line the gastric glands - parietal cells and chief cells. |
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| Parietal cells secrete dilute solution of hydrochloric acid. |
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| Chief cells secrete the inactive enzymes collectively called zymogens. The inactive enzymes present in the gastric juice are pepsinogen and prorenin. They are activated by the HCl and form pepsin and renin respectively. Both are proteases acting on proteins. Pepsin hydrolyses proteins into peptones, smaller chains of amino acids. Pepsin can digest many types of proteins including collagen of bones but not proteins like keratin present in hair, skin, horn or nails. |
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| Renin is present only in young children. It digests the milk protein, casein. Casein is hydrolysed into paracasein which gets precipitated as insoluble calcium paracaseinate. This is called coagulation. Renin secreted by the gastric mucosa of the calf is extracted to make rennet tablets used to commercially curdle milk. |
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| Apart from this, HCl in the gastric juice kills many bacteria, denatures proteins for easy digestion, loosens fibrous and cellular components of the tissue and creates an acidic environment needed for the gastric enzymes to function. |
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| Intestinal juice is secreted by the ileum region of the small intestine. Here the final stage of the digestion of different kinds of food takes place. It contains maltase to convert maltose to glucose, sucrase to convert sucrose to glucose and fructose, lactase to convert lactose to glucose and galactose and erepsin to convert the proteoses, peptones and peptides into amino acids. It also contains enteropeptidases that activate the pancreatic proteases like the trypsin. |
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| Two or more amino acids join together to form peptides. Many peptides together form polypeptides which in turn form proteins. Proteoses and peptones are compounds that are structurally intermediate between proteins and peptides |
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