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Digestion Review 2

This is a summary of the digestion of a meal including carbohydrates, proteins and fats. It is intended to give a brief overview of the digestive system.

Mouth

Proteins, Fats and Carbohydrates are mechanically digested by chewing
Carbohydrates are chemically digested into polysaccharides and disaccharides by salivary amylase, an enzyme which is secreted from the serous cells of the parotid and submandibular glands

Stomach

Proteins are denatured (unfolded) by HCl, which is secreted from the parietal cells in the lining of the stomach; HCl also provides an extremely acidic environment which kills most of the bacteria ingested with food
Proteins are chemically degraded into shorter polypeptides by the enzyme pepsinogen; Pepsinogen is secreted from the chief cells in the lining of the stomach; it is secreted in its inactive form pepsin, which is then activated by the acid in the stomach

Small Intestine

Fats are emulsified by bile salts which are produced in the liver; emulsification breaks up fats into tiny particles making them more accessible for digestive enzymes
Bicarbonate ions from the pancreas are secreted into the duodenum to neutralize the stomach acid and make the pH slightly alkaline

Proteins are chemically digested further into smaller polypeptides and amino acids by the enzymes trypsin and chymotrypsin, which are secreted from the pancreas; these enzymes, like pepsin, are released in their inactive forms trypsinogen and chymotrypsinogen, which are activated once they enter the small intestine; Trypsin is activated by enterokinase, an intestinal brush border enzyme, and chymotrypsin is in turn activated by trypsin

Fats are chemically digested into glycerol and fatty acids by lipase, another enzyme secreted by the pancreas
Polysaccharides and Disaccharides are chemically digested further into simple sugars (glucose, fructose and galactose) by the enzymes amylase, maltase, sucrase, and lactase
Virtually all absorption of nutrients (simple sugars, amino acids, glycerol and fatty acids) takes place along the small intestine

Large Intestine

The mixture entering the large intestine has very few nutrients so no further food breakdown takes place, with the exception of digestion done by the enteric bacteria
Some vitamins are reabsorbed and most of the remaining water is reclaimed
The remaining material is eliminated from the body

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