Digestion in the small intestine

1. Digestion in the small intestine Todd and Miho

2. The structure of the small intestine • The small intestine, despite its name, is the longest part of the human alimentary canal. • The small intestine can be up to 5m long. • It is called the small intestine because of its relative small diameter compared with the other parts of the alimentary canal. • The small intestine is made up of 3 regions: • Duodenum • Jejunum • ileum

3. Structure of the duodenum • The duodenum makes up the first 25 cm of the small intestine. • The duodenum is C shaped. • The duodenum begins at the pylonic sphincter. • The duodenum is the most fixed part of the small intestine. • The bile duct and the pancreatic duct open into the duodenum.

4. Structure of the jejunum • The jejunum is the central part of the 3 regions in the small intestine. • The jejunum in a fully grown adult is normally over 2 metres long. • The ph in the jejunum is between 7-8 (slightly alkaline). • The jejunum is suspended by a mesentary, giving the bowel more mobility within the abdomen. • A mesentary connects an internal organ ( like the small intestine )to the abdominal wall. • The inner surface of the jejunum is a mucus membrane covered in villi. This increases surface area of tissue so more nutrients can be absorbed from the gut.

5. Structure of the ileum • The ileum is the last part of small intestine. • The ileum wall contains numerous villi, each villi contain lots of microvilli. • The function of the ileum is to absorb vitamin B12 and bile salts. • The ileum is slightly alkaline. • The large surface area also helps to absorb enzyme molecules and products of digestion. • The cells lining the ileum contain the carbohydrase and protease enzymes needed for digesting carbohydrates and proteins. • These enzymes are located in the cytoplasm of epithelial cells. • The faeces is then moved out of the small intestine into the colon.

6. Digestion in the small intestine The various enzymes in the pancreatic juice act on their substrates in the ileum. • Protein molecules are hydrolysed first to peptides and then amino acids by tripsin, chymotripsin and carboxypeptidase. • Lipids are hydrolysed to fatty acids and glycerol by lipases. • Starch is broken down to maltose be amylase

7. Digestion on the epithelial cells of the villi The enzymes do not only act in the lumen of the small intestine. Overall it appears that only a very small proportion of digestion takes place in the lumen. Many enzymes act on their substrates while attached to the surface of the epithelial cells of the villi. E.g. Amylase becomes attached to the surface of these cells where they act on the carbohydrate chains of glycoproteins in their plasma membranes. This way ensures that the products of digestion are concentrated next to the cells that will absorb them.

8. Digestion in the epithelial cells of the villi Another source of enzymes in the ileum is the epithelial cells of the villi. Their plasma membranes contain several different enzymes, held in the membrane where their active site is exposed to the outside of the cell. These enzymes include exopeptidases, (converts peptides to amino acids) and carbohydrases such as maltose (converts disaccharides to monosaccharides.)

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