Digestion in the small intestine

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Digestion in the small intestine
Todd and Miho
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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

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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.

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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.

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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.

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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

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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.

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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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