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Digestion in the
small intestine
Todd and Miho
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
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.
Structure of the jejunum
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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.
Structure of the ileum
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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.
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
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.
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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