Page 293 - Biology_F5

Biology for Advanced Level Secondary Schools

sodium hydrogen carbonate (NaHCO ) more time for digestion and absorption to
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and digestive enzymes. Mucus and take place.
sodium hydrogen carbonate are secreted
by Brunner’s glands, which are found in The villi are numerous hair-like projections
the interstitial wall. The role of mucus is to found on the surface of the folds. The
lubricate the interstitial wall and prevent function of the villi is to increase the
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corrosion, whereas sodium hydrogen surface area of the intestinal epithelium
carbonate helps to neutralise the acidic for an effective absorption. Normally in
chyme.
1 mm there can be as many as 40 villi.
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The digestive enzymes produced by the Mucosal epithelium is composed of
epithelial cells of interstitial wall include absorptive cells that cover the villi. The
carbohydrase and proteases. Carbohydrase villi are equipped with a large number of
enzymes, such as amylase converts blood vessels that carry sugars, minerals,
remained starch into maltose, lactase vitamins, and amino acids to the liver
converts lactose to galactose and glucose, for processing. Each villus is supplied
and sucrase converts sucrose into fructose with venules, arterioles, capillaries and
and glucose. Protease (erepsin) enzymes lymphatic capillaries. Moreover, there
convert peptones and peptides into amino are lacteal vessels in the villi, which
acids. Additionally, the ileum performs are important for the transportation of
absorptive role as it absorbs vitamin B , fat soluble substances (fatty acids and
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bile salts and any other nutrients that are glycerol) into lymph vessels.
not absorbed in the jejunum.
The mucosa between the folds contains
The ileum as a site for absorption deep tissues, lined by cells that lead to
Ileum is a site for digestion and absorption a tubular intestinal gland called crypt of
of digested food in the alimentary canal. Lieberkühn. This gland secretes slightly
The absorption is done through diffusion alkaline juice, in response to irritation on
or active transport aided by various the mucosa caused by acidic chyme.
features, including presence of circular
folds, villi, and microvilli. These structures Microvilli are cylindrical extensions of
increase the surface area for absorption of the epithelial mucosa cells. They are
nutrients. Villi are finger-like projections much smaller than the villi, measuring
on the intestinal wall, which are equipped about 1µm. Due to their small size, a
with smaller folds called microvilli on mass of microvilli appears as fine bristle
their surface. The surface occupied by brush called brush borders. The surface
microvilli is called a brush boarder. Folds of microvilli contains enzymes that
are deep ridge-like structures found on the accomplish digestion of proteins and
mucosa and submucosa wall. These folds carbohydrates. The brush borders increase
are important in disrupting the straight the surface area of the plasma membrane
movement, of food into meander-like to increase absorption. Microvilli increase
movement, which ultimately delays food the surface area for absorption (Figure
movement in the ileum, thus availing 5.16). The surface of the villus is lined

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