KINESIOL 1Y03 Lecture Notes - Lecture 33: Methionine, Vitamin K, Chymotrypsin
22 Jun 2018
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Intestinal absorption: normal & abnormal
Mucosal surface area amplified by folds of Kerckring (x3), villi (x10), microvilli (x20) so total
surface area = 200m2
carbohydrates are mainly absorbed proximally (in the dudoenum), lipids and protein are
absorbed more distally (jejunum + ileum – 5% of fats and 20% protein not absorbed by colon
iron and calcium are absorbed proximally whilst bile salts and B12 (cobalamin) are
absorbed by specialized structures in the ileum
villus tip eneterocytes express genes for active transport:
villi have: abundant brush border hydrolases, high nutrient transport, net water and ion
absorption but low passive permeability
crypts have: sparse brush border hydrloases, low nutrient transport, net water and ion
secretion and high passive permeability
Epithelial cells are continually renewed with s 2-3 days turnover time from the bottom of
the crypt to the tip of the villus
crypt stem cells produce cells at the same rate at which cells are lost
brush border c.f. basolateral membrane of tip enterocytes are functionally distinct to allow
vectoral transport:
brush border has: glycocalyx, complex cytoskeleton and specific exchangers and channels
basolateral membrane has: simple cytoskeleton and different exchangers and ion pumps
Digestion in the stomach
Vagal activity (via Ach), gastric distension, gastrin and histamine cause:
1) parietal cells to secrete IF and H+
2) chief cells to secrete pepsinogen 1 and 2
3) fundic cells to secrete gastric lipase
Digestion of lipids N.B. lecithin = phosphatidylcholine
1) Gastric lipase carries out 20-30% of total triglyceride (TG) digestion
2) Jejunal lipolysis: pancreatic lipases:
-colipase lipase breaks down TG FA + MG
-phospholipase A2 breaks down phosholipids FA and lysolecithin
-cholesterol esterase breaks down cholesterol FA and cholesterol
-N.B. micelles have hydrophilic –OH group on outside and lipophilic parts in the inner
of the lipid bilayers. There are lipids e.g. FA, MG, cholesterol and PL that diffuse in
and are carried in the centre of the micelles
-Micelles carry the lipids to the brush border by diffusion and bind specific
transporters allowing lipids to move into enterocytes
-Inside gut enterocytes lipids are RE-ESTERIFIED i.e. FAs are reinstated
-Lipids vesicles come together fatty acid binding protein (FABP) and microsomal
triglyceride transport protein (MTTP) which carries them to the golgi. Here they
combine with apoproteins from the ER forming a secretory vesicle containing
chylomicrons and VLDL
-The enterocytes only form chylomicrons and VLDL (other forms of lipids e.g. LDL and
HDL are synthesized elsewhere (muscle / liver)
find more resources at oneclass.com
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Document Summary
Mucosal surface area amplified by folds of kerckring (x3), villi (x10), microvilli (x20) so total surface area = 200m2. Carbohydrates are mainly absorbed proximally (in the dudoenum), lipids and protein are absorbed more distally (jejunum + ileum 5% of fats and 20% protein not absorbed by colon. Iron and calcium are absorbed proximally whilst bile salts and b12 (cobalamin) are absorbed by specialized structures in the ileum. Epithelial cells are continually renewed with s 2-3 days turnover time from the bottom of the crypt to the tip of the villus. Crypt stem cells produce cells at the same rate at which cells are lost. Brush border c. f. basolateral membrane of tip enterocytes are functionally distinct to allow vectoral transport: brush border has: glycocalyx, complex cytoskeleton and specific exchangers and channels basolateral membrane has: simple cytoskeleton and different exchangers and ion pumps.
