o
Monosaccharides are formed by single saccharide
molecules.
o
We normally talk about glucose, fructose &
galactose as monosaccharides. There are so many except these three.
o
When we talk about starches, they are digested to
various types of monosaccharides, disaccharides by gut enzymes secreted from
various organs like pancreas.
o
There are same enzymes which digest disaccharides to
monosaccharides. They are placed on brush border of the mucosal cells.
Isomoltase, Moltase, Sucrose.
o
After the digestion final product is monosaccharides
& they are transported across the intestinal mucosal cells to the portal
system by various transport mechanisms.
Glucose/ Galactose
o
The transport of the glucose & galactose is
dependent in Na+ in the intestinal lumen.
o
High concentration of the Na+ facilitates
glucose & galactose transport & low concentration inhibits it.
o
This is because glucose & galactose & Na+
share the same co transport. It is called sodium – dependent glucose
transporter.
o
The members of the family are SGLT 1, SGLT 2.
o
They transport glucose in facilitated diffusion.
o
SGLT 1 is responsible for uptake of dietary glucose
from the gut.
o
The intracellular Na+ concentration is low
in intestinal cells. So that Na+ moves into the cell along its
concentration gradient.
o
Glucose moves in with Na+ & released in
the cell.
o
Then Na+ is transported into the lateral
intracellular spaces & the glucose is transported by GLUT – 2 into the
interstitium & hence to the capillaries.
o
Thus, glucose transport is an example of secondary
active transport. The energy for glucose transport is provided indirectly by
the active transport of Na+ out of the cell.
o
When Na+/ glucose co transporter is congenitally
defective the resulting glucose/galactose mal-absorption causes severe
diarrhoea that is often fatal of glucose & galactose are not promptly
removed from the diet.
o
SGLT – 1 also transports glucose, but fructose
utilizes a different mechanism. Its absorption is independent of Na+
o
It is transported instead by facilitated diffusion
from the intestinal lumen into the enterocytes by GLUT – 5 & out of the
enterocytes into the interstitium by GLUT – 2.
o
Insulin has little effect on intestinal transport of
sugars.
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