Mechanism for Nutrient Transport
mikaelhanell at hotmail.com
Thu Mar 20 02:46:38 EST 2003
"Dan Marquez" <dmarquez3 at socal.rr.com> wrote in message news:<AdU9a.46518$aa.13710447 at twister.socal.rr.com>...
> What is the mechanism that a nutrient gets transported from within a small
> intestine to the lymph or bloodstream? Are there pores of limited size?
> Once the nutrient is in the lymph or bloodstream, how does a "hungry" cell
> "grab" a desired nutrient? What makes the immune system think a nutrient is
> not an antigen?
The first step is that proteins, fats and carbohydrates are broken
down into smaller units that can be absored, the process of digestion.
This is made by enzymes and happen in the mouth (carbohydrate),
stomach (proteins and fats) and in the small intestine where some of
te enzymes is from the pancreas and other is bound to the luminal
membranes in the small intestine. The endproduct of the digestion is
Hexoses, pentoses (simple sugars), amino acids, di and tripeptides,
and free fatty acids.
The next step is called absorption and is the process by witch the
nutrient is taken from the intestine to the bloodstream (sugars,
aminoacids and free fatty acids whit less than 10-12 carbon atoms) or
the lymph other fatty acids.
The carbohydrates is taken into the cells lining the intestine by
three different mechanisms. 1. Glucose and galactose is cotransported
with sodium that move into the cells because it's concentration is
lower inside the cells. 22. Fructose is taken in by transporters (Glut
5) in the cells membrane that uses energi to take the fructose in. 3.
Pentoses is taken in by simple diffusion. From the cells to the
bloodstream the sugars uses another transporter, Glut 2.
Aminoacids and peptides i stransported in a simmiliar way as the
sugars. The aminoacids have several transport systems and the majority
is cotransporters with sodium. The di and tripeptides require protons
instead of sodium. In the cells is the peptids broken down into
aminoacids and these and the ones absorbed is realesed into the
The free fatty acids is taken in to the cells by diffusion and when
they are inside the cells the ones containing more than 10-12 carbons
are esterified into triglycerides and coated whit a layer of protein,
cholesterol and phospholipids to form what is known as chylomicrons
and enter the lympha.
The tansporters are closed when nothing is bound to them, and things
that should be transported have tofit into the transported have to fit
like a hand in a glove. When this thing is bound the transporter
change its configuration so that the bound substans is transported
trough the membrane.
The "hungry" cell is taken up glucose by another tranporter, Glut
1-Glut5. In the muscels and adipose tissue it is Glut 4 that take up
glucose and there are a pool of free glut 4 in the cytoplasm in these
cells. When insulin is realesed as an answer too high levels of
glucose in the blood, the insulin bind to receptors at the muscle and
adipose tissue and in response to that the pool of Glut 4 move to the
The answer to why the immunsysten doesn't think that a nutrient is a
antigen is becouse it is broken down into free fatty acids, amino
acids and simple sugars.
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