AQA AS Biology Unit 1: Absorption in the Small Intestine

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Absorption in the Small Intestine
Glucose is absorbed through the walls of the small intestine.
These are folded and possess finger-like projections called villi.
They have thin walls, lined with epithelial cells, on the other side of which is a blood rich
network of blood capillaries.
The villi considerably increase the surface area of the small intestine and therefore
accelerate the rate of absorption.
Villi are situated at the interface between the lumen of the intestines and the blood and
other tissues inside the body.
They are part of a specialised exchange surface adapted for the absorption of the products
of digestion, such as glucose.
Their properties increase the efficiency of absorption in the following ways:
They increase the surface area for diffusion
They are very thin walled, thus reducing the distance over which diffusion takes
place
They are able to move and so help to maintain a diffusion gradient
They are well supplied with blood vessels so that blood can carry away absorbed
molecules and hence maintain a diffusion gradient
The epithelial cells lining the villi possess microvilli.
These are finger-like projections of the cell-surface membrane.

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The Role of Diffusion in Absorption
Diffusion is the net movement of molecules or ions from a region where they are highly
concentrated to a region where their concentration is lower.
As carbohydrates are being digested continuously, there is normally a greater concentration
of glucose within the small intestine than in the blood.
There is therefore a concentration gradient down which glucose diffuses from inside the
small intestine into the blood.…read more

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The Role of Active Transport in Absorption
Diffusion only results in the concentrations either side of the intestinal epithelium
becoming equal.
This means that not all the available glucose can be absorbed in this way and some may pass
out of the body.
The reason why this does not happen is because glucose is also being absorbed by active
transport.
This means that all the glucose should be absorbed into the blood.…read more

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Sodium ions are actively transported out of the epithelial cells, by the sodium-
potassium pump, into the blood.
This takes place in one type of protein-carrier molecule found in the cell surface
membrane of the epithelial cells.
2. There is now a much higher concentration of sodium ions in the lumen of the
intestine than inside the epithelial cells.
3. The sodium ions diffuse into the epithelial cells down this concentration gradient
through a different type of protein carrier in the cell-surface membrane.…read more

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