6.10 Absorption of the products of digestion

Question 1

Why is the ileum adapted to the function of absorbing the products of digestion

Answer 1

The wall of the ileum is folded and possesses finger like projections called villi

villi have thin walls, lined with epithelial cells on the other side of which is a rich network of blood capillaries. The villi considerably increase the surface area of the ileum and therefore accelerate the rate of absorption

Question 2

Where are villi situated

Answer 2

Between the lumen of the intestine and the blood and other tissues inside the body

Question 3

What are the properties of villi that increase the efficiency of absorption

Answer 3

1. They increase the surface area for diffusion
2. They are very thin walled, thus reducing the distance over which diffusion takes place
3. They contain muscle so are able to move. This helps to maintain diffusion gradients because their movement mixes the contents of the ileum
4. They are well supplied with blood vessels so that blood can carry away absorbed molecules and hence maintain a diffusion gradient
5. The epithelial cells lining the villi possess microvilli. These are finger-like projections of the cell-surface membrane that further increase the surface area for absorption

Question 4

How are amino acids absorbed

Answer 4

Via diffusion and co-transport

Question 5

Describe the absorption of triglercerides

Answer 5

Once formed during digestion, monoglycerides and fatty acids remain in association with the bile salts that initially emulsified the lipid droplets

The structures formed are micelles. The micelles come in contact with epithelial cells lining the villi, here they break down, releasing monoglycerides and fatty acids.

Once inside the epithelial cells, monoglycerides and fatty acids are transported to the endoplasmic reticulum where they are recombined to form triglycerides.

Starting in the ER, and continuing in the Golgi apparatus, the triglycerides associate with cholesterol and lipoproteins to form chylomicrons.

Chylomicrons move out of epithelial cells via exocytosis. They enter lymphatic capillaries called lacteals that are found at the centre of each villus.

From here, the chylomicrons pass, via lymphatic vessels, into the blood system.

The triglycerides in the chylomicrons are hydrolysed by an enzyme in the endothelial cells of blood capillaries from where they diffuse into cells