6.10 Absorption of the products of digestion

Question 1

Define lumen

Answer 1

the hollow cavity inside a tubular structure (gut)

Question 2

List and explain the 5 properties of the ileum to increase efficiency of absorption

Answer 2

1. increase the surface area for diffusion
2. very thin walled thus reducing the distance over which diffusion takes place
3. contain muscle and so are able to move. This helps to maintain diffusion gradients because their movement mixes the contents of the ileum, This ensures that as the products of digestion re absorbed from the food adjacent to the villi, new material rich in the products of digestion replaces it
4. they are well supplied with blood vessels so that blood can carry away absorbed molecules hence maintaining a diffusion gradient
5. epithelial cells lining the villi possess microvilli. These finger-like projections of the cell-surface membrane further increase the surface area for absorption.

Question 3

How do you absorb amino acids and monosaccharides?

Answer 3

via co-transport and diffusion

Question 4

How are triglycerides absorbed?

Answer 4

• bile salts emulsify lipid droplets forming micelles
• through the movement of material within the lumen of the ileum, the micelles come into contact with epithelial cells lining the villi of the ileum.
• here the micelles breakdown releasing monoglycerides and fatty acids
• as these are non-polar molecules, they easily diffuse across the cell-surface membrane into the epithelial cells
• once in epithelial cells, monoglycerides and fatty acids are transported to the endoplasmic reticulum where they are recombined to form triglycerides

-starting in endoplasmic reticulum and continuing in Golgi apparatus, the triglycerides associate with cholesterol and lipoproteins to form CHYLOMICRONS
(chylomicrons are special particles adapted for the transport of lipids)

• Chylomicrons move out of epithelial cells via exocytosis
• they enter LYMPHATIC CAPILLARIES called LACTEALS found at the centre of each villus
• chylomicrons pass via lymphatic vessels into blood system. The triglycerides in them are hydrolysed by enzyme in the endothelial cells of blood capillaries from where they diffuse into cells.