3.3.3 Digestion and absorbtion

Question 1

How does lipid digestion occur?

Answer 1

• Bile salts emulsify lipids into micelles to increase surface area and solubility in water.

• Lipids/triglycerides are hydrolysed by lipases to form fatty acids and monoglycerides.

• Micelles contain fatty acids, monoglycerides and bile salts.

• They move through the ileum to the epithelium cells.

Question 2

How does lipid absorption occur?

Answer 2

• Micelles contain bile salts and fatty acids/monoglycerides, making them soluble in water.

• Fatty acids/monoglycerides are released to cell/lining of the ileum.

• This maintains a higher concentration of fatty acids/monoglycerides outside the cell, so they are absorbed by simple diffusion.

• Triglycerides are reformed in cells and form chylomicrons.

• The chylomicron vesicles fuse with the cell membrane and are released by exocytosis.

Question 3

How does carbohydrate (starch) digestion occur?

Answer 3

• Amylase in saliva hydrolyses starch by breaking the glycosidic bonds to maltose (alpha glucose
disaccharide).

• Amylase is denatured in the stomach – no carb digestion there.

• Pancreatic amylase is released and further hydrolyses any leftover starch.

• Maltose is hydrolysed to α-glucose by breaking the glycosidic bonds in the ileum by the enzyme maltase which is a membrane-bound enzyme.

• Glucose is absorbed in co-transport.

Question 4

How does protein digestion occur?

Answer 4

• It is the hydrolysis of peptide bonds.

• Exo/Endopeptidases are produced in the stomach.

• Endopeptidases act in the middle of the protein/polypeptide and produce shorter polypeptides, increasing the number of
ends.

• Exopeptidases act at the end of a protein/polypeptide and produce dipeptides.

• Dipeptidases are membrane bound enzymes in the ileum which act on dipeptides and produce single amino acids.

Question 5

Describe the complete digestion of starch by a mammal.
[2 marks]

Answer 5

1. Hydrolysis;
2. (Of) glycosidic bonds;
3. (Starch) to maltose by amylase;
4. (Maltose) to glucose by disaccharidase/maltase;
5. Membrane-bound (disaccharidase/maltase);

Question 6

Compare and contrast protein and lipid digestion.

Answer 6

• Both involve hydrolysis.

• In proteins peptide bonds are hydrolysed and in lipids ester bonds are hydrolysed.

• Exo-, endo- and dipeptidases are used in protein digestion but lipases are used in lipid digestion.

• Bile salts emulsify lipids to form micelles, but proteins do not form micelles.

• Protein digestion results in amino acid monomers, but lipid digestion results in three fatty acids and glycerol.