Digestion and absorption

Question 1

What is digestion?

Answer 1

When large biological molecules are hydrolysed into smaller molecules that can be absorbed across cell membranes

Question 2

What enzymes hydrolyse carbohydrates and what do they hydrolyse them into?

Answer 2

Amylase, membrane-bound disaccharidases, into monosaccharides

Question 3

How are carbohydrates digested?

Answer 3

1. Saliva in the mouth has salivary amylase which hydrolyses starch into maltose
2. Food enters the stomach, acid denatures amylase so no further hydrolysis of starch into maltose
3. Food enters small intestine, pancreas secretes pancreatic juices which contain pancreatic enzymes which hydrolyse remaining starch into maltose
4. Food passes along the ileum, maltase (membrane-bound disaccharide) hydrolyses maltose into alpha-glucose (monosaccharide)

Question 4

What are the other 2 common disaccharides that are hydrolysed and what are they hydrolysed into?

Answer 4

Sucrose - sucrase hydrolyses the single glycosidic bond into the 2 monosaccharides glucose and fructose

Lactose - lactase hydrolyses the single glycosidic bond into the 2 monosaccharides glucose and galactose

Question 5

What enzymes hydrolyse proteins?

Answer 5

Endopeptidases - hydrolyse peptide bonds between amino acids in the middle of a protein

Exopeptidases - hydrolyse peptide bonds between amino acids in the middle of a protein

Membrane-bound dipeptidases - hydrolyse peptide bonds between 2 amino acids

Question 6

What enzymes hydrolyse lipids, where are these enzymes found?

Answer 6

Lipases, found in pancreas

Question 7

How are lipids hydrolysed?

Answer 7

Lipases hydrolyse the ester bond found in triglycerides to form fatty acids and monoglycerides (glycerol with singe fatty acid molecule). Bile salts produced in the liver split up lipids into micelles (tiny droplets) in a process called emulsification which increases the surface area of the lipids so that the action of lipases is sped up.

Question 8

Tell me about the absorption of triglycerides

Answer 8

• The micelles formed from emulsification contain fatty acids and monoglycerides and bile salts
• Micelles carry the fatty acids and monoglycerides to the epithelial cells which creates a high concentration at the cell surface
• Fatty acids and monoglycerides diffuse into epithelial cells by diffusion.
• Once inside they recombine to form triglycerides which are packaged into vesicles and move to the membrane of epithelial cells and are released by exocytosis into the lymph vessels.