3.2-Digestion And Absorption

Question 1

1. What is the definition of digestion? And what is its purpose?

Answer 1

Digestion is the hydrolysis of large molecules to smaller molecules which allows the molecules to be absorbed across cell membranes

Question 2

2. What does it mean if an enzyme is ‘membrane-bound’?

Answer 2

It is attached to the surface membrane of a cell

Question 3

3. What is the benefit of some enzymes being ‘membrane-bound’?

Answer 3

• So that the enzymes are not lost (by being carried out of the small intestine and out of the body by the movement of food through the intestine)
• There is more effective absorption of the products of digestion (because the enzymes are next to transport proteins in the membrane which transport the products into the cells)

Question 4

4. What is the ileum? What type of cells line the ileum?

Answer 4

• The ileum is part of the small intestine

* It is lined by epithelial cells (they are called enterocytes which is a type of epithelial cell)

Question 5

5. How are these epithelial cells adapted for absorption of the products of starch and protein digestion?

Answer 5

• Microvilli provide a large / increased surface area;
o Reject villi on epithelial cells
o Accept large SA:vol ratio
• Many/lots mitochondria produce ATP / release or provide energy (for active transport);
• Carrier proteins for active transport;
• Channel / carrier proteins for facilitated diffusion;
• Co-transport of sodium ions and glucose/amino acids
OR symport / carrier protein for sodium ions and glucose/amino acids;
• Membrane-bound enzymes digest disaccharides / dipeptides;

Question 6

6. What does amylase digest? Which bond is hydrolysed? What is the product of its digestion?

Answer 6

• Digests starch
• Glycosidic bond
• Forms maltose

Question 7

7. What does maltase digest? Which bond is hydrolysed? What is the product of its digestion?

Answer 7

• Digests maltose
• Glycosidic bond
• Forms two glucose molecules

Question 8

8. What does sucrase digest? Which bond is hydrolysed? What is the product of its digestion?

Answer 8

• Digests sucrose
• Glycosidic bond
• Forms glucose and fructose

Question 9

9. What does lactase digest? Which bond is hydrolysed? What is the product of its digestion?

Answer 9

• Digests lactose
• Glycosidic bond
• Forms glucose and galactose

Question 10

10. Describe how monosaccharides are absorbed by the cells lining the ileum

Answer 10

1. Na+/K+ protein pumps sodium ions out of the cell and into the blood (by active transport)
2. So the concentration of Na+ in the epithelial cells decreases
3. So Na+ and glucose diffuse from the lumen into the epithelial cell through the symport protein
4. So the concentration of glucose increases in the epithelial cell
5. So glucose moves from the epithelial cell into the blood by facilitated diffusion

Question 11

11. What does an endopeptidase digest? Which bond is hydrolysed and where is this bond positioned? What is the product of its digestion?

Answer 11

• Proteins
• Hydrolyses a peptide bond
• In the middle of a long polypeptide chain
• Produces shorter polypeptides

Question 12

12. What does an exopeptidase digest? Which bond is hydrolysed and where is this bond positioned? What is the product of its digestion?

Answer 12

• Short polypeptide chains
• Hydrolyses a peptide bond
• On the terminal amino acid
• Produces dipeptides and shorter polypeptide chains

Question 13

13. What does a dipeptidase digest? What is the product of its digestion?

Answer 13

• Digests dipeptides

* single amino acids

Question 14

14. Describe how amino acids are absorbed by the cells lining the ileum

Answer 14

1. Na+/K+ protein pumps sodium ions out of the cell and into the blood (by active transport)
2. So the concentration of Na+ in the epithelial cells decreases
3. So Na+ and amino acids diffuse from the lumen into the epithelial cell through the symport protein
4. So the concentration of amino acids increases in the epithelial cell
5. So amino acids moves from the epithelial cell into the blood by facilitated diffusion

Question 15

15. What does lipase digest? Which bond is hydrplysed? What is the product of its digestion?

Answer 15

• Digests lipids (triglycerides)
• Ester bond
• Forms fatty acids and monoglycerides

Question 16

16. What is bile’s role in lipid hydrolysis?

Answer 16

• Bile salts are used to break up fats into micelles;
• This is called emulsification;
• Which increases the surface area of the lipids exposed to lipase
• To allow lipases to form more E-S complexes with lipid (in a set time);
• Rate of reaction/rate of lipid hydrolysis increases

Question 17

17. What is a micelle? Draw a labelled diagram to show a micelle

Answer 17

An aggregate of lipid molecules within an aqueous solution

Question 18

18. Describe the role of micelles in the absorption of lipids (HELPFUL HINT: use the diagram as you read through the steps)

Answer 18

1. Monoglycerides and fatty acids associate with bile salts to form micelles
2. Micelles release monoglycerides and fatty acids when they come into contact with epithelial cells
3. Monoglycerides & fatty acids diffuse across the phospholipid bilayer and enter epithelial cells
4. Enter the endoplasmic reticulum and are converted back into triglycerides
5. Move to the golgi apparatus
6. Associate with cholesterol and lipoproteins to form chylomicrons
7. Chylomicrons leave epithelial cells by exocytosis
8. Chylomicrons enter the lymphatic capillaries
9. The lymphatic vessels transport the chylomicrons to the blood (lymph joins blood at the subclavian vein)