3.3.2 - 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 by enzymes which allows the molecules to be absorbed across cell membranes

Question 2

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

Answer 2

It is attached to the surface membrane of a cell

Question 3

1. 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

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

Answer 4

• The ileum is the end part of the small intestine
• It is lined by epithelial cells (they are called enterocytes which is a type of epithelial cell)

Question 5

1. 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

1. 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

1. 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

1. 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

1. 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

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

Answer 10

• Sodium-potassium ion pump actively transports 3 sodium ions out of the epithelial cell into the blood and 2 potassium ions into the epithelial cell from the blood
• This reduces the concentration of sodium ions in the cell.
• So sodium ions move into the epithelial cell from the intestinal lumen down their concentration gradient through a symport protein by facilitated diffusion and bring glucose with them (this is called co-transport). Sodium ions move down their concentration gradient, glucose moves AGAINST its concentration gradient.
• The concentration of glucose increases in the epithelial cell
• So glucose moves into the blood from the epithelial cell by facilitated diffusion down the diffusion gradient through a carrier protein

Question 11

1. 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/ increases number of ends

Question 12

1. 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/ amino acids

Question 13

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

Answer 13

• Digests dipeptides
• Produces single amino acids

Question 14

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

Answer 14

• Sodium-potassium ion pump actively transports 3 sodium ions out of the epithelial cell into the blood and 2 potassium ions into the epithelial cell from the blood
• This reduces the concentration of sodium ions in the cell.
• So sodium ions move into the epithelial cell from the intestinal lumen down their concentration gradient through a symport protein by facilitated diffusion and bring amino acid with them (this is called co-transport). Sodium ions move down their concentration gradient, amino acid moves AGAINST its concentration gradient.
• The concentration of amino acids increases in the epithelial cell
• So amino acids moves into the blood from the epithelial cell by facilitated diffusion down the diffusion gradient through a carrier protein

Question 15

1. What does lipase digest? Which bond is hydrolysed? What is the product of its digestion?

Answer 15

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

Question 16

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

Answer 16

• Bile emulsifies fats
   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
• Bile salts join with lipids to form micelles so fatty acids are more soluble in water.
• Bile neutralises stomach acid to form the optimum pH for lipase activity

Question 17

1. 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

1. What is the role of micelles?

Answer 18

• make the fatty acids soluble in water
• transport fatty acids to the lining of the ileum

Question 19

1. Describe the absorption of lipids (include the function of micelles) (HELPFUL HINT: use the diagram as you read through the steps)

Answer 19

1. Monoglycerides and fatty acids associate with bile salts to form micelles
2. Micelles transport fatty acid and glycerol to the lining of the ileum
3. Micelles release monoglycerides and fatty acids when they come into contact with epithelial cells
4. Monoglycerides & fatty acids diffuse across the phospholipid bilayer via simple diffusion and enter epithelial cells
5. They then enter the smooth endoplasmic reticulum and are converted back into triglycerides
6. A vesicle buds off the SER and moves to the golgi apparatus and fuses with it
7. Triglycerides associate with proteins to form lipoprotein before combining with cholesterol to form a chylomicron (which are wrapped in a vesicle – from the golgi apparatus which buds off)
8. Chylomicrons leave epithelial cells by exocytosis
9. Chylomicrons enter the lymphatic capillaries
10. The lymphatic vessels transport the chylomicrons to the blood (lymph joins blood at the subclavian vein)