3.3.3 Digestion and absorption

Question 1

What is the role of amylase in digestion?

Answer 1

Amylase hydrolyses starch into maltose by breaking glycosidic bonds.

Question 2

What is the role of membrane-bound disaccharidases in digestion?

Answer 2

Membrane-bound disaccharidases hydrolyse disaccharides into monosaccharides by breaking glycosidic bonds.

Question 3

What is the role of lipase in digestion?

Answer 3

Lipase hydrolyses triglycerides into monoglycerides and fatty acids by breaking ester bonds.

Question 4

How do bile salts assist in lipid digestion?

Answer 4

Bile salts emulsify lipids into smaller droplets increasing the surface area for lipase action.

Question 5

What is the role of endopeptidases in protein digestion?

Answer 5

Endopeptidases hydrolyse peptide bonds within a protein molecule to produce smaller polypeptides.

Question 6

What is the role of exopeptidases in protein digestion?

Answer 6

Exopeptidases hydrolyse peptide bonds at the ends of protein molecules releasing single amino acids or dipeptides.

Question 7

What is the role of membrane-bound dipeptidases in protein digestion?

Answer 7

Membrane-bound dipeptidases hydrolyse peptide bonds in dipeptides to release individual amino acids.

Question 8

What type of bond does amylase break?

Answer 8

Amylase breaks glycosidic bonds.

Question 9

What type of bond does lipase break?

Answer 9

Lipase breaks ester bonds.

Question 10

What type of bond do endopeptidases exopeptidases and dipeptidases break?

Answer 10

Endopeptidases exopeptidases and dipeptidases break peptide bonds.

Question 11

Why is emulsification by bile salts important for lipid digestion?

Answer 11

Emulsification increases the surface area of lipids allowing lipase to hydrolyse triglycerides more efficiently.

Question 12

What is the role of co-transport in the absorption of monosaccharides?

Answer 12

Co-transport involves the absorption of glucose or galactose with sodium ions via a sodium-glucose co-transporter protein.

Question 13

What is the role of co-transport in the absorption of amino acids?

Answer 13

Co-transport involves the absorption of amino acids with sodium ions via specific co-transporter proteins in the ileum.

Question 14

How are micelles involved in the absorption of lipids?

Answer 14

Micelles transport monoglycerides and fatty acids to the epithelial cells where they diffuse through the phospholipid bilayer.

Question 15

What happens to lipids after they are absorbed by epithelial cells?

Answer 15

Monoglycerides and fatty acids are recombined into triglycerides packaged into chylomicrons and transported into the lymphatic system.

Question 16

Describe the processes involved in the absorption and transport of
digested lipid molecules from the ileum into lymph vessels. (5)

Answer 16

1. Micelles contain bile salts and fatty acids/monoglycerides;
   Ignore other correct components of micelles
2. Make fatty acids/monoglycerides (more) soluble (in water)
   OR
   Bring/release/carry fatty acids/monoglycerides to cell/lining (of the
   iluem)
   OR
   Maintain high(er) concentration of fatty acids/monoglycerides to
   cell/lining (of the ileum);
   Accept lipid/fat for fatty acid/ monoglyceride
3. Fatty acids/monoglycerides absorbed by diffusion;
   Reject if absorbed by facilitated diffusion
   Ignore if micelles themselves are being absorbed
4. Triglycerides (re)formed (in cells);
   Accept chylomicrons form
5. Vesicles move to cell membrane;
   Accept exocytosis for ‘vesicles move’

Question 17

The movement of Na+ out of the cell allows the absorption of glucose into
the cell lining the ileum.
Explain how. (2)

Answer 17

1. (Maintains/generates) a concentration/diffusion gradient
   for Na+ (from ileum into cell);
   Accept ‘(Maintains/generates) a lower concentration
   of Na+ inside the cell compared with outside the
   cell’.
2. Na+ moving (in) by facilitated diffusion, brings glucose with it
   OR
   Na+ moving (in) by co-transport, brings glucose with it;
   Accept ‘co-transporter’ for ‘co-transport’.

Question 18

Describe the role of micelles in the absorption of fats into the cells lining
the ileum. (3)

Answer 18

1. Micelles include bile salts and fatty acids;
   Ignore other correct components of micelles.
2. Make the fatty acids (more) soluble in water;
   For ‘fatty acids’ accept fats / lipids.
3. Bring/release/carry fatty acids to cell/lining (of the ileum);
   For ‘fatty acids’ accept fats/lipids.
4. Maintain high(er) concentration of fatty acids to cell/lining (of the
   ileum);
5. Fatty acids (absorbed) by diffusion;
   Reject if absorbed by facilitated diffusion
   Ignore if micelles themselves are being absorbed.

Ignore references to monoglycerides

Question 19

Describe the role of enzymes in the digestion of proteins in a mammal. (4)

Answer 19

1. (Reference to) hydrolysis of peptide bonds;
2. Endopeptidase act in the middle of protein/polypeptide
   OR
   Endopeptidase produces short(er) polypeptides
3. Exopeptidases act at end of protein/polypeptide
   OR
   Exopeptidase produces dipeptides/amino acids;
4. Dipeptidase acts on dipeptide/between two amino acids
   OR
   Dipeptidase produces (single) amino acids;

Accept chain/chain of amino acids/peptide for polypeptide

Accept digest/breakdown/ break for ‘act’

Mark points 2, 3 and 4 reject answers where substrate or product is incorrect eg ‘Endopeptidase produces
dipeptides’

Ignore references to source and location of enzymes

Question 20

Explain the advantages of lipid droplet and micelle formation. (3)

Answer 20

1. Droplets increase surface areas (for lipase /
   enzyme action);
2. (So) faster hydrolysis / digestion (of
   triglycerides / lipids);
3. Micelles carry fatty acids and glycerol /
   monoglycerides to / through membrane / to
   (intestinal epithelial) cell;
4. Context is important
5. Reject micelles increase surface area
6. Ignore ‘breakdown’
7. Ignore ‘small enough’
8. Accept description of membrane
9. Reject any movement through membrane
   proteins

Question 21

Suggest how the Golgi apparatus is involved in the absorption of lipids. (3)

Answer 21

1. Modifies / processes triglycerides;
2. Combines triglycerides with proteins;
3. Packaged for release / exocytosis
   OR
   Forms vesicles;

Ignore ‘processes and packages’ unqualified
2. Reject synthesises triglycerides
3. Accept ‘forms / are lipoproteins

Question 22

Cells lining the ileum of mammals absorb the monosaccharide glucose by
co-transport with sodium ions. Explain how. (3)

Answer 22

1. Sodium ions actively transported from ileum cell to blood;
2. Maintains / forms diffusion gradient for sodium to enter cells from gut
   (and with it, glucose);
3. Glucose enters by facilitated diffusion with sodium ions;

Question 23

Absorption - Describe the role of micelles in the absorption of fats into the cells lining the ileum (2)

Answer 23

1. Micelles include bile salts and fatty acids; Ignore other correct components of micelles.
2. Make the fatty acids (more) soluble in water; For ‘fatty acids’ accept fats / lipids.
3. Bring/release/carry fatty acids to cell/lining (of the ileum); For ‘fatty acids’ accept fats/lipids.
4. Maintain high(er) concentration of fatty acids to cell/lining (of the ileum);
5. Fatty acids (absorbed) by diffusion;

Question 24

Describe the action of these membrane-bound dipeptidases and explain their
importance.

Answer 24

1. Hydrolyse (peptide bonds) to release amino
   acids;
2. Amino acids can cross (cell) membrane;
   OR
   Dipeptides cannot cross (cell) membrane;
   OR
   Maintain concentration gradient of amino acids
   for absorption;
   OR
   Ensure (nearly) maximum yield from protein
   breakdown;