3.3.3 - DIGESTION & ABSORPTION

Question 1

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

Answer 1

1. micelles contain bile salts and fatty acids & glycerol
2. they make fatty acids/glycerol more soluble in water
   OR
   they bring/release/carry fatty acids & glycerol to the cells lining the ileum
   OR
   they maintain higher concentration of fatty acids & glycerol to the cells lining the ileum
3. fatty acids & glycerol are absorbed by (simple) diffusion
4. triglycerides are reformed in cells
5. vesicles move to cell membrane

Question 2

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

Answer 2

1. generates a concentration/diffusion gradient for Na+ from ileum into cell
2. Na+ moving in by co-transport/facilitated diffusion, brings glucose with it

Question 3

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

Answer 3

1. micelles contain bile salts and fatty acids
2. make the fatty acids more soluble in water
3. carry fatty acids to the lining of the ileum
4. maintain higher concentration of fatty acids to the lining of the ileum
5. fatty acids absorbed by simple diffusion

Question 4

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

Answer 4

1. reference to hydrolysis of peptide bonds
2. endopeptidases act in the middle of a protein/polypeptide (produces short polypeptides)
3. exopeptidases act at the end of a protein/polypeptide (produces dipeptides)
4. dipeptidases act on dipeptides

Question 5

name the process by which fatty acids and glycerol enter the intestinal epithelial cell (1)

Answer 5

diffusion

Question 6

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

Answer 6

1. sodium ions actively transported from ileum cells to blood
2. maintains diffusion gradient for sodium to enter cells from gut (and with it, glucose)
3. glucose enters by facilitated diffusion with sodium ions

Question 7

maltose is hydrolysed by the enzyme maltase.

explain why maltase catalyses only this reaction. (3)

Answer 7

1. active site of maltase has a specific shape/tertiary structure that is complementary to the maltose
2. only maltose can bind to it
3. to form the enzyme-substrate complex

Question 8

describe how proteins are digested in the human gut (4)

Answer 8

1. hydrolysis of peptide bonds
2. endopeptidases break polypeptides into smaller peptide chains
3. exopeptidases remove terminal amino acids
4. dipeptidases hydrolyse dipeptides into amino acids

Question 9

mammals have some cells that produce extracellular proteases. they also have cells with membrane-bound dipeptidases.

describe the action of these membrane-bound dipeptidases and explain their importance (2)

Answer 9

1. hydrolyse peptide bonds to release amino acids
2. amino acids can cross cell membrane OR dipeptides cannot cross the membrane

Question 10

what is digestion? (2)

Answer 10

1. hydrolysis of
2. large insoluble substances into smaller soluble substances

Question 11

suggest why endocellulases and exocellulases act at different places on cellulose molecules (2)

Answer 11

1. active sites are different shapes
2. so different ESCs are formed OR so complementary to different parts of the substrate

Question 12

describe the mechanism for the absorption of amino acids in the ileum (4)

Answer 12

1. facilitated diffusion of amino acid into cell when higher concentration in lumen
2. co-transport
3. sodium ions actively transported from cell to blood/capillary/tissue fluid
4. creating sodium ion concentration/diffusion gradient
5. facilitated diffusion of amino acid into blood/capillary

Question 13

explain the advantages of lipid droplet and micelle formation (3)

Answer 13

1. droplets increase SA for lipase action
2. so faster hydrolysis/digestion of lipids
3. micelles carry fatty acids and glycerol through membrane to intestinal epithelial cell

Question 14

some proteases are secreted as extracellular enzymes by bacteria.

suggest one advantage to a bacterium of secreting an extracellular protease in its natural environment.

explain your answer (2)

Answer 14

1. to digest protein
2. so they can absorb amino acids for growth/reproduction/protein synthesis OR so they can destroy a toxic substance/protein

Question 15

suggest and explain why the combined actions of endopeptidases and exopeptidases are more efficient than exopeptidases on their own (2)

Answer 15

1. endopeptidases hydrolyse internal peptide bonds OR exopeptidases hydrolyse bonds at ends
2. more ends or increase in surface area for exopeptidases

Question 16

describe the complete digestion of starch by a mammal (4)

Answer 16

1. hydrolysis
2. of glycosidic bonds
3. starch to maltose by amylase
4. maltose to glucose by maltase
5. membrane bound maltase

Question 17

name the process by which fatty acids and glycerol enter the intestinal epithelial cell (1)

Answer 17

diffusion

Question 18

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

Answer 18

1. modified/processes triglycerides
2. combines triglycerides with proteins
3. packaged for release/forms vesicles

Question 19

following digestion and absorption of food, the undigested remains are processed to form faeces in the parts of the intestine below the ileum.

the faeces of people with constipation are dry and hard. constipation can be treated by drinking lactulose. lactulose is soluble, but is not digested or absorbed in the human intestine.

use your knowledge of water potential to suggest why lactulose can be used to help people suffering from constipation (2)

Answer 19

1. lactulose lowers the WP of faeces/intestine/contents of intestine
2. water enters due to osmosis and softens the faeces

Question 20

describe the hydrolysis reactions involved in the digestion of triglycerides. do not write about the activity of lipase (2)

Answer 20

1. breaking of ester bonds
2. by addition of water

Question 21

all mammals produce a lipase called CEL which digests triglycerides, it is activated by bile salts binding to the enzyme.

describe two other functions of bile salts (2)

Answer 21

1. emulsify fats/lipids
2. increases surface area of lipid/fat for increased lipase activity
3. form micelles

Question 22

some hospital patients suffer from diarrhoea caused by infection with the bacterium C. difficile. This bacteria releases toxins which cause diarrhoea.

the toxins damage the cells lining the ileum, causing them to lose their microvilli.

this damage reduced the absorption of the products of digestion and reduced the absorption of water, resulting in diarrhoea.

explain why the damage to the cells lining the ileum reduces the absorption of the products of digestion and why this reduces absorption of water (3)

Answer 22

1. reduced surface area OR fewer co-transport/carrier/channel proteins
2. decreases water potential in the ileum
3. so less water absorbed/moves out of ileum into cells by osmosis

Question 23

to be used as passive immunity treatment, the anti-toxin antibody would be injected. if it was given by mouth, it would be digested.

describe how the anti-toxin antibody would be digested (3)

Answer 23

1. peptide bonds hydrolysed
2. endopeptidases break internal peptide bonds
3. exopeptidases break terminal peptide bonds
4. membrane-bound dipeptidases break dipeptides to amino acids