Topic 3C: Digestion And 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, fatty acids and monoglycerides;
2. Make fatty acids and monoglycerides soluble (in water);
3. Fatty acids and monoglycerides absorbed by diffusion;
4. Triglycerides reformed (in cells);
5. Vesicles move to cell membrane.

Question 2

Explain the advantages of lipid droplet and micelle formation. [3]

Answer 2

1. Droplets increase surface areas;
2. (So) fast(er) hydrolysis (of triglycerides);
3. Micelles carry fatty acids and monoglycerides to (intestinal epithelial) cells

Question 3

How is the Golgi apparatus involved in the absorption of lipids? [3]

Answer 3

1. Modifies triglycerides;
2. Combines triglycerides with proteins;
3. Forms vesicles

Question 4

Describe the role of micelles in the absorption of fats into the cells lining the ileum. [3]

Answer 4

1. Micelles include bile salts and fatty acids;
2. Make fatty acids (more) soluble in water;
3. Carry fatty acids to lining (of the ileum);
4. Fatty acids (absorbed) by diffusion.

Question 5

Describe the complete digestion of starch by a mammal. [4]

Answer 5

1. Hydrolysis;
2. (Of) glycosidic bonds;
3. (Starch) to maltose by amylase;
4. (Maltose) to glucose by disaccharides/maltase;
5. Membrane-bound (disaccharidase/maltase)

Question 6

Describe the role of enzymes in the digestion of proteins in a mammal. [4]

Answer 6

1. Hydrolysis of peptide bonds;
2. Endopeptidase acts in the middle of polypeptide;
3. Exopeptidase acts at the end of polypeptide;
4. Dipeptidase acts on dipeptide.

Question 7

Describe the mechanism for the absorption of amino acids in the ileum. [4]

Answer 7

1. Sodium ions actively transported from cell to blood;
2. Creating sodium ion concentration gradient;
3. Co-transport of amino acids with sodium ions (into cell);
4. Facilitated diffusion of amino acid into blood.

Question 8

Name the three enzymes involved in the hydrolysis of proteins. [1]

Answer 8

1. Endopeptidase, exopeptidase, dipeptidase

Question 9

Suggest and explain why the combined actions of endopeptidases and exopeptidases are more efficient than exopeptidases on their own. [2]

Answer 9

1. Endopeptidases hydrolyse internal (peptide bonds);
2. More ends (for exopeptidases)

Question 10

Describe the structure of glycogen. [2]

Answer 10

1. Polysaccharide of alpha glucose;
2. (Joined by) glycosidic bonds

Question 11

Suggest how glycogen acts as a source of energy. [2]

Answer 11

1. Hydrolysed to (glucose);
2. Glucose used in respiration.

Question 12

Suggest and explain two ways the cell-membranes of epithelial cells may be adapted to allow rapid absorption. [2]

Answer 12

1. Membrane folded so larger surface area;
2. Lots of protein channels for facilitated diffusion;
3. Lots of protein carriers for active transport;
4. Lots of protein (channels/carriers) for co-transport

Question 13

Respiratory inhibitors prevent the absorption of amino acids.

Suggest why. [3]

Answer 13

1. No/less ATP produced;
2. Sodium ions not moved (into/out of cell);
3. No concentration gradient for sodium

Question 14

Explain why amylase is unable to digest cellulose even though starch and cellulose are both polymers of glucose. [3]

Answer 14

1. Amylase has an active site only complementary to starch so can (only) form enzyme-substrate complexes with starch;
2. Starch contains alpha glucose but cellulose contains beta glucose;
3. Starch is helical, cellulose is straight.

Question 15

The concentration of glucose in the blood rises after eating a meal containing carbohydrates. The rise is slower if the carbohydrate is starch rather than sucrose.

Explain why. [3]

Answer 15

1. Starch digested to maltose (by amylase);
2. Maltose digested to glucose (by maltase);
3. Digestion of sucrose is a single step.

Question 16

Suggest one advantage if using a pH meter rather than a pH indicator. [1]

Answer 16

1. Numerical reading / not subjective / gives quantitative data;
2. Greater accuracy

Question 17

Explain why the pH decreases when lipase is added to milk. [1]

Answer 17

1. Fatty acids produced

Question 18

Name the part of a cell that produces enzymes. [1]

Answer 18

Ribosomes

Question 19

Trypsin is an endopeptidase. It is produced in an inactive form inside some of the cells of the pancreas. Suggest the advantage of producing trypsin in an inactive form. [2]

Answer 19

1. Does not digest protein inside cells;
2. So (pancreatic) cell not damaged

Question 20

Explain how competitive inhibitors stop enzymes from working. [3]

Answer 20

1. Inhibitor is a similar shape to the substrate.
2. (Inhibitor) is complementary to the active site;
3. Substrate can’t bind to active site

Question 21

Describe how bacteria divide. [2]

Answer 21

1. Binary fission;
2. Replication of (circular) DNA;
3. Division of cytoplasm to produce 2 daughter cells;
4. Each with single copy of (circular) DNA

Question 22

Suggest and explain one advantage to a bacterium of secreting an extracellular protease in its natural environment. [2]

Answer 22

1. To digest protein;
2. (So) they can absorb amino acids for growth

Question 23

Describe the action of membrane-bound dipeptidases and explain their importance. [2]

Answer 23

1. Hydrolyse (peptide bonds) to release amino acids;
2. Amino acids can cross (cell) membrane

Question 24

Explain how digestion of starch in the gut (small intestine) leads to an increase in the concentration of glucose in the blood. Details of co-transport are not requires. [3]

Answer 24

1. Hydrolysed by enzymes;
2. Produces glucose (in the gut);
3. Small enough to cross the gut walk (into the blood)

Question 25

Describe the difference between the structure of a triglyceride molecule and the structure of a phospholipid molecule. [1]

Answer 25

1. In phospholipids, one fatty acid replace by a phosphate group

Question 26

Describe how you would test for the presence of a lipid in a sample of food. [2]

Answer 26

1. Add ethanol then add water and shake;
2. White/milky emulsion

Question 27

Maltose is hydrolysed by the enzyme maltase.

Explain why maltase only catalyses this reaction. [3]

Answer 27

1. Active site complementary to maltose;
2. (Only) maltose can bind;
3. To form enzyme substrate complex