4. proteins and enzymes epqs redo

Question 1

Use Figure 1, Figure 2 and your knowledge of enzyme action to explain why the
arrows point in opposite directions. (4)

Answer 1

1. (Figure 1 shows) DNA has antiparallel strands / described;
2. (Figure 1 shows) shape of the nucleotides is different / nucleotides
   aligned differently;
3. Enzymes have active sites with specific shape;
4. Only substrates with complementary shape / only the 3’ end can bind
   with active site of enzyme / active site of DNA polymerase.

Question 2

Calculate the rate of reaction at 25 °C. (2)

Answer 2

1. 2.5 / 0.04;

1 mark for correct value
2. g dm–3 minute–1 / g dm–3 s–1;

Question 3

Describe and explain the differences between the two curves. (5)

Answer 3

1. Initial rate of reaction faster at 37 °C;
2. Because more kinetic energy;
3. So more E–S collisions / more E–S complexes formed;
4. Graph reaches plateau at 37 °C;
5. Because all substrate used up.

Question 4

A principle of homeostasis is the maintenance of a constant internal environment. An
increase in the concentration of carbon dioxide would change the internal environment
and blood pH.
Explain the importance of maintaining a constant blood pH. (3)

Answer 4

1. Named protein / enzyme (in blood) sensitive to / affected by change in
   pH;

Accept converse for MP2 and MP3.
Named example should be a protein that might be affected
(by change in pH) eg haemoglobin, carrier protein in plasma
membrane.
Accept ‘change in H+

concentration’ for ‘change in pH’.
2. (Resultant) change of charge / shape / tertiary structure;

The change in charge idea relates to the enzyme / protein
and not the blood (plasma) or red blood cells.
‘Denaturation’ alone is insufficient.
3. Described effect on named protein or enzyme.
e.g. less oxygen binds with haemoglobin / less transport across membranes /
fewer substrates can fit active site / fewer enzyme-substrate complexes.
Idea of ‘less’ or ‘fewer’ required. Ignore suggestion of ‘no’ or
‘none’.

Question 5

Maltose is hydrolysed by the enzyme maltase.
Explain why maltase catalyses only this reaction. (3)

Answer 5

1. Active site (of enzyme) has (specific) shape / tertiary structure / active site complementary to substrate / maltose;
2. (Only) maltose can bind / fit;
   Accept “substrate” for “maltose”
3. To form enzyme substrate complex.