Q1.
A student investigated the effect of lipase concentration on the hydrolysis of lipids.
He took a beaker containing a suspension of lipids. He placed a pH probe attached to a
data logger into the beaker. After 5 minutes, he added the lipase solution. The data logger
recorded the pH. The apparatus used is shown in the diagram below.
(a)
The student did not add a buffer to the lipase solution.
Explain why.
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(1)
(b)
Give two variables the student would have controlled in this investigation.
1. _________________________________________________________________
2. _________________________________________________________________
(2)
(c)
Give the suitable control for this investigation.
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(1)
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The data logger recorded the pH. The graph below shows what happened after he added
the lipase solution.
(d)
Draw a tangent on the graph and use it to calculate the rate of change at 5 minutes.
Rate of change at 5 minutes = _________________ pH minute−1
(2)
(e)
Explain the results shown in the graph.
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(2)
(f)
The student repeated the experiment with a higher concentration of lipase solution.
Describe and explain the results you would expect him to get.
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(3)
(Total 11 marks)
Q2.
A technician investigated the effect of temperature on the rate of an enzyme-controlled
reaction. At each temperature, he started the reaction using the same concentration of
substrate.
The following graph shows his results.
Time after start of reaction / minutes
(a)
Give two other factors the technician would have controlled.
1. _________________________________________________________________
2. _________________________________________________________________
(1)
(b)
Draw a tangent on each curve to find the initial rates of reaction.
Use these values to calculate the ratio of the initial rates of reaction at 60 °C : 37 °C.
Show your working.
Ratio = ____________________ :1
(2)
(c)
Explain the difference in the initial rate of reaction at 60 °C and 37 °C.
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(2)
(d)
Explain the difference in the rates of reaction at 60 °C and 37 °C between 20 and 40
minutes.
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(4)
(Total 9 marks)
Q3.
(a)
Describe the induced-fit model of enzyme action.
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(2)
(b)
A scientist investigated the hydrolysis of starch.
He added amylase to a suspension of starch and measured the concentration of
maltose in the reaction mixture at regular intervals.
His results are shown in the graph below.
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Determine the rate of the reaction at 10 minutes.
Show how you obtained your answer.
Rate of reaction ____________________ mg dm−3 min−1
(2)
(c)
Explain the results shown in the graph.
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(2)
(d)
A quantitative Benedict’s test produces a colour whose intensity depends on the
concentration of reducing sugar in a solution. A colorimeter can be used to measure
the intensity of this colour.
The scientist used quantitative Benedict’s tests to produce a calibration curve of
colorimeter reading against concentration of maltose.
Describe how the scientist would have produced the calibration curve and used it to
obtain the results in the graph.
Do not include details of how to perform a Benedict’s test in your answer.
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(3)
(Total 9 marks)
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Mark schemes
Q1.
(a)
Student was measuring change in pH
OR
Buffer would maintain a constant pH.
1 max
(b)
1.
Volume of suspension of lipids;
2.
Concentration of suspension of lipids;
3.
Volume of lipase solution;
4.
Temperature;
2 max
(c)
Boiled lipase solution;
1
(d)
–0.34 = 2 marks
0.34 = 1 mark
2
(e)
1.
Fatty acids produced;
2.
Curve levels off as all substrate used up.
accept the lower pH inactivates / denatures the enzyme
2
(f)
1.
Faster fall in pH and levels off at same point;
2.
More enzyme = substrate complexes formed;
3.
Same amount of fatty acids produced / product
3
[11]
Q2.
(a)
Any two of the following:
Concentration of enzyme
Volume of substrate solution
pH.
Allow same concentration of substrate
1
(b)
Ratio between 5.18:1 and 5.2:1
Initial rates incorrect but correctly used = 1 mark.
Allow 1 mark if rate at:
60°C = 0.83g dm–3 s–1 /49.8g dm–3 minute–1
OR
37°C = 0.16g dm–3 s–1 /9.6g dm–3 minute–1
2
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(c)
At 60 °C:
1.
More kinetic energy;
2.
More E–S complexes formed.
Allow converse for 37 °C
2
(d)
Different times:
1.
Higher temperature / 60 °C causes denaturation of all of enzyme;
Accept converse for 37 °C
2.
Reaction stops (sooner) because shape of active site changed;
Reject if active site on substrate
Different concentrations of product (at 60 °C)
3.
Substrate still available (when enzyme denatured);
4.
But not converted to product.
4
[9]
Q3.
(a)
1.
2.
3.
(before reaction) active site not complementary to/does
not fit substrate;
Shape of active site changes as substrate binds/as
enzyme-substrate complex forms;
Note. Points 1 and 2 may be made in one statement and
‘complementary’ introduced at any point.
Points 1&2 – active site mentioned once applies for both
points
Point 2 – Ignore references to how shape change is caused
Stressing/distorting/bending bonds (in substrate leading to reaction);
2 max
(b)
1.
2.
Tangent to curve drawn;
Tangent drawn at about 10 minutes
Value in range of 8 to 11;
1 mark only for correct answer
2
(c)
1.
(Rate of) increase in concentration of maltose slows as
substrate/starch is used up
OR
High initial rate as plenty of starch/substrate/more E-S complexes;
Reject ref. to amylase being used up
2.
No increase after 25 minutes/at end/levels off because no substrate/starch left;
Accept ‘little’
Ignore references to substrate a limiting factor
2
(d)
1.
2.
Make/use maltose solutions of known/different concentrations
(and carry out quantitative Benedict’s test on each);
(Use colorimeter to) measure colour/colorimeter value of each
solution and plot calibration curve/graph described;
Axes must be correct if axes mentioned, concentration on x-
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3.
axis and colorimeter reading on y-axis
Find concentration of sample from calibration curve;
3
[9]
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Examiner reports
Q3.
In (a), 29% obtained both marks. The idea of induced fit was generally known but often
poorly expressed. Some students wrote about the substrate having the same shape as
the active site, rather than a complementary shape. A minority had the active site on the
substrate.
The rate at 10 minutes in (b) was correctly calculated by 22% of students. These students
drew a tangent to the curve at 10 minutes and used it to calculate rate. 19% of students
obtained one mark after drawing an incorrect tangent, or making a mistake in their
calculation from a correct tangent. The remainder simply divided the y axis number by the
x axis number at 10 days and failed to score.
In (c), many students simply described the curve, rather than explaining. Some attempting
an explanation incorrectly wrote about the enzyme being used up, or all its active sites
being occupied. 13% obtained both marks for writing about a rapid rate at the start
because there is lots of starch and the rate levelling off/falling to zero after 25/30 minutes
as all the starch is used up. Some 43% obtained one mark for making one or other of
these points.
Question (d) was based around skills that students were expected to have developed in
required practical 3; that is to say the production and use of a calibration curve. The
purpose of the required practicals is, generally, to allow students to practise and develop
skills that can then be applied in many different settings. 21% of students obtained one
mark, either for noting that the scientist would need solutions of maltose of known
concentrations, or that a calibration curve would have concentration on the x axis and
colorimeter reading on the y axis. About 12% obtained both of these marks. Only 5% then
went on to (briefly) describe how the calibration curve would be used to determine the
concentrations in the experiment.
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