Biology 5090/32 — October/November 2022

Cambridge O-Level · Practical Test · worked solutions for every part, with the mark scheme

questions

marks

minutes

Topics Analysis, Conclusions and Evaluation · Experimental Contexts · Planning Experiments and Investigations · Observations and Measurements · Microscopy and Biological Drawing · Use of Techniques, Apparatus and Materials

Q118MObservations and MeasurementsAnalysis, Conclusions and EvaluationExperimental ContextsPlanning Experiments and InvestigationsFree sample

Yeast is a microorganism that can use the sugar sucrose to obtain energy for growth. When yeast is added to a sucrose solution it grows, producing bubbles of a gas that cannot escape from the mixture and so the volume of the mixture increases.

You are going to investigate the effect of varying the concentration of sucrose solution on the growth of yeast, by measuring this increase in volume.

You are provided with a 5% sucrose solution, distilled water and four packets which each contain $1\,\text{g}$ of dried yeast.

Label four test-tubes, A, B, C and D.
Add $15\,\text{cm}^3$ of 5% sucrose solution to test-tube A.
Add $9\,\text{cm}^3$ of 5% sucrose solution to test-tube B.
Add $3\,\text{cm}^3$ of 5% sucrose solution to test-tube C.

Use a clean measuring device to add distilled water to the test-tubes.

Add $6\,\text{cm}^3$ of distilled water to test-tube B to make a 3% sucrose solution.
Add $12\,\text{cm}^3$ of distilled water to test-tube C to make a 1% sucrose solution.
Add $15\,\text{cm}^3$ of distilled water to test-tube D.
Mark the side of each test-tube to indicate the level of the contents. This is the starting level for each test-tube.

You are provided with a beaker to use as a water-bath for the test-tubes.

Raise your hand when you are ready to be supplied with hot water for the beaker. Use cold water to adjust the temperature to approximately $45\,^\circ\text{C}$ .

Place test-tubes A, B, C and D in the beaker.

Leave the test-tubes in the water-bath for 5 minutes.

After 5 minutes the temperature of the water-bath should be between $35\,^\circ\text{C}$ and $40\,^\circ\text{C}$ . If necessary use hot or cold water to adjust the temperature.

(a)

(i)

Record the temperature of the water-bath.

temperature = ______ $^\circ\text{C}$

DifficultyEasy

Worked solution

Answer

Any reading between $35$ and $40\,^\circ\text{C}$ (candidate's own recorded value). No unit required.

Final answer

Any value between 35 and 40 °C

Detailed explanation

Walkthrough

The instructions tell you to adjust the water-bath to approximately $45\,^\circ\text{C}$ , then after 5 minutes the temperature should be between $35$ and $40\,^\circ\text{C}$ . You record the actual reading from the thermometer. The mark scheme accepts any value in that range; no unit is required, though writing $^\circ\text{C}$ is good practice.

Key Takeaways

Reading a thermometer and recording the value. In practical work, the expected range is given in the instructions, and any reading within it scores.

Common Mistakes

Recording a temperature outside $35$ – $40\,^\circ\text{C}$ — the mark is not awarded.
The mark scheme states "no unit required", so omitting $^\circ\text{C}$ does not lose the mark.

Things to Be Careful About

Read the thermometer at eye level to avoid parallax error. Record the value you actually see, not the target of $45\,^\circ\text{C}$ .

Techniques used

read a thermometer and record the temperature within the stated range

(ii)

Add the contents of one packet of yeast to each test-tube.
Use the glass rod to stir the yeast in each test-tube to mix it well. Wipe the rod clean between using it in each test-tube.

Record the time. This is your start time.

start time ______

Five minutes after the start time measure the distance from the starting level to the top of the yeast mixture in each test-tube. Record your results in the table.

After 10 minutes from the start time repeat the measurements from the starting level to the top of the yeast mixture for each test-tube. Record your results in the table.

test-tube	percentage sucrose solution	height of yeast mixture / $\text{mm}$ after 5 minutes	height of yeast mixture / $\text{mm}$ after 10 minutes
A	5
B	3
C	1
D	0

DifficultyMedium-Easy

Worked solution

Answer

Start time recorded. Eight measurements recorded in $\text{mm}$ . Expected pattern: for A, B and C the 10-minute height is greater than the 5-minute height; A > C at 10 minutes; C > D at 10 minutes; D is the same at 5 and 10 minutes ( $\pm 5\text{ mm}$ ).

Final answer

See working

Detailed explanation

Walkthrough

Record the start time when the yeast is added. Then measure the height of the yeast mixture from the starting level in each tube after 5 minutes, and again after 10 minutes. That gives 8 measurements. The mark scheme expects a specific pattern: growth continues, so the 10-minute readings exceed the 5-minute readings for A, B and C; more sucrose gives more growth, so A > C at 10 minutes; C > D at 10 minutes; and D (0% sucrose) shows no change between 5 and 10 minutes ( $\pm 5\text{ mm}$ ).

Key Takeaways

Recording results in a table with units; taking repeat measurements at two times; recognising the expected pattern of results.

Common Mistakes

Recording fewer than 8 measurements.
Omitting units — the mark scheme requires $\text{mm}$ .
Not recording the start time.

Things to Be Careful About

The marks are for recording the candidate's own readings consistently and in the correct units. The pattern of results (growth continuing, more sucrose → more growth) is what the scheme checks.

Techniques used

record the start timemeasure the height of the yeast mixture at two times in each tuberecord eight measurements in a table with units

(iii)

Any change in the height of the mixture in each test-tube indicates a change in its volume and so of how much the yeast has grown.

Use your results to describe the effect of sucrose concentration on yeast growth.

DifficultyMedium

Worked solution

Answer

More sucrose (concentration) gives more growth (height / volume / bubbles).
Little or no growth with 0% / no sucrose.
With sucrose, growth between 5 and 10 minutes was greater than between 0 and 5 minutes.

Final answer

See working

Detailed explanation

Walkthrough

The data should show that more sucrose gives more growth — state this first. Second, note that there is little or no growth with 0% sucrose. Third, compare the two time intervals: with sucrose present, growth between 5 and 10 minutes is greater than between 0 and 5 minutes, because the yeast population is growing.

Key Takeaways

Drawing conclusions from data; relating concentration to growth; comparing growth rates over time.

Common Mistakes

Giving only one observation instead of three.
Not mentioning the 0% tube.
Not comparing the two time intervals.

Things to Be Careful About

Each of the three marks is a distinct observation drawn from your own table. Use your data, not a generic statement.

Techniques used

compare growth across the four concentrationsrelate sucrose concentration to yeast growthcompare growth in the two time intervals

(iv)

Suggest an explanation for your results with 0% sucrose solution (distilled water).

DifficultyMedium-Easy

Worked solution

Answer

No growth: there is no sucrose, so no energy for the yeast to grow / respire. (If some growth was seen: a small amount of sucrose was already present in the yeast cells.)

Final answer

No energy for the yeast to grow or respire

Detailed explanation

Walkthrough

With 0% sucrose there is no sugar for the yeast to respire, so there is no energy for growth. If some growth was seen, the explanation is that a small amount of sucrose was already present in the yeast cells or in the dried yeast packet. The mark scheme accepts either, depending on the candidate's result.

Key Takeaways

Respiration requires a substrate (sugar); without it there is no energy for growth.

Common Mistakes

Giving "no food" without linking to energy or respiration.
Giving both explanations when only one matches the candidate's result.

Things to Be Careful About

The answer must match the candidate's own result: if no growth was seen, give the "no energy" explanation; if some growth was seen, give the "sucrose already in the yeast" explanation.

Techniques used

explain the result with no sucrose using energy availability

(b)

Describe two possible sources of error in this investigation and suggest how the method could be improved.

source of error 1 ______

improvement 1 ______

source of error 2 ______

improvement 2 ______

DifficultyMedium

Worked solution

Answer

Source of error 1: Yeast not completely mixed in the solutions.
Improvement 1: Stir for longer, or shake, until dissolved.

Source of error 2: The top of the mixture is not level, so the height is difficult to measure accurately.
Improvement 2: Take the average of the high and low points, or use a graduated test-tube / measuring cylinder.

Final answer

See working

Detailed explanation

Walkthrough

Identify two sources of error in the method and pair each with a specific improvement. Acceptable pairs include: yeast not fully mixed (stir longer or shake); start time not the same for each tube (record the start time for each separately); top of the mixture not level so height hard to measure (average high and low points, or use a graduated test-tube); test-tubes of different sizes (use identical tubes); temperature hard to control (use a thermostatically-controlled water-bath). Each pair is worth 2 marks.

Key Takeaways

Evaluating experimental method; pairing each limitation with a specific improvement; avoiding vague answers.

Common Mistakes

Giving a vague improvement such as "be more careful" — rejected.
Giving an error without a matched improvement.
Naming a source of error that is not a real limitation of this method.

Things to Be Careful About

Each pair must be specific and matched. If the error is "the top of the mixture is not level", the improvement must address that exact problem.

Techniques used

identify sources of error in the methodsuggest a specific improvement for each error

(c)

Yeast uses sucrose to obtain energy for growth.

Suggest a simple method to demonstrate that this process requires enzymes to be present in the yeast.

DifficultyMedium

Worked solution

Answer

Boil / heat a sample of yeast to denature the enzymes.
Add the boiled yeast to sucrose solution.
Observe that there is no growth / no bubbles / no increase in height, compared with unboiled yeast which does grow.

Final answer

See working

Detailed explanation

Walkthrough

To show that the process requires enzymes, use a control in which the enzymes are destroyed. Boil or heat a sample of yeast to denature the enzymes. Add the boiled yeast to sucrose solution. If the boiled yeast shows no growth (no bubbles, no increase in height) while unboiled yeast does grow, this demonstrates that enzymes are required. The three marks are: boiling/heat treatment, enzymes denatured, and no growth in the boiled sample.

Key Takeaways

Using a control to isolate the role of enzymes; the effect of heat in denaturing enzymes.

Common Mistakes

Saying "boil the yeast" without stating what happens to the enzymes (denatured).
Not comparing boiled and unboiled yeast.

Things to Be Careful About

The comparison is essential: you must show that the boiled yeast does not grow while the unboiled yeast does. State that boiling denatures the enzymes.

Techniques used

design a control experimentdenature enzymes by boilingcompare growth of boiled and unboiled yeast

The rest of this paper

2 more questions

Q2Analysis, Conclusions and Evaluation · Experimental Contexts · Planning Experiments and Investigations12M
Q3Microscopy and Biological Drawing · Analysis, Conclusions and Evaluation · Use of Techniques, Apparatus and Materials10M

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