Biology 5090/31 — October/November 2020
Cambridge O-Level · Practical Test · worked solutions for every part, with the mark scheme
Topics Experimental Contexts · Analysis, Conclusions and Evaluation · Observations and Measurements · Planning Experiments and Investigations · Use of Techniques, Apparatus and Materials · Microscopy and Biological Drawing
When tissue from some plants is cut, the cut surface turns brown. This is because enzymes are released which cause reactions between chemicals in the plant and oxygen to produce brown substances.
You are going to investigate the effect of different treatments on the rate at which bananas turn brown.
You are provided with half a banana which was cut several hours ago.
- Cut a slice wide from the banana.
Complete the table to compare the colour of the newly cut surface of the banana with the surface cut several hours ago.
| colour of surface | |
|---|---|
| new cut | old cut |
Answer
newly cut surface: pale yellow / cream / white
old cut surface: brown / pale brown
New cut: pale yellow/cream/white; old cut: brown/pale brown
Walkthrough
The question asks you to compare the colour of a newly cut surface with one cut several hours ago. The new cut has just been made, so little enzyme reaction has happened; it should be pale yellow, cream or white. The old cut has been exposed to oxygen for hours, so the enzyme-catalysed browning reaction has occurred, making it brown or pale brown.
Key Takeaways
Browning is caused by enzymes reacting with oxygen. Fresh plant tissue is pale; exposure to air leads to brown pigments.
Common Mistakes
- Writing 'yellow' for the old cut.
- Describing texture instead of colour.
- Using 'darker' without giving a colour.
Things to Be Careful About
Use colour words the mark scheme accepts. If your banana is slightly different, any pale shade for the new cut and brown for the old cut is fine.
- Label the white tile A, B and C. Cut three fresh slices of banana each approximately wide and place one slice next to each letter.
- Place slice A in the beaker labelled dilute hydrochloric acid (). Ensure that the slice is completely covered by the acid.
Record the time.
time = ______
Answer
time = [candidate's recorded time]
Time recorded (candidate-dependent)
Walkthrough
You need to record the exact time when you start the experiment because all later observations are timed from this moment. The mark is simply for writing the time.
Key Takeaways
Timing is essential for comparing rates of change.
Common Mistakes
- Forgetting to record the time.
- Recording the time after starting.
- Writing the time without units.
Things to Be Careful About
Use a clock or stopwatch, record immediately, and use minutes as the unit.
- Leave slice C untreated on the white tile.
- Cut slice B into small pieces and leave them on the white tile.
- After two minutes use forceps to carefully remove the slice from the hydrochloric acid and return it to the white tile.
Five minutes after the time you recorded, observe the three slices and record their colours in the table below.
| time / minutes | colour | ||
|---|---|---|---|
| slice dipped in (A) | slice cut into small pieces (B) | untreated slice (C) | |
| 5 | |||
| 10 | |||
| 20 |
Answer
Record your observed colours for all three slices at 5, 10 and 20 minutes. Expected pattern:
| time / minutes | A (HCl) | B (small pieces) | C (untreated) |
|---|---|---|---|
| 5 | pale yellow / white | pale yellow / light brown | pale yellow |
| 10 | pale yellow / white | brown | light brown |
| 20 | pale yellow / white or very slightly brown | darkest brown | brown (darker than A) |
B should be darker than C, and C darker than A, at 20 minutes.
Expected: A stays yellow/white; C brown (darker than A); B darkest brown (darker than C)
Walkthrough
At 5, 10 and 20 minutes you observe the three slices and write their colours in the table. The expected pattern is that A (HCl) stays pale because the acid denatures the enzyme, C (untreated) turns brown, and B (small pieces) turns brown fastest and darkest because cutting increases the surface area.
Key Takeaways
Enzyme activity depends on pH and surface area. More surface area means a faster reaction.
Common Mistakes
- Leaving cells blank.
- Writing 'same' instead of a colour.
- Confusing A and C.
- Not recording at the exact times.
Things to Be Careful About
Fill all nine cells. Use consistent colour terms. Compare at the same time. Note that B should be darker than C, and C darker than A.
Measure the pH of the upper surfaces of the three slices using universal indicator paper.
Record your results in the table below.
| pH | ||
|---|---|---|
| slice dipped in (A) | slice cut into small pieces (B) | untreated slice (C) |
- 10 minutes after your recorded time, observe the three slices and record their colours in the table at the top of this page.
- 20 minutes after your recorded time, observe the three slices and record their colours in the table at the top of this page.
You should begin Question 2 or 3 while you wait.
After completing the table, continue with Question 1(b), (c) and (d).
Answer
| slice | pH |
|---|---|
| A (dipped in HCl) | 2–4 |
| B (small pieces) | 5–7 |
| C (untreated) | 5–7 |
Record all three pH values from your universal indicator paper.
A: pH 2–4; B and C: pH 5–7
Walkthrough
Use universal indicator paper on the upper surface of each slice and compare the colour with the pH chart. A has been dipped in HCl, so its pH should be low (2–4). B and C are not acid-treated, so their pH should be around 5–7.
Key Takeaways
Universal indicator measures pH. HCl is an acid with a low pH. pH affects enzyme activity.
Common Mistakes
- Not recording all three pH values.
- Writing 'acidic' instead of a number.
- Using the wrong slice.
Things to Be Careful About
Press the paper on the upper surface, read it immediately, and record the numbers. The mark scheme accepts pH 2/3/4 for A and pH 5/6/7 for B and C.
Describe the effect of dilute hydrochloric acid by comparing the results for slices A and C.
______
Answer
The slice dipped in HCl (A) stayed yellow/white, while the untreated slice (C) turned brown, so the HCl stopped/slowed browning.
HCl stopped/slowed browning
Walkthrough
Compare A and C at the same time. A stayed pale, C turned brown. Therefore HCl prevented or slowed browning.
Key Takeaways
Conclusions must be based on comparison of results. Acid inhibits this enzyme-catalysed browning.
Common Mistakes
- Describing colours without drawing a conclusion.
- Saying 'A is lighter' without adding 'so HCl stopped browning'.
Things to Be Careful About
Use 'stopped/slowed browning' or 'prevents colour change'.
Suggest a reason for the result with dilute hydrochloric acid.
______
Answer
The acid denatured the enzyme, so it could no longer catalyse the browning reaction.
Enzyme denatured
Walkthrough
Enzymes have an optimum pH. Strong acid changes the shape of the active site, so the enzyme is denatured and cannot catalyse the reaction.
Key Takeaways
Extreme pH denatures enzymes. Denatured enzymes lose their function.
Common Mistakes
- Saying 'acid kills the enzyme'.
- Saying 'stops the reaction' without using 'denatured'.
Things to Be Careful About
The mark scheme requires the word 'denatured'.
State the effect of cutting up slice B into small pieces by comparing the results with C.
______
Answer
B turned darker brown / browned more quickly than C.
B turns darker brown / browns more quickly than C
Walkthrough
Compare B and C. B was cut into small pieces, so it has more surface area. It turned darker brown or browned more quickly than C.
Key Takeaways
Increasing surface area increases the rate of an enzyme-catalysed reaction.
Common Mistakes
- Saying 'B is darker' without comparing to C.
- Saying 'more browning' without 'than C'.
Things to Be Careful About
Use 'darker' or 'more quickly' and include the comparison with C.
Suggest a reason for this effect in slice B.
______
Answer
Cutting into small pieces increases the surface area, so more enzyme and substrate are exposed and there is more contact with oxygen, speeding up browning.
Increased surface area / more contact with oxygen
Walkthrough
Cutting into small pieces breaks more cells, releasing more enzyme and exposing more substrate. It also increases the surface area in contact with oxygen, so the reaction is faster.
Key Takeaways
Surface area, enzyme availability and oxygen contact all affect the rate of browning.
Common Mistakes
- Giving only 'more pieces' without a mechanism.
- Saying 'more oxygen' without mentioning surface area.
Things to Be Careful About
Any one of the mark scheme reasons scores: increased surface area, more enzyme/substrate available, or increased contact with oxygen.
It can be difficult to describe your observations in this investigation. Suggest a reason why it can be difficult and an improvement to the method to overcome this.
reason = ______
improvement = ______
Answer
reason: the colour change is difficult to assess / subjective
improvement: compare with a colour chart / record over a longer time
Reason: colour change subjective; improvement: compare with a colour chart
Walkthrough
Colour changes are gradual and different people judge colours differently, so observations are subjective. An improvement is to compare with a standard colour chart, or to record over a longer time so differences are clearer.
Key Takeaways
Evaluation involves identifying a limitation and suggesting a matching improvement.
Common Mistakes
- Giving a vague 'human error' or 'to avoid mistakes'.
- Suggesting an improvement not linked to the reason.
Things to Be Careful About
Pair the improvement with the reason. The mark scheme accepts 'colour chart' or 'record over a longer time'.
Using a method similar to the one you carried out for this investigation design an experiment to determine the optimum (best) pH for a banana to turn brown.
Answer
- Use freshly cut banana slices of the same thickness (about 1 cm).
- Prepare buffer solutions of different pH values, e.g. pH 2, 4, 6, 8 and 10.
- Place one fresh slice in each buffer, making sure it is completely covered.
- Leave all slices for the same time (e.g. 2 minutes), then remove them with forceps and place them on a white tile.
- Measure and record the time taken for each slice to turn brown, checking at regular intervals.
- Keep all other variables constant: same banana, same slice thickness, same temperature, same time in the solution.
- The pH at which the slice turns brown fastest is the optimum pH for browning.
See working (method with buffers, control variables and measurement of browning time)
Walkthrough
To find the optimum pH, you need to vary pH while keeping everything else constant. Use buffers of different pH, place fresh banana slices in each for the same time, then measure how long each takes to turn brown. The pH with the fastest browning is the optimum.
Key Takeaways
A fair test needs one independent variable (pH), a measurable dependent variable (time to brown), and controlled variables (same banana, thickness, temperature, time).
Common Mistakes
- Using only two pH values.
- Not controlling variables.
- Not removing slices from the acid before observing.
- Not stating how to decide the optimum.
Things to Be Careful About
Include buffers, the same time in solution, measurement of time to brown, and a clear conclusion. Mention safety with acid, such as wearing goggles and handling carefully.
The rest of this paper
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