Planning investigations: variables, controls and validity - AQA A-Level Biology (Paper 3) Exam Questions

Practise 18 exam-style questions on Planning investigations: variables, controls and validity for AQA A-Level Biology (Paper 3). Write your answer as you would in the exam and get it marked in seconds against the mark scheme, point by point, with the Study Helper on hand if you get stuck.

18 questions
1–7 marks each
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18 questions, marked as you go

Sample questions with model answers

Here are 4 of the questions in this topic, each with a model answer, an explanation of why it earns the marks and the mark scheme an examiner would use.

Question 1

2 marks

A student investigated whether adding nitrate fertiliser to soil affects the number of root nodules on clover plants. Root nodules contain nitrogen-fixing bacteria.

She grew clover plants in pots of soil with added nitrate fertiliser and in pots of the same soil without it. After 8 weeks, she counted the nodules on the roots of 20 plants from each type of soil and calculated the mean number of nodules per plant for each.

Give two reasons why she counted the nodules on 20 plants from each type of soil, rather than on one plant.

Model answer

1. Individual clover plants vary in the number of nodules they have, so a mean from 20 plants is more representative of all the plants in that soil.
2. Anomalous plants can be identified, and their effect on the mean is reduced.

Explanation

Living organisms vary, genetically and because of their surroundings, so a single plant may be untypical. A larger sample gives a mean that is more representative, reduces the effect of anomalies and random error, and allows a statistical test of whether the difference between the two means is significant. 'To make it fair' or 'to make it accurate' on its own does not earn a mark.

Mark scheme

1 mark
Plants vary (in the number of nodules), so a larger sample is more representative (of all the plants grown in that soil)
Max 2. List rule: if more than two reasons are given, each wrong or contradictory extra answer cancels one correct answer. Accept 'reliable' for representative. Accept 'there is variation between plants, e.g. genetic variation'.
1 mark
To reduce the effect of anomalous results / so anomalies can be identified
Accept 'to reduce the effect of an unusual plant on the mean'.
1 mark
So that a statistical test can be carried out / so that she can find whether the difference between the means is significant (or due to chance)
Accept 'so a standard deviation can be calculated'. Accept 'a t-test'.
1 mark
To reduce the effect of random error (in counting) on the mean
Ignore 'to make it more accurate' or 'to make it a fair test' unqualified. Ignore 'so a reliable mean can be calculated' with no reason.

Question 2

3 marks

A student planned to investigate the effect of water temperature on the ventilation rate of a species of small freshwater fish kept in the laboratory. The fish normally live in water between 15 °C and 25 °C.

Her plan:
1. Catch a fish in the home tank and put it in a 100 cm³ beaker of water at 5 °C.
2. Immediately count the movements of its operculum (gill cover) for one minute.
3. Lift the fish out by hand and put it into a beaker of water at 15 °C, then 25 °C, then 35 °C, counting the movements at each temperature.

Suggest three changes to her plan that would reduce harm to the fish.

Model answer

1. Use temperatures within the fish's normal range, for example 15, 18, 21 and 24 °C, rather than 5 °C and 35 °C.
2. Change the temperature of the water gradually with the fish in it, and let the fish acclimatise for a few minutes before counting, instead of lifting it by hand from beaker to beaker.
3. Use a larger, aerated container of water and return the fish to its home tank as soon as the readings are taken, or straight away if it shows signs of distress.

Explanation

AQA expects organisms to be used safely and ethically (apparatus and technique h). Temperatures far outside the fish's normal range, sudden changes, handling, a small volume of water and low oxygen all cause stress or harm. Sudden transfers would also make the results less valid, because a stressed fish ventilates faster. Each change should reduce a different source of harm.

Mark scheme

1 mark
Use a narrower range of temperatures within the fish's normal range / do not use 5 °C or 35 °C
Max 3. List rule: if more than three changes are given, each wrong extra answer cancels one correct answer. Accept 'use temperatures between 15 and 25 °C'.
1 mark
Change the temperature gradually / let the fish acclimatise (for a set time) before counting
Accept 'warm the water slowly with the fish in it' or 'leave the fish for 10 minutes at each temperature before counting'.
1 mark
Do not handle the fish / transfer it in a net or in a container of water / keep it out of water for as little time as possible
Accept 'move the fish in water' or 'change the temperature of the water around the fish instead of moving it'.
1 mark
Use a larger volume / container of water
Accept 'a tank rather than a small beaker'.
1 mark
Aerate the water / keep the oxygen concentration high
Accept 'bubble air through the water (warm water holds less oxygen)'.
1 mark
Stop / return the fish to its tank if it shows signs of distress
Accept 'return the fish to its home tank as soon as the readings are taken'. Ignore 'use fewer fish' unless explained (the plan uses one fish).

Question 3

4 marks

A class planned to find out what proportion of the students can taste a bitter chemical called PTC. The ability to taste PTC is inherited. Each student would put a paper strip containing a small amount of PTC on their tongue and record whether it tasted bitter. The results would be displayed on a poster in the laboratory.

PTC is toxic in large doses.

Give two ethical considerations and two safety precautions for this investigation.

Model answer

Ethical considerations: each student should give informed consent, knowing that the test reveals inherited (genetic) information, and should be free to withdraw at any time; the results on the poster should be anonymous, with no names.

Safety precautions: use commercially prepared strips that contain only a tiny, known amount of PTC, and do not chew or swallow the strip; each student uses a new strip of their own, which is put in the bin afterwards.

Explanation

When people are the subjects of an investigation, they must give informed consent, take part voluntarily with the right to withdraw, and have their data kept confidential; this matters here because tasting ability is genetic information about the student and their relatives. Safety precautions reduce the risk from a hazard: PTC is toxic in large doses, so the dose must be tiny and not swallowed, and strips must not be shared.

Mark scheme

2 marks
Two ethical considerations, 1 mark each, from: informed consent - students are told what is involved (including that the result gives genetic information) and agree to take part; taking part is voluntary / students can withdraw at any time / no pressure to take part; results are kept anonymous / confidential / not displayed with students' names (because they are genetic information about the student and their family)
1 mark for each different consideration, max 2. List rule: if more than two ethical considerations are given, each wrong extra answer cancels one correct answer. A point given under safety may be credited here if it is an ethical point, but never credit the same point twice.
2 marks
Two safety precautions, 1 mark each, from: use commercially prepared strips containing a very small, known amount of PTC; do not swallow or chew the strip / spit it out (and rinse the mouth); each student uses their own new strip (hygiene) / do not share strips; exclude students with allergies to the strip or chemical (or ask about allergies first); wash hands / dispose of used strips in a bin
1 mark for each different precaution, max 2. List rule: if more than two precautions are given, each wrong extra answer cancels one correct answer. Ignore 'wear goggles' and 'wear gloves'. Ignore 'do not eat or drink in the laboratory' unless linked to the strip.

Question 4

6 marks

Discs cut from a leaf float on water because the air spaces in the leaf contain air. If this air is removed and replaced with a solution, the discs sink. When the sunken discs are lit, they photosynthesise, the oxygen they release collects in the air spaces, and the discs float again.

Describe how you could use this method to investigate the effect of carbon dioxide concentration on the rate of photosynthesis.

You are given fresh spinach leaves, a cork borer, a 10 cm³ plastic syringe, 0.50 mol dm⁻³ sodium hydrogencarbonate solution (a source of carbon dioxide), distilled water, a lamp, a light meter and normal laboratory apparatus.

Model answer

1. Dilute the 0.50 mol dm⁻³ sodium hydrogencarbonate solution with distilled water to make 0.05, 0.10, 0.20, 0.30, 0.40 and 0.50 mol dm⁻³ (for example, 2 cm³ of stock with 8 cm³ of water gives 0.10 mol dm⁻³).
2. Use the cork borer to cut discs of the same size from leaves of the same plant, avoiding the large veins.
3. Put 10 discs and some of the first solution into the syringe, block the nozzle and pull the plunger to remove the air from the air spaces; repeat until the discs sink.
4. Put the 10 discs into 20 cm³ of the same solution in a beaker, standing in a water bath at 25 °C.
5. Place the lamp at a fixed distance, checking with the light meter that the light intensity is the same for every concentration, in a darkened room.
6. Switch on the lamp and record the time taken for five of the discs to float; rate = 1 ÷ time.
7. Repeat three times at each concentration with fresh discs and calculate the mean rate for each concentration.

Explanation

A good plan gives the range of the independent variable and how to make it (a dilution series), how the dependent variable is measured (time for a set number of discs to float, converted to a rate as 1 ÷ time), the variables that must be controlled (disc size and source, volume, light intensity, temperature) and how repeats are processed. Light and temperature both affect the rate of photosynthesis, so they must be kept constant when carbon dioxide concentration is the independent variable.

Mark scheme

1 mark
Dilute the 0.50 mol dm⁻³ sodium hydrogencarbonate solution with distilled water to make at least five different concentrations
Max 6. Accept a correct dilution described, e.g. 2 cm³ of 0.50 mol dm⁻³ solution and 8 cm³ of distilled water for 0.10 mol dm⁻³. Accept a stated range of five or more concentrations, e.g. 0.05, 0.10, 0.20, 0.30, 0.40 and 0.50 mol dm⁻³.
1 mark
Cut discs of the same size with the cork borer, from the same leaf / leaves of the same plant, avoiding large veins
Need same size AND one of: same leaf / same plant / avoiding veins.
1 mark
Put the discs and the test solution in the syringe, block the nozzle and pull the plunger to remove the air from the air spaces, until the discs sink
Accept a description of creating a vacuum / reduced pressure with the syringe.
1 mark
Put the same number of discs (e.g. 10) into the same volume of each solution
Both 'same number' and 'same volume' are needed.
1 mark
Keep the light intensity the same: lamp at a fixed distance, checked with the light meter (in an otherwise darkened room)
Ignore 'same lamp' alone.
1 mark
Keep the temperature constant, e.g. with a water bath / a heat filter (a beaker or tank of water) between the lamp and the discs
Accept 'use an LED lamp, which gives off little heat'.
1 mark
Record the time taken for a set number of discs (e.g. half of them) to float, and calculate the rate as 1 ÷ time
Accept 'count the number of discs floating after a set time'.
1 mark
Repeat at least three times at each concentration AND calculate a mean
Both parts needed. Ignore 'repeat to make it reliable' with no mean.

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