Questions on variables and controls: what an experiment can show

Question 1

Describe how you would investigate the effect of caffeine concentration on the heart rate of Daphnia, observed through a light microscope.

Accepted Answer

B1 make at least five caffeine concentrations by diluting a stock solution with pond water, with pond water alone as the control B1 place one Daphnia in a small drop on a cavity slide, with a few cotton wool strands to restrict its movement, and leave it for a fixed time in each solution before counting B1 count heartbeats for a fixed time such as 15 seconds, scale up to beats per minute, and repeat the count to give a mean B1 control temperature, because the lamp warms the drop and heart rate rises with temperature: keep counts short, turn the lamp off between counts, and measure the water temperature B1 control the size of the animals, record each one's rate in pond water first and report the change from that baseline, and use a fresh animal for each concentration

Question 2

Discuss the ethical issues raised by using invertebrates such as Daphnia in research.

Accepted Answer

B1 for: invertebrates such as Daphnia have much simpler nervous systems than vertebrates, so their capacity to suffer is probably lower B1 for: using an invertebrate in place of a vertebrate is replacement under the 3Rs, and observing the heart through a transparent body without surgery is refinement B1 against: whether invertebrates are sentient is not known, so a precautionary view treats them as able to suffer; restraint, heat from the lamp and the drug may cause stress and some animals die B1 against: Daphnia is not protected by the Animals (Scientific Procedures) Act, which covers vertebrates and cephalopods, so the care taken depends on the person doing the work; or: some hold that all living organisms have value whatever their nervous system B1 a judgement linked to the arguments, such as that the use is justified if the benefit outweighs a probably small harm and the 3Rs are followed, with the fewest animals used and each returned to recover

Question 3

A student measures oxygen production by pondweed at five distances from a filament lamp, and the rate rises as the lamp is brought closer. Explain why repeating the whole investigation ten times would not make the conclusion valid.

Accepted Answer

B1 temperature is a confounding variable here: a filament lamp is mostly a heater, so bringing it closer raises the temperature of the water as well as the light intensity B1 either the light or the warmth could have raised oxygen production, and the readings collected cannot separate the two effects B1 every repeat changes both quantities together in exactly the same way, so more repeats trace the same two routes again rather than telling them apart B1 repeats improve precision, not validity; the fix is a change of design, such as a water-filled flask acting as a heat filter or an LED source, with the temperature measured at each distance to show it stayed put

Question 4

Compare accuracy with precision, and compare repeatability with reproducibility, as descriptions of a set of measurements.

Accepted Answer

B1 accuracy is closeness to the true value, whereas precision is how closely repeat readings agree with one another B1 a random error such as misjudging a meniscus scatters readings either side of the true value and damages precision, whereas a systematic error such as an unzeroed balance shifts every reading the same way and damages accuracy B1 readings can therefore be precise and inaccurate at the same time, which is why agreement between repeats is no evidence at all of being right B1 repeatability is the same person getting the same results with the same method and apparatus, whereas reproducibility is somebody else, or different apparatus or a different method, getting them

Question 5

A student tests six extracts of seaweed for antibacterial activity by soaking paper discs in each and placing them on a lawn of bacteria. No clear zone appears around any of the six discs. Suggest two control experiments the student should have set up, and suggest what each would have shown.

Accepted Answer

B1 a negative control: a disc soaked in the solvent the extracts were made up in, and nothing else B1 if that disc produces a zone, the solvent was killing the bacteria and any zone around an extract could not be credited to the seaweed B1 a positive control: a disc carrying an antibiotic the bacterium is known to be sensitive to B1 a zone around it shows the bacteria were alive, the plate was seeded properly and a zone would have been visible had there been one to see; with no zone anywhere the honest conclusion is that the investigation failed, not that the extracts have no effect

Question 6

A survey of 5000 adults finds that those with a higher intake of oily fish have fewer heart attacks over the following ten years. A newspaper reports that eating oily fish prevents heart attacks. Evaluate that conclusion.

Accepted Answer

B1 the study is observational rather than experimental: nobody set how much fish each person ate, so the groups differ in many ways at once and the correlation is consistent with the conclusion without establishing it B1 a common cause is the obvious alternative, since people who can afford oily fish and cook at home also tend to differ in income, exercise and smoking, any of which could lower heart attack rate on its own B1 reverse causation deserves a sentence: people already diagnosed with heart disease may have been told to change their diet, which would put the arrow the other way round B1 the argument the paper needs is a stack of separate points: a mechanism, a dose response, survival of adjustment for the known confounders and repetition elsewhere. On this evidence alone the causal claim is not yet supported

Question 7

A student assigns leaf discs to five sucrose concentrations by drawing lots rather than by picking each disc up and choosing a tube for it. Explain what this randomisation achieves, and explain why it does not guarantee an even spread of disc types in this one experiment.

Accepted Answer

B1 discs differ in ways nobody measured, such as thickness, age or distance from the midrib, and choosing by hand could pile one kind into one treatment B1 assigning by chance means those unmeasured variables tend to balance out between the treatments on average, so a confounder nobody thought of is less likely to line up with the independent variable B1 randomisation balances confounders in expectation, across many runs, not in any single one, so an uneven spread can still happen by chance, which is one reason sample size still matters

Question 8

State what is meant by interpolation and what is meant by extrapolation, and state which of the two a valid conclusion may rest on.

Accepted Answer

B1 interpolation is reading a value off the line between two measured points B1 extrapolation is reading a value off the line extended beyond the range that was measured B1 only interpolation is safe: a conclusion holds over the range tested, and a value read from an extended line is guesswork because nothing was measured there

Question 9

A student plans to investigate the effect of temperature on the rate of an enzyme-catalysed reaction using only four temperatures: 30 °C, 33 °C, 36 °C and 39 °C. Suggest why this choice of temperatures is a poor design for finding the enzyme's optimum temperature.

Accepted Answer

B1 the four temperatures are bunched together in a narrow, comparatively low range, so the investigation may not reach the temperature at which the enzyme's rate is greatest B1 with the optimum possibly lying above the highest temperature tested, the results could show only a rising trend and never capture the point where the rate turns over and falls, which denaturation would be expected to produce B1 a higher temperature would be needed to be confident the curve had been followed past its peak, since too few points spread over too narrow a range can be joined by a line a wider range would rule out

Question 10

Explain what is meant by the validity of an investigation, and explain why a confounding variable destroys validity however carefully every measurement was made.

Accepted Answer

B1 an investigation is valid if its design allows its conclusion to actually follow from the results, that is, if it tests what it claims to test B1 a confounding variable changes alongside the independent variable and could itself explain the change in the dependent variable, so the data cannot show which of the two produced the effect B1 no amount of care in measuring removes this problem, because the fault lies in the design of the investigation rather than in how precisely the readings were taken

Question 11

An investigation finds that the rate of an enzyme-catalysed reaction falls above 45 °C. Explain why the conclusion 'this shows the enzyme denatured' claims more than the data support, and explain the more careful sentence a scientist should write instead.

Accepted Answer

B1 the investigation measured rate against temperature, so directly it shows only that rate falls above 45 °C, not any change to the enzyme's structure B1 denaturation is an explanation for the pattern, brought in from what is known about protein structure elsewhere, rather than something the readings themselves establish B1 the careful sentence states what happened and offers denaturation only as an interpretation: 'this is consistent with the enzyme denaturing above 45 °C', rather than asserting it as shown

Question 12

A student takes five leaf discs from a single ivy leaf and concludes from the results that 'ivy leaves lose water fastest through the lower surface.' Suggest why this conclusion claims more than the investigation can support, and suggest what would be needed to support the wider claim.

Accepted Answer

B1 the five discs all came from one leaf, so strictly the result describes that leaf and not necessarily the species as a whole B1 leaves from different plants, or different parts of the same plant, could vary in ways not sampled here, such as age, exposure to sun or thickness of cuticle B1 supporting the wider claim needs a sample deliberately drawn to represent the species, such as leaves taken from several different ivy plants in different conditions, decided before the investigation was carried out

Question 13

A student investigating the effect of substrate concentration on the initial rate of an enzyme reaction keeps the pH, temperature and enzyme concentration the same in every tube, and also runs one extra tube with the enzyme replaced by boiled, inactivated enzyme. Identify three control variables in this investigation, and identify the one control experiment, saying what it is for.

Accepted Answer

B1 the three control variables are pH, temperature and enzyme concentration, each held constant across every tube B1 the control experiment is the tube with boiled, inactivated enzyme B1 it is a negative control, testing whether any change seen is really due to the active enzyme rather than to the apparatus, the substrate itself or the handling

Question 14

A Daphnia gives 52 heartbeats in 15 seconds in pond water and 61 heartbeats in 15 seconds after five minutes in a caffeine solution. Calculate the percentage change in heart rate. Give your answer to three significant figures.

Accepted Answer

M1 heart rates are 52 × 4 = 208 beats min⁻¹ and 61 × 4 = 244 beats min⁻¹, or the counts are compared directly since both cover the same time M1 percentage change = (244 − 208) ÷ 208 × 100, or (61 − 52) ÷ 52 × 100 A1 +17.3 per cent, an increase

Question 15

A student writes in a risk assessment for the Daphnia heart-rate practical: 'Caffeine is a hazard, so the risk is high.' Suggest why this sentence confuses hazard with risk, and suggest one control measure for a hazard in this practical.

Accepted Answer

B1 a hazard is anything that could cause harm; caffeine is one, as a solid that is harmful if swallowed B1 risk is the likelihood and severity of harm under the conditions of use, and dilute solutions in small volumes, handled with care, carry a low risk, so a hazard does not by itself make the risk high B1 any one control measure matched to its hazard, such as washing hands after handling pond water that may carry microorganisms, keeping liquids away from the microscope's mains plug, or handling slides and coverslips carefully to avoid cuts

Question 16

Explain how each of the 3Rs could be applied to a class practical measuring the heart rate of Daphnia.

Accepted Answer

B1 replacement: an invertebrate is used in place of a vertebrate, or a video of a beating heart replaces live animals for practising the counting method B1 reduction: students work in groups and pool results, so the fewest animals that give a reliable mean are used B1 refinement: each animal is kept in pond water, out of the lamp's heat, for as short a time as possible, then returned to a recovery tank

Question 17

State what a negative control is designed to rule out, and state what a positive control is designed to rule out.

Accepted Answer

B1 a negative control, in which the factor being tested has been removed or destroyed, rules out a false positive: an effect produced by the apparatus, the solvent or the handling rather than by the factor B1 a positive control, using something already known to produce the effect, rules out a false negative: a method that failed, so that flat readings mean nothing

Question 18

A student measures the volume of oxygen released in five minutes by catalase-soaked potato discs in hydrogen peroxide solutions of five different concentrations, all held at 25 °C. Identify the independent variable and the dependent variable, giving the unit of each.

Accepted Answer

B1 the independent variable is the concentration of the hydrogen peroxide solution, in mol dm⁻³ or per cent, because it is the quantity changed on purpose B1 the dependent variable is the volume of oxygen released in five minutes, in cm³, because it is the quantity measured to see what happened

Question 19

A student investigates the rate of photosynthesis in five species of pondweed, each at the same light intensity. Suggest whether a bar chart or a line graph should be used to present the results, and justify your choice.

Accepted Answer

B1 a bar chart, because species is a categorical variable rather than a continuous one B1 a line graph would wrongly imply that the species form a continuous, ordered scale with intermediate values between them

Question 20

State how a control variable such as temperature should be described in a method, so that the description is reproducible.

Accepted Answer

B1 give the set value of the variable, such as a stated temperature of 35 °C B1 give the method used to hold it there, such as a water bath, for a stated time before starting