All subjects · Biology · WBI16 Practical Skills in Biology II

Planning a full investigation at A2

Edexcel IAL Biology · WBI16 Practical Skills in Biology II

The long Unit 6 planning question carries a large share of the paper's marks across safety, preliminary work, method, analysis and limitations. The October 2023 mark scheme gave 9 marks for the method alone.

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Preliminary work

A small-scale trial before the main investigation to decide:

a suitable range and interval of the IV (for example which light intensities give measurable gas);
a suitable time for each reading (long enough to measure, short enough not to run out of substrate);
concentrations or volumes of reagents;
an acclimatisation time for organisms;
whether the method gives quantitative results (for example whether a colour change can be timed).

The January 2024 report: answers that engaged with the context scored; generic 'do a trial run' did not.

The method (aim for 9 or more creditworthy points)

From the October 2023 CP10 mark scheme, credit went to:

1. a clear DV (volume of oxygen released);

2. how the DV is measured (photosynthometer, gas syringe);

3. standardising the organisms (same mass or length of pondweed, same species);

4. how the IV is produced (lamp at measured distances);

5. a key reagent (sodium hydrogencarbonate so CO₂ is not limiting);

6. an acclimatisation time;

7. a standard collection time;

8. a controlled variable with a method (water bath or heat shield for temperature);

9. repeats giving a mean and standard deviation;

10. how the rate is calculated.

At A2, also give apparatus details (a 1 cm³ syringe reading to 0.01 cm³; a 10 cm long capillary of 1 mm bore), and say how instruments are calibrated.

Recording, analysis and limitations

A raw data table with headings and units, plus means.
A graph matched to the test: bar chart and t-test for two conditions; scatter graph and Spearman's rank for several values.
A null hypothesis in context and the comparison with a critical value at p = 0.05.
Limitations specific to the method: small volumes are hard to measure; temperature drifts; leaf area hard to control; oxygen used in respiration or dissolved in water; field conditions change through the day.

Worked example

Give a null hypothesis, test and graph for an investigation comparing the rate of oxygen production at two light intensities, with 5 repeats each.

H₀: there is no significant difference between the mean rate of oxygen production at the two light intensities.
Two means of continuous data: Student's t-test.
Bar chart of the two means with ± 1 standard deviation bars.

Answer: No significant difference between the means; t-test; bar chart with error bars.

Key idea: Plan: preliminary work for range, time and concentrations; specific method with DV, IV, standardised organisms, controlled variables with methods, acclimatisation, repeats, rate calculation; table, graph, test with H₀; limitations specific to the method.

Check yourself

Which is the best example of preliminary work in a respiration experiment with seeds?
- Trying different masses of seeds to find one that moves the fluid a measurable distance in 10 minutes
- Doing the full investigation twice
- Writing a risk assessment
- Drawing the graph
Show answer
Trying different masses of seeds to find one that moves the fluid a measurable distance in 10 minutes. It sets a parameter so results are measurable.
Why is pondweed left at each light intensity for a few minutes before gas is collected?
- So the pondweed can respire
- So the water evaporates
- So the rate of photosynthesis adjusts to the new conditions
- So the lamp warms the water
Show answer
So the rate of photosynthesis adjusts to the new conditions. Acclimatisation gives a steady rate.
Why is sodium hydrogencarbonate added in CP10 plans?
- To provide carbon dioxide so it does not limit the rate
- To provide oxygen
- To keep the temperature constant
- To kill microorganisms
Show answer
To provide carbon dioxide so it does not limit the rate. Listed in the October 2023 mark scheme.
Which description of apparatus meets the Unit 6 requirement to give details?
- A syringe
- Some measuring equipment
- A large container
- A 1 cm³ syringe graduated in 0.01 cm³
Show answer
A 1 cm³ syringe graduated in 0.01 cm³. Range and resolution are given.
In a photosynthesis plan, which dependent variable is accepted?
- Volume of carbon dioxide released
- Volume of oxygen released per unit time
- Mass of glucose in the leaf, measured by eye
- Colour of the leaves
Show answer
Volume of oxygen released per unit time. The October 2023 mark scheme did not accept carbon dioxide as the gas.
Which is a realistic limitation of measuring oxygen from pondweed?
- Pondweed cannot photosynthesise in water
- Light has no effect on pondweed
- Oxygen is too heavy to collect
- Some oxygen is used in respiration or dissolves in the water
Show answer
Some oxygen is used in respiration or dissolves in the water. So the gas collected underestimates photosynthesis.
A plan uses six concentrations of gibberellin with three grains at each. Which analysis fits?
- t-test between six groups
- Scatter graph of mean zone area against concentration and Spearman's rank
- Chi-squared on zone areas
- Pie chart
Show answer
Scatter graph of mean zone area against concentration and Spearman's rank. Several values of a continuous IV: correlation.

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Last updated 8 November 2026. Spotted a mistake? Tell us.