International A Level Biology / Practical Skills in Biology II
Planning an investigation for Unit 6 revision notes
Edexcel International A Level Biology (YBI11), chapter 26: Practical Skills in Biology II, specification point U6.1. Free lesson.
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A Unit 6 planning question asks you to design an investigation, explain how you would collect and analyse data, and identify limitations. A clear plan gives specific details for variables, controls, ethics and safety, so the results can be measured and trusted.
What the four parts ask for
Planning questions nearly always follow the same order. Read all four parts first, because the statistical test in part (c) must suit the data your method collects. (a) Preliminary work: what you would try first, and why. (b) Method: variables, repeats, safety and ethics. (c) Record and analyse: a table, graph and statistical test. (d) Limitations: problems with this method in this particular setting.
How to make preliminary work specific
Preliminary work is a small trial run. It helps you choose conditions that make the method work and produce measurements you can use. State exactly what you would find out and why it matters. For a respirometer investigation, find the mass of seeds that moves the drop a measurable distance in 10 minutes. Find a temperature at which the seeds respire fast enough to produce a measurable volume. For an agar plate investigation, find an extract concentration that makes a clear zone, where bacteria have not grown, large enough to measure. Avoid saying only that you would practise the method or…
Choose and control the variables
For the Daphnia investigation, the hypothesis is that heart rate rises as water temperature rises, up to a point. The independent variable is the one you change; the dependent variable is the one you measure. Each control needs both a method and a value. Change temperature using water baths from 10 to 30 °C, in 5 °C intervals. Measure heart rate by counting beats for 15 s with a tally counter, then multiply by 4 to calculate beats per minute. Use Daphnia of about the same length from one culture, because heart rate varies with size and age. Allow 10 minutes at each…
Collect the Daphnia heart-rate measurements
Use a cavity slide, which has a small hollow, and cotton wool fibres to hold the Daphnia still. Its heart is a small oval on its back, just behind the head. A digital thermometer measures slide-water temperature to 0.1 °C.
Record, analyse and evaluate the results
Record the raw data clearly, then choose analysis that matches the question. Since the investigation tests whether two variables are correlated, use a test for correlation. Make a table with temperature in °C and the beats in 15 s for each of five Daphnia. Add a beats-per-minute column. Calculate the mean heart rate at each temperature and plot a line graph of mean heart rate. Use Spearman's rank to test for a correlation. The null hypothesis is that there is no significant correlation between temperature and Daphnia heart rate. For ethics, explain that Daphnia are invertebrates with…