International A Level Biology / Practical Skills in Biology I
Planning an experiment for Unit 3 revision notes
Edexcel International A Level Biology (YBI11), chapter 13: Practical Skills in Biology I, specification point U3.1. Free lesson.
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A strong Unit 3 method gives enough detail for someone else to carry it out and get a result. Include specific values, measurements, controls, repeats and safety steps that fit the practical.
What a planning answer needs
A planning question is marked against a checklist. Make each instruction specific enough to follow, including the apparatus and its size where that matters. Give at least five values for the independent variable, with the range from lowest to highest and the interval between values. Match the values to any graph requested. State the dependent variable as a measurement, with the instrument used and when it is measured. Include only relevant controlled variables, and say how each is kept constant. Give at least three repeats at each value. Explain that repeats allow you to…
How to investigate garlic extract and bacteria
To test whether garlic extract affects bacterial growth, change the extract concentration and measure the clear zone around each disc. A larger clear zone means bacteria did not grow close to the disc.
What the results should show, and how to stay safe
The control shows what happens without garlic extract. Comparing it with the garlic discs helps you judge whether garlic affects bacterial growth. If garlic extract is antimicrobial, higher concentrations are expected to produce wider clear zones. The sterile-water disc should have no clear zone. Because bacterial cultures may contain harmful microbes, do not open plates after incubation. Autoclave them, using pressurised steam to sterilise them, before disposal. For a heart dissection, a sharp scalpel is a hazard. The risk is cutting your hand. Reduce the risk by cutting away from your…
Make each instruction precise
General phrases often do not show what you would actually do. Give the value, instrument or action that fits the practical. Instead of “leave for a known time”, state “incubate for 48 hours”. Instead of “keep the temperature the same”, say “incubate every plate at 25 °C in an incubator”. Instead of “observe the zone”, measure the clear zone’s diameter with a ruler. Instead of “repeat to make it reliable”, state that you will use three plates to calculate a mean and spot anomalies. Build variables into the method steps. A list of variables, irrelevant controls such as humidity, or a blank…