International A Level Biology / Practical Skills in Biology I
Variables, controls and a fair test
Edexcel International A Level Biology (YBI11), chapter 13: Practical Skills in Biology I, specification point U3.1. Free lesson.
Read the revision notes · International A Level Biology past papers
A fair test changes one independent variable, measures a quantitative dependent variable and keeps relevant controlled variables the same. You also need to explain how variables are controlled, and distinguish a control set-up from a controlled variable.
What the three variables mean
The independent variable is what you change. The dependent variable is what you measure, so state it as a number rather than a vague word such as “growth”. Controlled variables are kept the same because they could also affect the result. In the beetroot practical, change the water temperature and measure the water’s absorbance after 30 minutes. Keep disc size, water volume and time the same. In the trifluralin seedling practical, change the trifluralin concentration, not the solvent, and measure the mitotic index: the fraction of root-tip cells that are dividing. Give the…
How to explain keeping a variable constant
Naming a controlled variable is only part of an answer. Say what apparatus you use and give a stated value or method. This explains how the factor is kept steady and why it matters. Temperature: use a thermostatic water bath set to a stated value, such as 30 °C. Enzyme activity and membranes depend on temperature. pH: add the same volume of buffer solution at a stated pH, such as pH 7. A buffer resists changes in pH, and enzymes have an optimum pH. Volume: measure the same volume each time with a graduated pipette or syringe. Different volumes can dilute a solution or add different…
Why a pH meter is not a control
A pH meter tells you the pH, but it does not change the solution or stop the pH changing. Measuring can check the pH; a buffer is what controls it. Add the same volume of buffer at a stated pH to every tube.
How a control differs from a controlled variable
A control is an extra set-up without the factor being tested. A controlled variable is a factor kept the same in every set-up, including the control. The control provides results to compare with and helps show that nothing else caused the effect. In an enzyme experiment, use the same volume of water instead of enzyme as the control. Keep temperature constant with a water bath. In the trifluralin experiment, use seedlings in solvent alone as the control. Keep the number and age of seedlings the same. Write “a control”, not “a control variable”. A controlled variable is a different thing.
How to standardise a step
Some steps cannot be set with a dial or controlled by a buffer. Standardising means doing them in exactly the same way each time. Give the details, including the amounts used. For a vitamin C extraction, blend 10 g of fruit with 50 cm³ of distilled water.
Applying the ideas to bacteria on agar
For garlic extract and bacterial growth, measure the clear zone around each paper disc. Then describe how you keep other factors the same, with a method and values. Measure the diameter of the clear zone around each disc with a ruler. Cut all discs to the same size with a hole punch. Incubate every plate at 25 °C for 48 hours.