Planning An Investigation

Topic Explainer Video

Check out this @lauradoesGCSEbiology video that explains planning an investigation, then read the study notes. Once you’ve gone through them, don’t forget to try the practice questions!

Key Steps to Planning an Investigation

When planning an investigation for GCSE Biology, you should be able to:

1. Identify variables:

• Independent variable: What you will change.
• Dependent variable: What you will measure.
• Control variables: What you will keep the same.

2. Write a clear method:

• Describe how you will change the independent variable.
• Say how you will measure the dependent variable.
• Explain how you will make sure results are valid (control variables).

3. Repeat and calculate a mean:

• Repeat the test for each condition (ideally you want at least three repeats).
• Use repeats to get a reliable mean.
• If testing on humans / volunteers, the larger the sample size the better – so use large groups of people.

4. Record and present results:

• Use a table with clear headings that contain any units.
• Think about how you will plot a graph (e.g., line graph for continuous data, bar chart for discontinuous data).

Example 1: Investigating Photosynthesis Rate

Past exam context: How does light intensity affect the rate of photosynthesis in pondweed?

Plan:

• Independent variable: Light intensity (change distance of lamp, e.g. 10, 20, 30, 40, 50 cm).
• Dependent variable: Volume of oxygen produced per minute.
• Control variables: Type of plant, size of pondweed, temperature, volume of water and volume of sodium hydrogen carbonate.
  • Sodium hydrogen carbonate releases CO₂ for the pondweed so it does not become a limiting factor.
  • Allow the pondweed time to equilibrate at each light intensity by waiting for 10 minutes before any measurements are taken.
  • Temperature can be controlled by using an LED bulb that does not give out heat energy or by placing a Perspex block between the lamp and the pondweed- as this will act as a heat shield.

Method: 

• Place the lamp at different distances from pondweed.
• Collect the oxygen produced by the pondweed in a gas syringe and record the volume.
  • This could be done over a period of 10 minutes and the volume divided by 10 to calculate the volume produced per minute.
• Repeat 2 further times at each different light intensity.
• Calculate the mean volume of oxygen produced at each distance.

Example 2: Investigating Enzyme Activity

Past exam context: How does temperature affect the activity of amylase on starch?

Plan:

• Independent variable: Temperature of water bath, e.g. 20, 30, 40, 50, 60 ^oC.
• Dependent variable: Time for starch to be broken down into glucose.
• Control variables: Volume of starch and amylase, concentration of starch and amylase, pH, volume of iodine.
  pH can be controlled by adding a pH solution (called a buffer) to each test tube.

Method: 

• Place amylase and starch solutions into water baths at different temperatures.
• Allow time for them to reach the correct temperature before mixing them together- for example, leave for 10 minutes.
• Mix together and then test small samples every 30 seconds with iodine until no starch is left.
  • You will know that no starch is left when the iodine stops going blue / black and remains yellow.
• Repeat 2 further times at each temperature.
• Calculate a mean time taken for starch to be broken down.

Practice Question