Lesson 1 of 8 / Practical and data skills
Plan an investigation that answers the question
What makes a comparison fair enough to explain?
In this lesson: Choose variables, controls, repeats and analysis that address a biological hypothesis.
About 6 min
The key ideaChange a defined independent variable, measure a useful response and explain how other influences will be controlled.
Work with the evidence
Change one cause and measure its response
Change: Choose buffered pH values over a useful range. This is the independent variable.
Schematic of comparable treatments, not a complete apparatus setup. Each pH needs independent repeats and suitable gas measurement.
Explanation
Start with a testable relationship, such as how pH affects the initial rate of a catalase-catalysed reaction. The independent variable is what you deliberately change. The dependent variable is the response you measure. Make both operational: state the pH values used and how gas volume over time will give a rate.
Identify variables that could also affect the response. For this enzyme investigation, keep temperature, enzyme amount, substrate concentration and total reaction volume consistent. A buffer controls pH, a water bath controls temperature, and fixed measured volumes make the comparisons reproducible. State how, not simply "keep everything constant".
Use a suitable range and intervals, with repeats at each value. A preliminary trial can identify a workable range, a measurable time interval and any practical difficulty. Repeating independent reaction mixtures estimates variation better than repeatedly reading one unchanged measurement. Keep biological replicates distinct from repeated readings of the same specimen.
A control treatment answers a specific alternative explanation. A mixture without active enzyme can reveal gas production unrelated to the intended enzyme reaction. Keep its other relevant conditions comparable. A controlled variable and a control treatment have different roles.
Plan the analysis before collecting data: calculate a rate for each repeat, summarise the repeats, plot rate against pH and evaluate the pattern. This ensures that the measurements can answer the original question.
Assess the actual hazards and exposure routes. For example, splashes of an irritant hydrogen peroxide solution can harm eyes. Using the school-specified dilute concentration and small volumes, wearing suitable eye protection and transferring carefully reduce exposure. Link each precaution to its hazard and follow the school's material-specific risk assessment. Accurate apparatus handling also needs supervised laboratory practice.
Step by step
- 1
Define the comparison
Name the biological question, independent variable and measurable response.
- 2
Protect the comparison
Specify controls, a useful range and independent repeats.
- 3
Plan the conclusion
Choose calculations, presentation and risk precautions before collecting data.
Worked example
Work through the evidence
A student compares catalase at pH 5 and pH 8, but uses twice as much enzyme at pH 8. Can a higher rate at pH 8 be attributed to pH?
One way to explain it
No. Enzyme amount is a confounding variable. Use the same enzyme concentration and volume at both pH values, keep other relevant factors consistent, and repeat independent reactions before comparing their rates.
Why this answer works
- Name the specific confounder.
- Explain why it can affect the dependent variable.
- Specify a correction that isolates the intended comparison.
Is this true? "A control means making every tube identical."
The independent variable must differ. Relevant other conditions should be held consistent, while a control treatment is chosen to test a particular alternative explanation.