A fair enzyme test changes one factor and keeps every other condition the same. Your plan must also say how the rate is measured and how you make the result reliable.
This lesson is part of metabolism and enzymes. It applies the ideas in distinguishing limiting factors.
What is the six-line planning frame?
Use these lines in order. Each one gets one sentence in your answer.
- Aim or question: what effect are you testing?
- Manipulated variable: what you change, and the values you use.
- Responding variable: what you measure, and how.
- Fixed variables: what you keep the same.
- Procedure in short steps.
- Repeats and how you use them.
A plan that covers all six lines is complete, even in simple language.
Worked plan: temperature and catalase
Aim: To find out how temperature affects the rate at which catalase breaks down hydrogen peroxide.
Manipulated variable: Temperature, using water baths at 10 °C, 20 °C, 30 °C, 40 °C and 50 °C.
Responding variable: The volume of oxygen collected in 2 minutes, measured with an upturned measuring cylinder over water.
Fixed variables: The volume of hydrogen peroxide (10 cm³), its concentration, the volume and source of catalase (1 cm³ of liver extract), the pH, and the time allowed.
Procedure:
- Place 10 cm³ of hydrogen peroxide in a boiling tube and warm it in the water bath for 5 minutes.
- Add 1 cm³ of liver extract and close the tube with a delivery tube.
- Start the stopwatch and collect the gas for 2 minutes.
- Record the volume of oxygen.
- Repeat at each temperature.
Repeats: Do each temperature three times and calculate the mean volume of oxygen. A mean is fairer than a single reading.
You can add a prediction too: the rate should rise up to about the optimum and fall beyond it. State it, then compare it with your results.
Why does the plan need a warming step?
If the enzyme and substrate are mixed before they reach the water bath temperature, the reaction begins at the wrong temperature. Warming each solution separately for the same time before mixing keeps the temperature fair.
Similarly, the pH must stay the same in every tube. A plan that changes temperature but leaves pH uncontrolled has two variables, so the result could not be traced to temperature alone.
The mistake that costs marks
The common slip is to write “keep everything the same” for the fixed variables. Name each item instead: volume, concentration, source and time.
| Student wrote | Problem | Better version |
|---|---|---|
| “Keep other things fair” | Nothing is named | “Keep the volume of hydrogen peroxide at 10 cm³” |
| “Measure the reaction” | No method | “Collect oxygen for 2 minutes and measure its volume” |
| “Do it once” | Cannot judge reliability | “Repeat three times and calculate the mean” |
| “Change temperature and pH” | Two variables change | “Change temperature only; keep pH fixed” |
Check yourself
A student plans to test how the concentration of hydrogen peroxide affects the rate of reaction with catalase. Name the manipulated variable, the responding variable, and two fixed variables.
Answer
Manipulated variable: concentration of hydrogen peroxide.
Responding variable: the volume of oxygen collected in a fixed time.
Fixed variables (any two): temperature, volume of hydrogen peroxide, volume and source of catalase, pH, and collection time.
What to study next
Practise on realistic data in interpreting enzymes and experimental rates. Then attempt the metabolism and enzymes practice set.
If you want a teacher to question each line of your plan, see online one-to-one Biology tuition or the one-hour trial class (from RM50).