The investigation · Temperature and catalase activity
How does temperature (10, 20, 30, 40, 50 °C) affect the initial rate of catalase activity in potato extract, measured as the volume of oxygen produced in the first 60 seconds?
Biology worked example: what becomes more explicit?
The comparison below is instructional, not examiner-scored. Use it to see which evidence or reasoning is missing, then check the current criterion wording for your session.
How does the example make research design evidence more explicit? (6-mark criterion)
The more explicit version defines the variables and measurement choices and makes the repeat strategy visible. The appropriate number of repeats still depends on the investigation.
Under-specified
I will test how temperature affects enzymes. I will put potato in hydrogen peroxide at different temperatures and see how much gas comes out.
More explicit
Independent variable: water-bath temperature (10 to 50 °C in 10 °C steps, checked with a thermometer ±0.5 °C). Dependent variable: O₂ volume collected in a 50 cm³ gas syringe over 60 s. Controlled: extract volume (2.0 cm³), H₂O₂ concentration (1.0%, freshly diluted each session), pH 7 buffer, equilibration time (5 min). Five trials per temperature so outliers can be identified.
How does the example make data analysis evidence more explicit? (6-mark criterion)
Processed data, uncertainty and a clear reading of what the numbers show. The weak version only describes the shape of the graph.
Under-specified
The table shows the results. At 40 °C the most gas was made. The graph goes up then down.
More explicit
Mean initial rate rose from 0.21 cm³ s⁻¹ at 10 °C to 0.68 cm³ s⁻¹ at 40 °C, then fell to 0.19 cm³ s⁻¹ at 50 °C. Error bars show ± one standard deviation; the 30 °C and 40 °C bars do not overlap, so the increase is unlikely to be random variation. One 50 °C trial (0.41 cm³ s⁻¹) was excluded as an outlier, and the reason is stated.
How does the example make conclusion evidence more explicit? (6-mark criterion)
The strong conclusion answers the research question with the data, links it to accepted biology, and states how far the evidence actually goes.
Under-specified
In conclusion, temperature affects catalase and the optimum is 40 °C.
More explicit
Rate increased to a peak near 40 °C and dropped sharply at 50 °C, consistent with more frequent enzyme-substrate collisions up to the point where the active site denatures. Because only 10 °C steps were tested, the true optimum could lie anywhere between 30 and 50 °C; the data support 'about 40 °C', not an exact value.
How does the example make evaluation evidence more explicit? (6-mark criterion)
Specific weaknesses are connected to the results they could affect and to targeted improvements; a generic 'human error' statement does not identify that mechanism.
Under-specified
There could have been human error. Next time I would be more careful.
More explicit
The main weakness was temperature drift once the extract left the bath (up to 3 °C in 60 s), which most affects the 10 °C and 50 °C readings. Running the reaction inside the bath would remove this. Potato samples also varied in catalase content; blending one large batch per session would reduce that source of spread.
How do you evaluate each criterion?
Compare the strength and relevance of evidence, reasoning, and evaluation against the criterion's mark-band descriptors.
- Research question with no range or measured quantity.
- Raw data only, no processing or uncertainty.
- Conclusion that repeats the hypothesis instead of the data.
- Evaluation that lists 'human error' and 'more trials'.
- Going past the 3,000-word maximum; content after it is not assessed.
Where can you find official IB Biology IA exemplars?
The IB publishes marked exemplar IAs with examiner comments in its Programme Resource Centre. Access is through your school, so ask your Biology teacher for the current exemplar pack for the first-assessment-2025 criteria.
Sources & verification
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