H2 Biology Enzyme Kinetics Practical: Catalase Experiment Guide

Safety: Wear goggles/gloves; handle hydrogen peroxide carefully; secure glassware; keep workstation dry.

3 Serial dilution plan (substrate concentration)

Target 5-6 concentrations with equal volume per trial. Example (10 mL reaction mix):

Tips

• Mix gently; prepare fresh substrate ($\ce{H2O2}$ decomposes over time).
• Label clearly; pre-equilibrate all tubes to $\pu{25 ^\circ C}$ in the water bath.

4 Method (MMO focus)

1. Place the gas syringe (or inverted measuring cylinder setup) and ensure an airtight seal.
2. Pipette a fixed volume of catalase (e.g., $\pu{2.0 mL}$) into each conical flask; equilibrate at temperature.
3. Add the substrate mix (e.g., $\pu{8.0 mL}$) to start the reaction. Immediately bung the flask and start the stopwatch.
4. Record gas volume at consistent $\pu{10 s}$ intervals for 1-2 minutes (initial phase).
5. Repeat for all concentrations; run at least one repeat for a key concentration to estimate variability.
6. Controls: a heat-denatured catalase tube or a zero-substrate tube to confirm baseline.

Notes

• Keep total volume constant (e.g., $\pu{10.0 mL}$) so comparisons are valid.
• Maintain temperature; re-check water bath and log readings.
• Rinse/replace syringes between runs to avoid cross-contamination.

5 Data treatment (PDO) with spreadsheets

• Tabulate time ($\pu{s}$) and gas volume ($\pu{mL}$). Use the first $\pu{30-60 s}$ where the trace is most linear.
• Compute initial rate from slope (e.g., change in volume per second). Do not force the line through the origin unless justified.
• Plot rate vs substrate concentration with axes labelled and units shown.
• Add error bars (e.g., standard deviation across repeats) where available.
• Optional: If concentration saturates the enzyme, note the plateau behaviour qualitatively.

Example rate statement: “Initial rate = $\pu{0.18 mL.s-1}$ at $0.8\% \space \ce{H2O2}$, $\pu{25 ^\circ C}$, pH 7.”

6 Evaluation (ACE): uncertainties and improvements

• Gas leakage or delays when bungs are fitted → pre-seat bungs; practise sealing to reduce dead time.
• Temperature drift → use a water bath and work quickly; log actual readings for each run.
• Biological variability (enzyme prep differences) → mix thoroughly; prepare a single batch for all runs.
• Reading resolution on gas syringe or cylinder → choose appropriate scale; read at eye level (avoid parallax).
• Control of independent variable → confirm fresh substrate and verify volumes with calibrated pipettes.

Improvements

• Use a gas pressure sensor for continuous data; export CSV to speed up slope calculation.
• Standardise catalase concentration using optical density (optional extension for school labs with spectrometers).

7 60-minute exam-pace schedule

8 Links & next steps

• Rehearse microscope measurement and drawings before or after this practical so Paper 4 feels cohesive: https://eclatinstitute.sg/blog/h2-biology-experiments/H2-Biology-Optical-Microscope-Mastery-Guide
• Extend rate investigations with the photosynthesis and respiration practicals: https://eclatinstitute.sg/blog/h2-biology-experiments/H2-Biology-Photosynthesis-and-Respiration-Rate-Practical-Guide
• If you want an invigilated mock with spreadsheet templates and feedback, book a lab slot via our Biology practical coaching page: https://eclatinstitute.sg/blog/h2-biology-experiments/H2-Biology-Practical-2026-Lab-Mastery-Guide
• Write up the catalase experiment as a full Paper 4 planning response using the H2 Biology Planning and Evaluation (Paper 4) template.

Assessment anchors (MMO/PDO/ACE)

• MMO (Manipulation, Measurement, Observation)
  • Correct serial dilution, accurate volumetric transfer (fixed total volume), airtight gas path, immediate timing on mixing.
  • Temperature/pH control maintained and logged; repeats at a key concentration.
  • Appropriate controls (heat‑denatured enzyme or zero substrate) included.
• PDO (Presentation of Data and Observations)
  • Time (s) and volume (mL) tables with units and consistent s.f.; clear note of the initial linear window used.
  • Rate calculation shown (e.g., Δvolume/Δtime) with units; graph of rate vs concentration with axes/units; error bars where available.
  • Legend/caption explains conditions (temperature, pH, enzyme source).
• ACE (Analysis, Conclusions, Evaluation)
  • Trend described (rate increases then plateaus) with biological reasoning (active site saturation).
  • Limitations discussed (leaks, H2O2 decomposition, temperature drift); improvements proposed (logger, standardised enzyme batches, more repeats around plateau).

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References

• SEAB. (2024). Biology (Syllabus 9477) GCE A-Level 2026 (first year of examination 2026). Singapore Examinations and Assessment Board - Scheme of Assessment and Core Idea 3 practical expectations (PDF last modified 2025-11-28). https://www.seab.gov.sg/files/A%20Level%20Syllabus%20Sch%20Cddts/2026/9477_y26_sy.pdf
• Campbell, N. A. et al. (2020). Campbell Biology (12th ed.). Pearson - enzyme catalysis laboratory practices.

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