Evaluate repeat yeast trials and rate uncertainty
Use repeat data without hiding anomalies or confounders.
Section 12.1 · Respiration
Exam scope and assessment notes
Core and Extended · Biology 0970, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked; human subject-teacher review, supervised practical competence and cumulative assessment remain pending. Cambridge states that IGCSE 0970 has the same subject content as IGCSE 0610, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Recall cell processes, enzymes and the distinction between gas exchange and breathing.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does a repeated trial help reveal?
What you will learn
- Summarise repeat measurements
- Judge reliability and validity
Evaluate repeat yeast trials and rate uncertainty
| Feature | Meaning |
|---|---|
| 30 °C · four minutes | 18 · 20 · 22 cm³ |
| 40 °C · four minutes | 26 · 28 · 30 cm³ |
| Mean rate | 5 versus 7 cm³/min |
| Limit | Repeats do not fix a systematic leak |
Summarise repeat measurements
At 30 °C, three invented independent trials collect 18, 20 and 22 cm³ in four minutes. Their mean is 20 cm³ and their rates are 4.5, 5 and 5.5 cm³/min. Repeating a controlled method shows the spread of the observations. It does not remove a systematic leak or automatically prove that the method measures only biological respiration.
At 40 °C, the corresponding invented values are 26, 28 and 30 cm³, with a mean rate of 7 cm³/min. The ranges in this small supplied sample do not overlap, supporting the observed comparison under this method. Do not turn three readings into a precise population claim or a guaranteed threshold; more repeats and controlled measurement strengthen the evidence.
Judge reliability and validity
If a later reading is much lower, investigate recording, apparatus leaks, mixture temperature, yeast quantity and glucose supply rather than deleting it just to improve the mean. A faulty trial can be repeated with the reason recorded. Bubble counting may be less consistent than volume collection because bubble sizes can differ; it measures a count proxy rather than a known gas volume.
A zero-gas control with no yeast can help detect background gas effects under the same conditions, but it cannot prove every active sample had identical living cell numbers. Equal handling, consistent timing, actual-temperature checks and comparable oxygen conditions remain necessary. Written analysis practises planning and interpretation; completing these questions does not certify practical laboratory competence.
Pause and explain
Why can bubble counts be a weak volume comparison?
Worked example
Find the mean of 18, 20 and 22 cm³ collected in four minutes.
Show the worked solution
- The sum is 60 cm³; 60 ÷ 3 = 20 cm³.
- 20 ÷ 4 = 5 cm³/min.
Answer Mean volume 20 cm³; mean rate 5 cm³/min.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Find the mean gas rate from 26, 28 and 30 cm³ collected in four minutes.
Give me a hint
Average the volumes, then divide by time.
Compare my reasoning
- The mean volume is 84 ÷ 3 = 28 cm³.
- 28 ÷ 4 = 7 cm³/min.
7 cm³/min.
Look for these in your work
- Use all three values
- Include rate units
A trial gives 2 cm³ while repeats give 18 and 20 cm³. Should the low value simply be erased?
Give me a hint
Look for a documented reason.
Compare my reasoning
- Investigate leaks, temperature and quantities.
- Record the anomaly and repeat any demonstrably faulty trial.
No. Investigate and record the discrepancy; repeat a faulty trial with its reason rather than silently deleting the result.
Look for these in your work
- Retain the evidence
- Name a justified follow-up
Two temperatures give equal bubble counts but different bubble sizes. Is equal respiratory gas volume established?
Give me a hint
A count needs a volume relationship.
Compare my reasoning
- Bubble size changes volume per bubble.
- A calibrated volume measurement is needed for that comparison.
No. Equal counts do not establish equal volume when bubble sizes differ; use comparable calibrated gas-volume collection.
Look for these in your work
- Distinguish count and volume
- Improve the measurement
Common mistakes
- Deleting inconvenient readings without investigation.
- Calling equal bubble counts equal gas volumes.
What can you explain now?
Why can bubble counts be a weak volume comparison?
Compare with the explanation
Bubble sizes may differ
Equal counts need not mean equal volume.
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Make it stick
Explain repeat variation and distinguish a count proxy from collected gas volume.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.