Plan a controlled oxygen requirement investigation
Compare stated oxygen availability while keeping moisture and temperature matched.
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Recall gametes, nuclear fusion, cellular respiration and the distinction between reproduction and growth.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What must stay matched to isolate oxygen?
What you will learn
- Identify the oxygen variable without inventing complete absence
- Separate evidence for respiration from germination counts
Plan a controlled oxygen requirement investigation
Invented day-5 record: oxygen available 16/20, oxygen limited 2/20; matched moisture and 22°C. Limited is not zero; 80% − 10% = 70 percentage points. Original teaching data.
| Condition | Starting | Germinated | Percentage |
|---|---|---|---|
| Dry, oxygen available, 22°C | 20 | 0 | 0% |
| Moist, oxygen available, 22°C | 20 | 16 | 80% |
| Moist, oxygen limited, 22°C | 20 | 2 | 10% |
| Moist, oxygen available, 5°C | 20 | 1 | 5% |
| Feature | Meaning |
|---|---|
| Changed factor | Stated oxygen availability |
| Controls | Moisture, temperature, seed stock and timing |
| Mechanism | Oxygen supports aerobic respiration |
| Limit | Counts are not direct respiration rates or death records |
Identify the oxygen variable without inventing complete absence
A controlled oxygen investigation needs oxygen availability to differ while other germination conditions remain suitable and matched. In the supplied teaching setup, prepared chambers have stated oxygen-available or oxygen-limited atmospheres. Each group receives the same viable seed stock, starting total, moist support, temperature and observation time. Relevant species-specific light or pretreatment conditions are also satisfied equally.
Use independently prepared repeat chambers and random seed allocation. A proposed classroom practical must be planned and supervised with the appropriate apparatus; written interpretation here is not certification that the gas conditions were achieved. If oxygen levels are not stated or measured, do not assume a covered container or submerged seed group contains absolutely no oxygen.
Separate evidence for respiration from germination counts
Oxygen supports aerobic respiration and the energy release needed for growth. A reduced count in the oxygen-limited treatment is consistent with that requirement under otherwise matched conditions. Counts alone do not measure respiration rate, and limited oxygen is different from proven complete absence. Some seeds may emerge using available oxygen before it becomes limiting, so a non-zero count is not evidence that oxygen is unnecessary.
The invented day-5 record gives 16 germinations among 20 oxygen-available seeds and 2 among 20 oxygen-limited seeds at matched moisture and 22°C. The percentages are 80% and 10%, a difference of 70 percentage points. The record supports the role of oxygen availability within this setup but does not prove all ungerminated seeds died or that the same outcome occurs in every species.
Sources for this lesson
Pause and explain
Does 2/20 emergence in an oxygen-limited group prove oxygen is unnecessary?
Worked example
Compare 16/20 and 2/20 germination percentages.
Show the worked solution
- The percentages are 80% and 10%.
- Subtract percentages for the percentage-point difference.
Answer 80% versus 10%; difference 70 percentage points.
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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.
State an oxygen comparison and four appropriate controls.
Give me a hint
Use stated gas availability with equal suitable hydration.
Compare my reasoning
- Compare oxygen-available and oxygen-limited prepared chambers.
- Match water, temperature, seed stock and observation time.
Compare stated oxygen availability; keep moisture, temperature, viable seed stock and timing matched, with independent repeat chambers.
Look for these in your work
- Make oxygen the intended variable
- Keep the four major controls
Calculate germination percentages and their difference for 16 of 20 versus 2 of 20.
Give me a hint
Use each starting total, then subtract percentages.
Compare my reasoning
- 16/20 × 100 = 80%; 2/20 × 100 = 10%.
- 80 − 10 = 70 percentage points.
80% and 10%; difference 70 percentage points.
Look for these in your work
- Use the correct denominators
- Name the difference as percentage points
Why are “no oxygen” and “all seeds died” unsupported descriptions of the supplied oxygen-limited record?
Give me a hint
Check what the conditions and counts actually state.
Compare my reasoning
- Limited oxygen is not complete absence.
- No radicle by day 5 does not establish death.
The condition is oxygen-limited, not proven oxygen-free; the counts record emergence by day 5, not permanent viability or death.
Look for these in your work
- Retain the stated oxygen condition
- Retain the timed observation boundary
Common mistakes
- Calling oxygen-limited completely oxygen-free without evidence.
- Treating germination counts as direct respiration-rate measurements.
What can you explain now?
Does 2/20 emergence in an oxygen-limited group prove oxygen is unnecessary?
Compare with the explanation
No; limited does not mean proven absent
Available oxygen and timing may still permit some emergence.
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Make it stick
Interpret an oxygen comparison without inventing an oxygen-free atmosphere.
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.