Interpret germination data with correct denominators and limits
Compare a suitable control with one restricted condition at a time.
Cross-referenced to the 2028–2029 0653 section using original teaching from the related single-science courses. A 0653-specific objective-by-objective and Core/Supplement review is in progress. Written practice is self-checked and 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 denominator calculates germination percentage for each group?
What you will learn
- Calculate timed germination percentages
- Use controlled contrasts without claiming permanent failure
Interpret germination data with correct denominators and limits
Invented day-5 counts out of 20: 0/16/2/1 → 0%/80%/10%/5%. Compare the suitable control with one restricted condition at a time. 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 |
|---|---|
| Denominator | Starting total: 20 per group |
| Suitable control | Moist, oxygen available, 22°C |
| Single-factor contrasts | Dry; oxygen limited; or 5°C |
| Difference to 5°C | 75 percentage points, not 75% relative increase |
Calculate timed germination percentages
The invented day-5 table uses the same starting total of 20 seeds in every group. The dry oxygen-available group at 22°C has 0 germinations; the moist oxygen-available group at 22°C has 16; the moist oxygen-limited group at 22°C has 2; and the moist oxygen-available group at 5°C has 1. Counts use radicle emergence, not mature seedling establishment.
Calculate each percentage using germinated seeds divided by starting seeds, multiplied by 100. The four percentages are 0%, 80%, 10% and 5%. Using 16 as the denominator for all groups would compare a suitable control’s successes rather than the actual number of seeds exposed to each condition, and would not calculate germination percentage.
Use controlled contrasts without claiming permanent failure
Use the moist oxygen-available 22°C group as the suitable control. Compare it with the dry group to investigate water, with the oxygen-limited group to investigate oxygen availability, and with the 5°C group to investigate temperature. Each comparison changes one intended factor. Directly comparing the dry 22°C group with the moist 5°C group changes water and temperature together and cannot isolate either effect.
The differences from the suitable control are 80, 70 and 75 percentage points respectively. These are absolute differences between percentages, not relative percentage increases. Conclusions concern this seed stock and day-5 criteria under the stipulated conditions. The record supports the three requirements but does not prove that every ungerminated seed is dead, that all other species match these percentages or that the conditions were verified in an actual practical.
Sources for this lesson
Pause and explain
Which pair isolates water?
Worked example
Which comparison isolates temperature in the table?
Show the worked solution
- Keep moisture and oxygen available in both groups.
- Compare 22°C with 5°C under those matched conditions.
Answer Moist oxygen-available seeds at 22°C versus 5°C.
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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.
Calculate the four germination percentages in table order: 0, 16, 2 and 1 out of 20.
Give me a hint
Use the same starting denominator for each.
Compare my reasoning
- Multiply each count by 100/20 = 5.
- The percentages are 0, 80, 10 and 5.
0%, 80%, 10% and 5%, all by day 5.
Look for these in your work
- Use 20 for every group
- Retain the observation time
Identify the suitable control and the single-factor comparison for each required condition.
Give me a hint
Keep the other two conditions matched.
Compare my reasoning
- Control is moist, oxygen available, 22°C.
- Compare with dry for water, oxygen-limited for oxygen, and 5°C for temperature.
Use moist/oxygen-available/22°C as control; contrast dry/available/22°C, moist/limited/22°C and moist/available/5°C respectively.
Look for these in your work
- Identify the control
- Match each restricted condition correctly
Calculate the suitable-control versus 5°C difference and explain one limit of the conclusion.
Give me a hint
Subtract 5% from 80%, not from a seed count.
Compare my reasoning
- 80 − 5 = 75 percentage points.
- The record is time-limited for one stock and does not establish seed death.
Difference 75 percentage points; it concerns emergence by day 5 in this stock, not permanent death or a universal species response.
Look for these in your work
- Use percentage points
- State a relevant evidence boundary
Common mistakes
- Using the control’s success count as every denominator.
- Comparing two changed conditions as if only one changed.
What can you explain now?
Which pair isolates water?
Compare with the explanation
Dry versus moist at matched oxygen and 22°C
Only water availability is the intended difference.
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Make it stick
Interpret controlled germination contrasts with correct denominators and percentage-point language.
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.