A respiration experiment gives you numbers, and the exam task is to describe the pattern with data, calculate a rate and draw a conclusion the results can support. The common apparatus is a respirometer, which measures oxygen uptake. All the data in this lesson are invented for teaching.
This lesson applies the ideas from comparing aerobic and anaerobic models to results.
How does a simple respirometer work?
Germinating seeds sit in a sealed tube with soda lime, which absorbs the carbon dioxide they release. The seeds use oxygen in aerobic respiration. With no carbon dioxide left to replace it, the gas volume falls and the coloured liquid in a capillary tube moves towards the seeds.
The distance moved, multiplied by the cross-sectional area of the capillary bore, gives the volume of oxygen taken up. A control tube uses killed seeds of the same mass, so any movement not caused by respiration shows up.
How do you work with the data, step by step?
- Read the table and say what was changed, measured and kept the same.
- Check the control first, to see whether any change is caused by something else.
- Calculate the rate as distance or volume divided by time.
- Compare the rates with numbers, such as “twice as fast”.
- Write a conclusion that links to respiration and stays within the tested range.
Worked example
Germinating seeds were put in a respirometer at three temperatures.
The capillary bore has a cross-sectional area of 0.8 mm². The liquid movement was recorded over 20 minutes. These are invented data.
| Condition | Distance moved in 20 min (mm) |
|---|---|
| Living seeds at 15 °C | 12 |
| Living seeds at 25 °C | 24 |
| Living seeds at 35 °C | 40 |
| Killed seeds at 25 °C (control) | 0 |
Step 1, control: the killed seeds gave 0 mm, so the movement in the other tubes is due to living seeds.
Step 2, rate of movement: 15 °C: 12 ÷ 20 = 0.6 mm/min. 25 °C: 24 ÷ 20 = 1.2 mm/min. 35 °C: 40 ÷ 20 = 2.0 mm/min.
Step 3, volume rate: multiply by the area of 0.8 mm². At 15 °C: 0.6 × 0.8 = 0.48 mm³/min. At 25 °C: 1.2 × 0.8 = 0.96 mm³/min. At 35 °C: 2.0 × 0.8 = 1.6 mm³/min.
Step 4, compare: the rate at 25 °C is double the rate at 15 °C (0.96 ÷ 0.48 = 2). The rate at 35 °C is about 1.7 times the rate at 25 °C (1.6 ÷ 0.96 ≈ 1.67).
Step 5, conclusion: in this range, a higher temperature gave a faster oxygen uptake, so the rate of aerobic respiration increased. The data do not show what happens above 35 °C, so no claim should be made about that.
The mistake to watch for
A plausible mistake is to read the movement of the liquid as carbon dioxide production, or to extend the trend beyond the data.
Mistaken answer: “The seeds made more carbon dioxide at 35 °C, so respiration will keep getting faster at higher temperatures.”
The carbon dioxide was absorbed, so the movement shows oxygen uptake, and there are no results above 35 °C to support the prediction.
The correction: the liquid moves because oxygen is used and the carbon dioxide is absorbed, and the conclusion is limited to 15 °C to 35 °C. To predict beyond that, the experiment would need more temperatures.
Check yourself
1. Using the table above, calculate the volume rate of oxygen uptake at 25 °C in mm³ per minute.
Show answer
24 ÷ 20 = 1.2 mm/min, then 1.2 × 0.8 = 0.96 mm³/min.
2. Explain what the killed seeds show in this experiment.
Show answer
The killed seeds do not respire, and the liquid did not move. This shows the movement in the other tubes was caused by the living seeds’ respiration, not by leaks or temperature changes in the apparatus.
3. A student says the results prove that 35 °C is the ideal temperature for respiration. Give a limited response.
Show answer
The data show the rate was highest of the three temperatures tested, but other temperatures were not tested, so no optimum can be claimed. One experiment with a single run at each temperature is also limited, and repeats would help.
Where this leads next
Try the mixed practice set, which includes a data question, and record any slips in the mistake log and retest queue. Go back to the module overview to plan your revision.
Students who lose marks on conclusions can rehearse writing them aloud with a teacher in online one-to-one Biology tuition.