Biology · 8.3 Transpiration · Paper 5/6 practical
Transpiration. Track the bubble.
A potometer measures how fast a leafy shoot takes up water. As water is lost from the leaves, an air bubble moves along the capillary tube. Change wind, humidity, temperature and light and measure how the rate changes.
0610 Topic 8.3 — Transpiration
potometer · environmental factors
Paper 5/6 — Practical
0 s
bubble 0.0 mm
Conditions
off
normal
20
bright
Live readouts
Distance moved
0.0 mm
Rate
— mm/min
Why
—
Faster water uptake (bubble moves further) means a faster transpiration rate.
Results — compare conditions
Record the rate under different conditions.
📋 Method (Cambridge practical procedure)
- Cut a leafy shoot under water and fit it into the potometer so it is airtight (assemble under water to avoid air locks).
- Introduce a single air bubble into the capillary tube and note its starting position against the ruler.
- Time how far the bubble moves in a set time — this measures the rate of water uptake (≈ transpiration).
- Reset the bubble (open the reservoir tap) and repeat under a new condition, changing one factor at a time.
- Compare the rates: wind, heat and bright light speed it up; high humidity slows it down.
⚠ Control variables & precautions
- Use the same shoot with the same leaf area; change only one environmental factor at a time.
- Set up under water and seal joints with petroleum jelly so no air leaks in.
- Let the shoot acclimatise before each measurement; repeat and take a mean.
- The potometer measures water uptake, which is slightly more than water lost in transpiration.
🎯 Syllabus reference (0610)
- 8.3 Transpiration — investigate and describe the effects of temperature, wind, humidity and light intensity on transpiration rate using a potometer.