Biology 11–12 · Year 11

Water uptake by a shoot in a bubble potometer under changing conditions

Module 2: Organisation of Living Things (Transport)

Practical, model not builtLow risk

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The idea

Water lost from leaves by transpiration is replaced through the xylem, so the rate at which a bubble travels along a capillary measures the pull generated by evaporation.

What you need

Leafy shoot of a woody plant, 20 to 30 cm, cut under water
Potometer (capillary tube of known bore, for example 1.0 mm, with a reservoir and tap) assembled under water; clamp stand; ruler or millimetre scale; marker
Stop clock; thermometer; fan; lamp; clear plastic bag; petroleum jelly
Beaker of water for the capillary end

How to do it

Cut the shoot and assemble the potometer under water so no air enters the xylem; seal joints with petroleum jelly and check for leaks.
Introduce a bubble by lifting the capillary end out of the water briefly and blotting it, then return it to the beaker.
Mark the capillary at 10 mm intervals; record the distance the bubble travels in 5 minutes, then reset the bubble from the reservoir.
Repeat three times under still air, then with the fan, then with the lamp, then with the shoot enclosed in a plastic bag (high humidity).
Convert distance to volume: volume = pi x (bore radius)^2 x distance; express the rate in microlitres per minute.
Count or trace the leaves to estimate leaf area and express the rate per square centimetre.

What you should see

For a 1.0 mm bore each millimetre of bubble travel is 0.785 microlitres (computed); 25 mm in 5 minutes is 3.93 microlitres per minute. The fan and lamp raise the rate, and the bag lowers it as the air inside becomes humid. The learner knows it worked when the bubble travels steadily with no leak, the reset returns it cleanly, and the order of the four conditions is the same in repeat runs.

What changes

What you change: air flow, light or humidity around the leaves
What you measure: volume of water taken up per minute
What you keep the same: shoot
temperature
capillary bore
time interval
leaf area

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

The potometer measures transpiration directly; it measures uptake, which is slightly larger than water lost because some water is used in the shoot.
Water is pushed up from the roots; the pull comes from evaporation at the leaves.
Wind cools the leaf and so slows transpiration; wind removes humid air and speeds it.

Safety card

Low riskLearners carry it out

Hazards

Fragile glass capillary
Scalpel when cutting the shoot
Plant sap may irritate skin

Controls

Handle the apparatus gently and clamp it firmly
Cut away from the body
Wash hands after handling

Note

No hazardous chemicals; see RiskAssess (https://www.riskassess.com.au/).

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Biology Stage 6 Syllabus (2017), current: Year 11 taught to the end of 2026 and Year 12 to Term 3 2027BIO11-9 BIO11/12-4 BIO11/12-6
Biology 11–12 Syllabus (2025), Year 11 focus area Cells to systems; new syllabus not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027BI-11-02
Biology 11–12 Syllabus (2025), Year 11 Working scientifically; new syllabus not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027BI-11WS-03 BI-11WS-04 BI-11WS-06
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities