Science 7–10 · Year 8

Transpiration with a straw potometer: water uptake against air movement

Science understanding: Biological sciences

PracticalLow risk

The idea

Water that evaporates from a shoot is replaced by water drawn up the stem, so the rise of the lower end of the water column in a narrow straw measures how fast the shoot takes up water, which closely tracks transpiration.

What you need

Three woody shoots from the same shrub with stems about 3 mm across and leaves without a thick waxy cuticle (SAPS suggests rosemary or lavender), each placed in water as soon as it is cut
Three clear 3 mm "sip" straws, as in the SAPS straw potometer, petroleum jelly, scissors, masking tape and a pen
A sink or large bowl of water, paper towel, a ruler, a stopwatch and 1 cm grid paper for leaf area
A desk fan and a large clear plastic bag for the moving-air and humid treatments

How to do it

Put a straw and a shoot in the bowl of water. Recut the stem under water at an angle and push it into the straw while both are still under water.
Put a finger over the other end of the straw, lift the assembly out and smear petroleum jelly around the join, keeping it off the cut end of the stem. When the finger is removed the water should stay in the straw; if it does not, there is a leak, so reassemble under water.
Dry the straw and leaves, tape the straw upright beside a window with the shoot at the top and the open end of the straw at the bottom, as the SAPS photographs show, and mark the lower end of the water column on masking tape. Make three potometers the same way.
Leave one in still air, one in the airflow of the fan and one inside the clear bag. Mark the lower end of the water column every five minutes for 30 minutes; it moves up the straw as the shoot takes up water, and SAPS notes a change is visible in about 10 minutes.
Convert each rise of the lower end into volume: volume = pi times r^2 times distance, with r = 1.5 mm for a 3 mm internal diameter (measure your own straw).
Because each condition uses a different shoot, trace its leaves on the grid paper and divide each rate by the leaf area. Compare rates (mm3 per minute per cm2) across the three conditions and pool class results.

What you should see

The lower end of the water column moves steadily up the straw as the shoot takes up water, because the shoot seals the top of the straw and surface tension holds the water in at the open bottom end. The fan shoot is expected to take up water fastest per square centimetre of leaf and the bagged shoot slowest, because moving air carries water vapour away from the leaves and the bag keeps the air around them humid. For a 3 mm internal diameter, a 10 mm rise is 70.7 mm3 (0.0707 mL) of water taken up. The learner knows it worked when the lower end moves steadily in every potometer and the class ranking of the three conditions is consistent.

What changes

What you change: Air movement (still, fan, bagged)
What you measure: Water uptake per minute per cm2 of leaf
What you keep the same: Same shrub and similar shoots
Straw diameter
Light and temperature at the window
Timing
Leaf area corrected for

Common misconceptions

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

The water moves because the plant drinks like an animal; the pull comes from evaporation at the leaves.
Water goes up the stem through the whole stem; it moves in xylem vessels.
Plants lose no water when it is cloudy.

Safety card

Low riskLearners carry it out

Hazards

Scissors used under water
Irritant sap from some plants
Fan near water

Controls

Cut away from the hand
Gloves for anyone who reacts to plant sap, as SAPS suggests
Fan on a dry bench away from the bowl

Note

No hazardous chemicals; record a RiskAssess risk assessment for the activity as school procedure requires.

Curriculum references

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

Science 7–10 Syllabus (2023)SC4-LIV-01 SC4-WS-03 SC4-WS-04 SC4-WS-06
Australian Curriculum v9AC9S8U02AC9S8I02AC9S8I03AC9S8I05

Sources

The pages the author read to write this activity.

All Concept Studio activities