Plant Transport Virtual Lab: Potometer, Xylem and Phloem

Updated 2026-10-07

This plant transport virtual lab runs the potometer practical from GCSE and A-level Biology: a cut shoot sealed into a capillary tube, an air bubble that moves as the shoot takes up water, and a stopwatch. Students take three readings at each of three air temperatures, calculate the rate and the volume of water taken up with V = πr²d, and explain the trend with cohesion–tension. A second practical stands celery in colored water to show that water moves in the xylem. One period gives a full results table, a clear trend and a calculation students meet in exams.

Transport in plants – root uptake, xylem and phloem

Curriculum links

AQA GCSE Biology 4.2.3.1–4.2.3.2 (plant tissues and the plant organ system): xylem and phloem, transpiration, and the effect of temperature, humidity, air flow and light on the rate of transpiration.
AQA A-level Biology 3.3.4.2 (mass transport in plants): cohesion–tension, the potometer, and translocation by mass flow in the phloem.
IB Biology B3.2 (transport) and Cambridge IGCSE Biology transport in plants.

Simulic is not affiliated with or endorsed by AQA, the IB, Cambridge or any exam board.

Before the lab (5 min)

Ask students to commit to a prediction, on paper or as question 1 of the class link:

"A leafy shoot in a potometer is moved from 15 °C air to 35 °C air. Light, humidity and wind stay the same. What happens to the rate of water uptake?"

Some students expect no change ("the plant only needs so much water"). Others expect a small rise. Few expect it to more than triple.

Method in the simulation

Open the Practicals tab and choose Potometer. Set Time to 2 min.
Keep the control conditions: Light 70%, Humidity 50%, Wind 1 m/s.
Set Temperature to 15 °C. Press Start reading three times; each reading starts with the bubble at the start mark. The table records the distance, the rate in mm/min and the volume in mm³/min. The box shows the mean of the readings at these conditions.
Repeat at 25 °C and 35 °C.
For one reading, calculate the volume yourself: V = πr²d, with r = 0.5 mm.
Celery: choose Celery in coloured water, 30 min, Temperature 25 °C. Run once with Leaves on ticked and once without, then press Cut sections to see which tissue is stained.

Temperature (°C)	Distance in 2 min (mm): 1 / 2 / 3	Mean rate (mm/min)	Volume taken up (mm³/min)
15
25
35

the potometer at 25 °C with the bubble 22 mm from the start mark and three readings in the results table

Expected results

All values come from the simulation. Each reading has a random error of about ±6%, so groups differ slightly.

Temperature (°C)	Example readings (mm/min)	Model rate (mm/min)	Volume (mm³/min)
15	6.15, 6.10, 6.25	5.9	4.6
25	11.56, 11.04, 11.00	11.0	8.6
35	19.73, 18.79, 20.01	19.5	15.3

The rate roughly doubles every 10 °C: from 15 to 35 °C it more than triples.
At 25 °C the bubble moves about 22 mm in 2 min, so V = π × 0.5² × 22 ≈ 17 mm³ in 2 min, or about 8.6 mm³/min.
Other factors at 25 °C: 90% humidity drops the rate to 2.2 mm/min; still air gives 5.7; darkness gives 1.5 because the stomata close.
Celery, 30 min: with leaves the dye rises about 87 mm (2.9 mm/min); without leaves only about 7 mm. The stained dots in the cross-sections are xylem in the vascular bundles.

Questions for students

(Prediction, asked again after the lab) What happens to the rate of water uptake from 15 °C to 35 °C?
Which conditions must stay the same while you test temperature?
At 25 °C, what is the mean rate of three 2-min readings?
At 35 °C, what volume of water does the shoot take up per minute?
Explain why the rate rises with temperature and how the water reaches the leaves.

Answers for teachers: (1) It more than triples, from about 5.9 to 19.5 mm/min. (2) Light, humidity and wind (and the same shoot). (3) About 11.0 mm/min (accept 10.2 to 11.8). (4) About 15.3 mm³/min (accept 14.0 to 16.6). (5) Warmer air holds more water vapor and water evaporates faster from the leaf cells, so the vapor gradient out of the stomata is steeper and more water is lost. Evaporation pulls the continuous water column up the xylem, held together by cohesion between water molecules.

Common misconceptions

"A potometer measures transpiration." It measures water uptake. Most of that water is transpired, but a little is used in photosynthesis and to keep cells turgid.
"Water is pumped up the stem." Xylem vessels are dead tubes. Water is pulled from above by evaporation, not pushed by the roots, except for a small root pressure.
"Phloem carries water up from the roots." Phloem carries sucrose from sources to sinks, both up and down. Try Ring the stem on the Whole plant tab: the leaves stay fresh, because the xylem is intact.

Extension

On the Phloem (pressure flow) tab, change the sucrose loading and the sink, and follow the pressure difference that drives mass flow.
On the Whole plant tab, press Give one leaf ¹⁴CO₂ and see where the labeled sucrose ends up.

FAQ

How is this different from the transpiration virtual lab?

The transpiration virtual lab uses a whole plant and stomata to compare wind, light, humidity and soil water. This lab uses a potometer, adds temperature, and covers xylem and phloem.

Why does every reading differ a little?

The simulation adds a random error of about ±6%, like a real potometer. That is why the method takes three readings at each temperature.

Does this replace a real potometer?

No. Real potometers teach skills such as cutting the shoot under water and sealing joints. Use the simulation to plan, to practice the calculation, or when a shoot fails on the day. See virtual labs vs physical labs.