IGCSE · Practice questions · Transpiration

Evaporate, diffuse, pull.

Six original Cambridge-style questions on how water leaves a leaf, potometer data and calculations, the effects of temperature, wind and humidity, the transpiration pull and why plants wilt.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Two steps, three factors.

Steps: water evaporates from mesophyll cell surfaces into air spaces, then water vapour diffuses out through the stomata.
Faster with: higher temperature, higher wind speed; slower with higher humidity (Extended explanations: kinetic energy, gradient).
Potometer: measures water uptake; rate = distance ÷ time; change one factor at a time.
Extended: transpiration pull on a column held together by attraction between water molecules; wilting = flaccid cells.

Recall

[3 marks]

Describe how water leaves a leaf during transpiration.

Water evaporates from the surfaces of the mesophyll cells. ✓
Into the air spaces inside the leaf. ✓
Water vapour then diffuses out of the leaf through the stomata. ✓

Calculation

[4 marks]

A student used a potometer to time an air bubble moving 50 mm at different temperatures.

Temperature / °C	Time / s
15	500
25	300
35	170

(a) Calculate the rate of water uptake in mm per minute at each temperature, to one decimal place. (b) Describe the trend.

(a) 15 °C: 50 ÷ 500 × 60 = 6.0 mm per minute. ✓
25 °C: 50 ÷ 300 × 60 = 10.0 mm per minute. ✓
35 °C: 50 ÷ 170 × 60 = 17.6 mm per minute. ✓
(b) As temperature increases, the rate of water uptake (and so transpiration) increases, and it rises more steeply at higher temperatures. ✓

Analysis

[4 marks]

Plan an investigation, using a potometer, to find the effect of wind speed on the rate of transpiration.

Cut a leafy shoot under water, fit it to the potometer under water and seal the joints with petroleum jelly. ✓
Place a fan at a fixed distance and use several settings (off, low, medium, high) to vary wind speed. ✓
For each setting, time how long the bubble takes to move a fixed distance (or measure distance moved in a fixed time) and calculate the rate; reset the bubble with the reservoir between readings. ✓
Keep temperature, light intensity and the same shoot constant; repeat each setting and calculate a mean. ✓

Explain

[3 marks]

Extended. Explain why the rate of transpiration is higher on a hot, dry, windy day than on a cool, humid, still day.

Higher temperature: water molecules have more kinetic energy, so they evaporate from mesophyll surfaces and diffuse out faster. ✓
Dry air (low humidity): less water vapour outside, so a steeper concentration gradient from leaf to air. ✓
Wind: removes water vapour from around the stomata, keeping the concentration gradient steep. ✓

Explain

[4 marks]

Extended. Explain how water moves up the xylem from the roots to the leaves of a tall tree.

Water evaporates from mesophyll cells and diffuses out of the leaves as water vapour (transpiration). ✓
Water moves from the xylem in the leaf into the mesophyll cells to replace it, creating a pull (tension) on the water in the xylem. ✓
Water molecules are held together by forces of attraction between them, forming a continuous column. ✓
So the whole column of water is drawn up the xylem from the roots: the transpiration pull. ✓

Explain

[4 marks]

Extended. A pot plant left on a sunny windowsill without water wilts by the afternoon. Explain how and why wilting occurs, and suggest how wilting can help the plant.

Water is lost by transpiration faster than it is taken up from the dry soil. ✓
The cells lose water, so their vacuoles shrink and they no longer press outwards on their cell walls: they become flaccid. ✓
Flaccid cells cannot support the leaves and stem, so the plant droops. ✓
Drooping leaves expose less surface to the sun and the stomata close, reducing further water loss. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.