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B3.1.9—Transpiration

Transpiration is water loss from leaves, driven by evaporation and diffusion through stomata and altered by humidity, temperature, wind and light.

Syllabus
First assessment 2025
Objective
B3.1.9
Level
HL

Exam analysis

Chance of appearing15%of analysed past papers
Latest appearanceMay 2025
Most common paperPaper3
Typical marks1–2

Common command terms

  • Explain
  • Outline
  • Suggest
  • Define
  • Compare

Scoring notes

Common mistake
Assuming a potometer directly measures water lost rather than estimating it from water uptake.

Recent exam appearances

May 2025Paper1A ["HL"] · TZ124[ 1 ]B3.1.9—Transpiration
November 2024Paper2 ["HL"] · TZ01(i)[ 2 ]B3.1.9—Transpiration
November 2024Paper2 ["HL"] · TZ01(d)[ 2 ]B3.1.9—Transpiration
November 2023Paper1 ["HL"] · TZ228[ 1 ]B3.1.9—Transpiration
May 2023Paper3 ["HL"] · TZ13(d)[ 2 ]B3.1.9—Transpiration
Practice this objective

Coverage 2012–2025 · Updated 15 Jul 2026

Transpiration Pulls Water Through Leaves

Transpiration is water loss from a plant: water evaporates from moist mesophyll cell walls and the vapour diffuses through leaf air spaces and out through stomata.

Gas exchange exposes moist internal surfaces to the atmosphere, so stomatal opening for carbon dioxide entry also permits water-vapour loss. A steeper water-vapour gradient or more open stomata increases the rate.

Higher temperature increases evaporation; lower humidity steepens the vapour gradient; wind removes the humid boundary layer; light commonly promotes stomatal opening. Water stress can close stomata and reduce transpiration.

A warm, dry, windy leaf usually transpires faster than a cool leaf in still, humid air because evaporation is faster and the external boundary layer is continually replaced.

A potometer measures water uptake as an estimate, not transpiration directly; water may also be used in growth or photosynthesis. State which factor changes evaporation, gradient or stomatal aperture.

Transpiration exam focus

Assessment in practice

1–2 marks
How it is assessed

This objective is assessed through structured response, commonly using Explain / Outline / Suggest.

Command terms

Explain / Outline / Suggest / Define / Compare

What earns marks

Build the answer around this relationship: Transpiration involves evaporation from leaf surfaces followed by diffusion through stomata.

Watch for

Assuming a potometer directly measures water lost rather than estimating it from water uptake.

Representative question

Question 1

[Maximum number: 8]

Explain how abiotic factors affect the rate of transpiration in terrestrial plants.

Gas Exchange Across Animals And Leaves

Core gas-exchange answers link exchange surfaces to diffusion gradients. For animals, exchange surfaces are explained by diffusion properties, ventilation, and blood flow. For plants, leaves allow carbon dioxide entry and oxygen/water vapour exit while controlling water loss through stomata. Spirometry, transpiration, and stomatal density data provide evidence of gradient and surface-area effects.

  • Core animal answer: large, thin, moist, permeable surface plus ventilation and blood flow.
  • Core plant answer: stomata, guard cells, mesophyll air spaces, cuticle, and transpiration factors.
  • Data questions usually test rate, gradient, volume, or density per area.

Concept essentials

  • Transpiration involves evaporation from leaf surfaces followed by diffusion through stomata.
  • Low humidity, higher temperature and wind usually increase the rate of transpiration.
  • Light can increase transpiration by promoting stomatal opening for photosynthesis.
  • Potometers estimate water uptake over time as an indirect measure of transpiration.
  • Drought can reduce transpiration when stomata close to conserve water.