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Learn4BI1Paper 1B · Paper 1 (core content)Structure and functions in living organismsTranspiration and translocation
Structure and functions in living organisms · Subtopic 18 of 21

Transpiration and Translocation

Transpiration is the loss of water vapour from leaves: water evaporates from mesophyll cell surfaces into air spaces, then diffuses out through the stomata. This pulls a continuous column of water up the xylem (the transpiration stream), because water molecules stick together (cohesion). Transpiration is faster when it is hotter, drier (low humidity), windier and brighter (more stomata open). It's measured with a potometer. Translocation moves sucrose and amino acids in the phloem from sources (leaves, storage organs) to sinks (growing points, roots, fruits).

Cambridge 5090 Paper 1Cambridge 5090 Paper 2Cambridge 0610 Papers 2, 4Cambridge 0610 Papers 1, 3Edexcel 4BI1 Paper 1BEdexcel 4BI1 Paper 2B≈ 40 minRevision 1 · updated 2026-10-30
Section 1 of 14

Learning objectives

  • Describe transpiration and the transpiration stream.
  • Explain how temperature, humidity, wind and light affect the rate.
  • Use potometer results to calculate rates.
  • Describe translocation from sources to sinks.
Pause and think

Washing dries fastest on a warm, windy, dry day. Why is the same true for transpiration?

Section 2 of 14

Core idea

Start hereLeaves that sweat

Put a plastic bag over a leafy branch on a sunny day and within an hour droplets collect inside. That water left the leaves as vapour.

The big idea

Evaporation from leaves pulls water up the xylem. Conditions that speed up evaporation or steepen the vapour gradient speed up transpiration.

1. The transpiration stream

How water is pulled up

Evaporation at the top drives the flow.

2. Factors and potometers

Worked exampleComparing conditions

A potometer bubble moves 30 mm in 5 min in still air and 70 mm in 5 min with a fan.

Goal
Rates and explanation.
Strategy
Distance ÷ time.
  1. 30/5=6 mm/min; 70/5=14 mm/min30/5=6\ \text{mm/min};\ 70/5=14\ \text{mm/min}
    Rates.
  2. fan removes humid air\text{fan removes humid air}
    Steeper water vapour gradient, faster diffusion.
AnswerWind more than doubled the rate.
✓ CheckConsistent with theory ✓.
Key takeawayTranslocation Phloem moves sucrose from sources (leaves, storage organs) to sinks (roots, buds, fruits, seeds), which can change with the seasons.
Pause and think

Why do many plants close their stomata in the middle of a hot day?

Summary

  • Evaporation + diffusion.
  • Hot, dry, windy, bright = faster.
  • Source → sink.
Key words
Transpiration
Loss of water vapour from leaves.
Potometer
Apparatus that measures water uptake by a shoot.
Translocation
Movement of sucrose and amino acids in phloem.
HistoryCohesion theory. In 1894 Henry Dixon and John Joly proposed that water is pulled up trees by evaporation from leaves and held together by cohesion, the explanation still accepted today.
Fun factThirsty oak. A large oak tree can transpire over 400 litres of water on a hot summer day.
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