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LearnYBI11WBI12 · Unit 2: Cells, Development, Biodiversity and ConservationPlant structure and function, biodiversity and conservationXylem, phloem and transpiration
Plant structure and function, biodiversity and conservation · Subtopic 3 of 4

Xylem, Phloem and Transpiration

Xylem vessels are dead, hollow tubes with no end walls and lignified walls (strength, waterproof) with pits. Water evaporates from mesophyll cells and diffuses out through stomata (transpiration), creating tension; cohesion between water molecules (hydrogen bonds) and adhesion to vessel walls pull a continuous column up: cohesion–tension. Water crosses the root via the apoplast (cell walls) and symplast (cytoplasm), but the Casparian strip forces it into the symplast at the endodermis. Transpiration increases with light, temperature and wind and decreases with humidity; measured with a potometer (water uptake). Xerophytes reduce water loss (sunken stomata, hairs, rolled leaves, thick cuticle). Phloem sieve tubes with companion cells move sucrose by mass flow from source to sink: sucrose loaded by co-transport lowers ψ, water enters, hydrostatic pressure pushes sap.

Cambridge 9700 Paper 1Cambridge 9700 Paper 2Edexcel YBI11 WBI12AQA 7402 Paper 1≈ 50 minRevision 1 · updated 2026-11-02
Section 1 of 14

Learning objectives

  • Relate xylem and phloem structure to function.
  • Explain cohesion–tension.
  • Describe factors affecting transpiration and use a potometer.
  • Explain mass flow in phloem.
Pause and think

Trees over 100 m tall move water to their tops without a pump. How?

Section 2 of 14

Core idea

Start herePumpless plumbing

A giant redwood lifts hundreds of litres of water a day up 100 m with no pump.

The big idea

Evaporation from leaves pulls a cohesive water column up dead xylem; living phloem moves sugars by pressure from source to sink.

Water from soil to air

Water moves down a water potential gradient.

Worked examplePotometer rate

Bubble moves 30 mm in 5 min; capillary radius 0.5 mm. Uptake rate?

Goal
mm³ min⁻¹.
Strategy
V = πr²l.
  1. V=π×0.52×30=23.6 mm3V=\pi\times0.5^2\times30=23.6\ \text{mm}^3
    Volume of water taken up.
  2. 23.65=4.71 mm3 min−1\frac{23.6}{5}=4.71\ \text{mm}^3\ \text{min}^{-1}
    Per minute.
Answer4.71 mm³ min⁻¹.
✓ CheckUptake, not exactly transpiration ✓.

Summary

  • Cohesion–tension.
  • Transpiration factors.
  • Mass flow in phloem.
Key words
Transpiration
Loss of water vapour from leaves.
Translocation
Movement of assimilates in phloem.
HistoryDixon and Joly. Henry Dixon and John Joly proposed the cohesion–tension theory in 1894 at Trinity College Dublin.
Fun factAphid probes. Scientists sample phloem sap by letting aphids feed, then cutting off the aphid and collecting sap from its mouthparts.
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