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Learn4BI1Paper 1B · Paper 1 (core content)Structure and functions in living organismsXylem, phloem and water uptake
Structure and functions in living organisms · Subtopic 9 of 21

Xylem, Phloem and Water Uptake

Plants have two transport tissues in vascular bundles. Xylem carries water and mineral ions up from the roots; its vessels are dead, hollow tubes with no end walls and thick walls strengthened with lignin, which also support the plant. Phloem carries sucrose and amino acids, both up and down, from where they're made or stored (sources) to where they're needed (sinks). Water enters root hair cells by osmosis, crosses the root cortex to the xylem, and travels up to the leaves.

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

Learning objectives

  • State the functions of xylem and phloem.
  • Relate the structure of xylem to its function.
  • Describe the pathway of water from soil to leaf.
Pause and think

Celery placed in red-coloured water develops red dots in its cut stem after an hour. What are the dots?

Section 2 of 14

Core idea

Start herePlumbing without a pump

Plants have no heart, yet water travels from root tips to the highest leaves. Two tissues, xylem and phloem, carry everything a plant needs.

The big idea

Xylem carries water and minerals upwards in dead, hollow, lignified tubes. Phloem carries sugars and amino acids in living cells, in whichever direction they're needed.

1. Two transport tissues

Xylem vs phloem

Different cargo, different direction.

2. Water's journey

Worked exampleSoil to air

Describe the route of a water molecule from soil to air.

Goal
Sequence with processes.
Strategy
Follow the tissues.
  1. soil→root hair\text{soil}\rightarrow\text{root hair}
    Osmosis: soil water is more dilute than cell sap.
  2. →cortex→xylem\rightarrow\text{cortex}\rightarrow\text{xylem}
    Across the root, cell to cell.
  3. →leaf mesophyll→air spaces\rightarrow\text{leaf mesophyll}\rightarrow\text{air spaces}
    Up the stem; evaporates from cell surfaces.
  4. →stomata→air\rightarrow\text{stomata}\rightarrow\text{air}
    Water vapour diffuses out.
AnswerRoot hair → cortex → xylem → mesophyll → stomata.
✓ CheckEach step follows a water potential gradient ✓.
Key takeawayRoot hairs Their long extensions give a large surface area for absorbing water (osmosis) and minerals (active transport).
Pause and think

Why is lignin useful to a tree beyond water transport?

Summary

  • Xylem: water, up.
  • Phloem: sugar, both ways.
  • Root hair → xylem → leaf.
Key words
Xylem
Tissue carrying water and minerals up the plant.
Phloem
Tissue carrying sucrose and amino acids.
Lignin
Strengthening substance in xylem walls.
HistoryStephen Hales. In 1727 Stephen Hales measured the pressure of sap rising in vines and the water lost by sunflowers, founding plant physiology.
Fun factTallest drinking straw. Coast redwoods over 110 m tall lift water from their roots to their top leaves through xylem, with no pump at all.
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