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Cambridge IGCSE Biology · 0610

Chapter 8: Transport in plants (Part 1)

Functions of xylem and phloem

Plants possess a transport system consisting of two main types of vessels that move substances between the roots, stems, and leaves:

Xylem
Transports water and mineral ions from the roots to the rest of the plant and provides support.
Phloem
Transports sucrose and amino acids.

Examiner Report Insights

  • Examiners often test whether you can state the substance each tissue transports without swapping them.

Exam Traps

  • Do not say phloem transports water or xylem transports sucrose.
  • Avoid describing xylem as living cells with nuclei — vessels are dead and hollow.

Positions of transport tissues

The arrangement of xylem and phloem varies depending on the part of the plant:

  • In roots: The xylem is located in the center in a characteristic X-shape, with the phloem positioned on the outside of the xylem.
  • In stem: The xylem is on the inside and the phloem is on the outside.
  • In leaf: The xylem is positioned on top of the phloem.
Cross-sections of a root, stem, and leaf showing the positions of xylem and phloem
Diagram 1: Cross-sections of a root, stem, and leaf showing the positions of xylem and phloem. The X-shape xylem in the root and the vascular bundles in the stem and leaf are labelled.

Exam Traps

  • Do not place phloem in the centre of a root cross-section — xylem is central.
  • Avoid using the same relative positions for stem and leaf bundles without checking the diagram.

Xylem structure and adaptations

Xylem vessels are highly specialised for the efficient transport of water:

  • Thick walls with lignin: Provides structural support and prevents the tubes from collapsing.
  • No cell contents: The cells are hollow, providing more room to transport water.
  • No cross walls: Cells are joined end-to-end to form a long continuous tube for water to pass through without obstruction.

Water uptake and root hair cells

Root hair cells
Adapted with a large surface area to increase the rate of osmosis and maximise water uptake.
Adaptations
They have thin walls to shorten the diffusion distance. Mineral ions are also taken up here via active transport.
Pathway of water
Water moves from the root hair cells → root cortex cells → xylem → mesophyll cells (where it is used for metabolic reactions like photosynthesis).
Investigation
Placing a plant in water with stain added. Only the xylem vessels will be stained, proving they are the pathway for water transport.
Structure of a root hair cell showing the large vacuole, cell wall, cell membrane, and hair-like extension
Diagram 2: Structure of a root hair cell. The large vacuole, cell wall, cell membrane, and the long hair-like extension that increases surface area are labelled.

Exam Traps

  • Do not say minerals enter by osmosis — that process moves water, not ions against a gradient.
  • Avoid reversing the pathway or claiming the stain colours phloem as well as xylem.

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