Clock it with SCIBook a tutor
QualificationsGCSE AQA Biology (Triple)GCSE AQA Combined ScienceGCSE Edexcel Biology (Triple)GCSE Edexcel Combined ScienceIGCSE coming soonIB MYP coming soonIB Diploma coming soonA-level coming soon
ResourcesVideosAboutBook a tutor
Plant structures and their functions, subtopic 2 of 4Spec 6.7–6.10

Transport in plants

Root hair cells, xylem and phloem, transpiration and the stomata, and translocation of sucrose.

4 sections, with a quick check at the end.

Download notes PDF6 pages

Root hair cellsSpec 6.7

Root hair cells are found near the tips of roots. They take in water and mineral ions from the soil. Their structure is adapted for this job.

Adaptations of a root hair cell
AdaptationHow it helps
A long, thin projection (the root hair)Gives a large surface area for absorbing water and mineral ions
A thin cell wallA short distance for water and mineral ions to cross
Many mitochondriaRelease energy by respiration for the active transport of mineral ions into the cell
Cell sap more concentrated than the soil waterWater moves into the cell by osmosis

Mineral ions, such as nitrate ions, are usually at a lower concentration in the soil water than inside the cell. They are taken in by active transport, which needs energy from respiration.

A root hair cell in soil, labelled thin cell wall, cytoplasm, cell membrane, vacuole with cell sap, nucleus, mitochondria and the long root hair, with arrows for water entering by osmosis and mineral ions by active transport.Tap to enlarge
A root hair cell: water enters by osmosis; mineral ions enter by active transport using energy from the mitochondria.

Xylem and phloemSpec 6.8

Xylem and phloem are the two transport tissues in a plant. They run together through the roots, stem and leaves.

Xylem

The xylem carries water and mineral ions from the roots to the rest of the plant. It is made of lignified dead cells.

  • The cells are dead, with no cytoplasm or nucleus, and the end walls between them have broken down, so they form a continuous hollow tube. Water can flow through easily.
  • The walls are strengthened with lignin, which is strong and waterproof. This stops the tubes from collapsing and supports the plant.

Phloem

The phloem carries sucrose around the plant. It is made of living cells that use energy to transport sucrose.

  • Sieve tube cells are joined end to end. The end walls have holes (sieve plates), so dissolved sucrose can pass from cell to cell.
  • Companion cells are next to the sieve tube cells. They have many mitochondria to release the energy that is needed for the transport of sucrose.
Xylem and phloem compared
XylemPhloem
Substances carriedWater and mineral ionsSucrose (dissolved in water)
CellsDeadLiving
Cell wallsThickened with ligninCellulose, with holes in the end walls (sieve plates)
Energy needed for transportNoYes
A stem cross-section with a ring of vascular bundles (phloem on the outside, xylem on the inside), next to longitudinal sections of a xylem vessel (hollow tube, lignin rings, water and mineral ions moving up) and phloem (sieve tube cells, sieve plates, companion cells with mitochondria, sucrose moving up and down).Tap to enlarge
Xylem: dead, hollow, lignified tubes carrying water up. Phloem: living sieve tube cells and companion cells carrying sucrose both ways.

Transpiration and stomataSpec 6.9

Transpiration is the loss of water vapour from a plant, mainly through the stomata in the leaves. It pulls a continuous stream of water, called the transpiration stream, through the plant. This is how water and mineral ions are transported from the roots to the leaves.

  1. Water is absorbed from the soil by the root hair cells.
  2. Water and dissolved mineral ions move up through the xylem to the leaves.
  3. Water evaporates from the surface of the cells inside the leaf, into the air spaces.
  4. Water vapour diffuses out of the leaf through the stomata, into the air.
A plant showing water taken in by root hair cells, moving up the xylem in the root, stem and leaf, then a magnified leaf section where water evaporates from cells into the air spaces and water vapour diffuses out through a stoma.Tap to enlarge
The transpiration stream: water moves up the xylem, evaporates into the air spaces and diffuses out through the stomata.

Stomata

Stomata are tiny pores, found mainly on the underside of a leaf. Each stoma is surrounded by two guard cells which control whether the stoma is open or closed.

  • Open stomata let carbon dioxide diffuse into the leaf for photosynthesis, and let oxygen and water vapour diffuse out.
  • When the guard cells take in water they swell and the stoma opens.
  • When the guard cells lose water they become limp and the stoma closes. This reduces the amount of water lost by transpiration, for example when the plant is short of water.
Surface views of a stoma: open, with swollen guard cells, carbon dioxide going in and oxygen and water vapour going out; closed, with limp guard cells and no pore.Tap to enlarge
Guard cells that take in water swell and open the stoma; guard cells that lose water go limp and close it.

TranslocationSpec 6.10

Translocation is the transport of sucrose around the plant. Sucrose is made from the glucose produced in photosynthesis, and it is carried dissolved in water in the phloem.

  • Sucrose moves from the leaves, where it is made, to the parts of the plant that use it or store it, such as growing shoot tips, roots, flowers and fruits.
  • It can move up and down the plant, depending on where it is needed.
  • Translocation needs energy, which is released by respiration in the living cells of the phloem.
A plant with arrows showing sucrose moving in the phloem from the leaves, where it is made, up to the growing shoot tip, along to a fruit and down to the root.Tap to enlarge
Translocation: sucrose moves in the phloem from the leaves to where it is used or stored, up and down the plant.

Quick check

  1. Give two adaptations of a root hair cell.

    Show answer

    Any two of: a long, thin projection for a large surface area; many mitochondria for active transport; a thin cell wall.

  2. Why is xylem made of lignified dead cells?

    Show answer

    Dead cells with no end walls form a continuous hollow tube for water, and lignin strengthens the walls.

  3. What is transpiration?

    Show answer

    The loss of water vapour from the leaves, which pulls water through the plant.

  4. What is translocation?

    Show answer

    The transport of sucrose around the plant in the phloem.