AQA · GCSE · Biology

    Course version: 8461 · Version from Sept 2018

    It is composed of hollow tubes strengthened by lignin adapted for the…

    Full specification wording

    Plant organ system: It is composed of hollow tubes strengthened by lignin adapted for the transport of water in the transpiration stream.

    Xylem tissue is built from dead, elongated cells joined end to end to form hollow tubes.

    Checked against the AQA 8461 specification (opens in a new tab) on .

    • Point 9 of 13
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Xylem tissue is built from dead, elongated cells joined end to end to form hollow tubes. Because the end walls break down, there is an uninterrupted lumen through which water and dissolved ions travel. The cell walls are thickened with lignin, a tough waterproof polymer that prevents collapse under the tension created when water is pulled upwards. Lignin also makes the walls impermeable, so water stays inside the lumen rather than leaking sideways. Pits in the walls allow some lateral movement between neighbouring xylem vessels. These features are adaptations: hollow tubes reduce resistance to flow, lignin gives strength, and dead cells remove cytoplasmic obstacles. In the transpiration stream, evaporation from leaves creates tension that pulls the water column up the stem. This cohesion-tension mechanism depends on the continuous, reinforced plumbing that xylem provides.

    What earns the marks

    • Xylem cells are dead and elongated, joined end to end to form continuous tubes.
    • The end walls between xylem cells break down, creating a hollow lumen with little resistance to water flow.
    • Lignin thickens and strengthens cell walls, preventing the tubes from collapsing under tension.
    • Lignin is waterproof, so it helps keep the water column inside the xylem lumen.
    • Pits in lignified walls permit some sideways movement of water between xylem vessels.

    Common mistakes and how to put them right

    • Stating that xylem cells are living.

      Put it right by saying they are dead at maturity, which leaves a clear hollow tube.

    • Confusing lignin with cellulose.

      Put it right by describing lignin as a strengthening, waterproof wall material in xylem.

    • Saying lignin pushes water upwards.

      Put it right by explaining that lignin strengthens walls while evaporation and cohesion pull the water column.

    Exam tips

    • Link each structural feature to a transport benefit, for example hollow lumen to reduced resistance and lignin to support.
    • Use the phrase transpiration stream when describing the upward movement of water through xylem.
    • When asked about adaptations, give the feature and then explain how it helps transport rather than listing features alone.
    • Sketch a simple xylem vessel with a hollow centre and thickened walls to support your written answer.

    Try the lesson

    The start of the Plant organ system: It is composed of hollow tubes strengthened by lignin adapted for the transport of water in the transpiration stream. lesson, made from checked teaching notes for this specification point.

    Have a go first

    Which substance strengthens the walls of the hollow tubes that make up xylem tissue?

    Listen

    Xylem tissue is built from dead, elongated cells joined end to end to form hollow tubes. Because the end walls break down, there is an uninterrupted lumen through which water and dissolved ions travel. The cell walls are thickened with lignin, a tough waterproof polymer that prevents collapse under the tension created when water is pulled upwards. Lignin also makes the walls impermeable, so water stays inside the lumen rather than leaking sideways. Pits in the walls allow some lateral movement between neighbouring xylem vessels. These features are adaptations: hollow tubes reduce resistance to flow, lignin gives strength, and dead cells remove cytoplasmic obstacles. In the transpiration stream, evaporation from leaves creates tension that pulls the water column up the stem. This cohesion-tension mechanism depends on the continuous, reinforced plumbing that xylem provides. A strong answer earns marks for these points. Xylem cells are dead and elongated, joined end to end to form continuous tubes. The end walls between xylem cells break down, creating a hollow lumen with little resistance to water flow. Lignin thickens and strengthens cell walls, preventing the tubes from collapsing under tension. Lignin is waterproof, so it helps keep the water column inside the xylem lumen. A common slip: Stating that xylem cells are living. Instead, saying they are dead at maturity, which leaves a clear hollow tube. Exam tip: Link each structural feature to a transport benefit, for example hollow lumen to reduced resistance and lignin to support.

    See it

    A top-down flowchart. At the top, a node reads 'Dead elongated cells joined end to end'. An arrow leads down to 'End walls break down', which leads to 'Hollow lumen: little resistance to water flow'. A second arrow from the top node leads to 'Walls thickened with lignin', which branches into three nodes: 'Strengthens walls: prevents collapse under tension', 'Waterproof: keeps water column inside lumen', and 'Pits allow sideways movement between vessels'. Arrows from 'Hollow lumen: little resistance to water flow', 'Strengthens walls: prevents collapse under tension' and 'Waterproof: keeps water column inside lumen' all lead to a final node: 'Water moves upwards in the transpiration stream'.
    It shows how the structural features of xylem — dead joined cells, broken end walls, a hollow lumen and lignified walls — adapt it for water transport in the transpiration stream.

    Key words

    Say what each word means before you turn the card.

    Structure and function

    Try answering aloud or in your head before turning the card.

    Adaptation of transport tissues

    Try answering aloud or in your head before turning the card.

    Water movement in plants

    Try answering aloud or in your head before turning the card.

    Cohesion and tension

    Try answering aloud or in your head before turning the card.

    See every definition
    Structure and function
    Hollow tubes strengthened by lignin are adapted for the transport of water in the transpiration stream.
    Adaptation of transport tissues
    Hollow tubes reduce resistance to flow, lignin gives strength, and dead cells remove cytoplasmic obstacles.
    Water movement in plants
    Water moves upwards in the transpiration stream from roots to leaves.
    Cohesion and tension
    Evaporation from leaves creates tension that pulls the water column up the stem, depending on the continuous, reinforced plumbing that xylem provides.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A top-down flowchart. At the top, a node reads 'Dead elongated cells joined end to end'. An arrow leads down to 'End walls break down', which leads to 'Hollow lumen: little resistance to water flow'. A second arrow from the top node leads to 'Walls thickened with lignin', which branches into three nodes: 'Strengthens walls: prevents collapse under tension', 'Waterproof: keeps water column inside lumen', and 'Pits allow sideways movement between vessels'. Arrows from 'Hollow lumen: little resistance to water flow', 'Strengthens walls: prevents collapse under tension' and 'Waterproof: keeps water column inside lumen' all lead to a final node: 'Water moves upwards in the transpiration stream'.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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