AQA · A-Level · Biology

    Course version: 7402 · Version from Sept 2016

    Phloem as the tissue that transports organic substances in plants.

    Full specification wording

    Mass transport in plants: Phloem as the tissue that transports organic substances in plants.

    Phloem is living tissue.

    Checked against the AQA 7402 specification (opens in a new tab) on .

    • Point 2 of 4
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Phloem is living tissue. It is made of sieve tube elements joined end to end through perforated sieve plates; each element has lost its nucleus and most organelles, and is kept alive by an adjacent companion cell packed with mitochondria. Phloem carries assimilates, chiefly sucrose but also amino acids, from sources to sinks. A source is anywhere assimilates are made or released, such as a photosynthesising leaf or a storage organ mobilising its reserves; a sink is anywhere they are used or stored, such as respiring cells, root and shoot tips, developing fruits and seeds. Sucrose is the transport sugar because it is soluble but metabolically fairly inert. Loading is active: the companion cell uses ATP to pump hydrogen ions out, and sucrose re-enters with those hydrogen ions through a co-transporter protein.

    What earns the marks

    • Sucrose being actively transported, or co-transported with hydrogen ions, into the phloem or sieve tube.
    • Naming the companion or transfer cell as the site at which loading occurs.
    • Movement from source to sink, with a named sink such as respiring cells, a storage organ, a root tip or a developing fruit.
    • Sucrose being unloaded at the sink by active transport or by facilitated diffusion.
    • Naming sieve tube elements and sieve plates as the structures through which transport occurs.

    Common mistakes and how to put them right

    • Saying phloem transports glucose when the sugar carried is sucrose.

    • Describing phloem as dead tissue, which confuses it with xylem.

    • Assuming transport is always downwards from the leaves.

    • Omitting the companion cell, so the answer contains no source of ATP for loading.

    • Writing transports food where the mark requires a named organic substance.

    Exam tips

    • Name sucrose every time, and name a specific sink rather than saying the rest of the plant.
    • Contrast phloem with xylem in one line: living against dead, two-way against one-way, sucrose against water and mineral ions.
    • Sieve tube and sieve element are both accepted for phloem cell, so use whichever term you are confident with.

    Try the lesson

    The start of the Mass transport in plants: Phloem as the tissue that transports organic substances in plants. lesson, made from checked teaching notes for this specification point.

    Have a go first

    Which tissue transports organic substances such as sucrose in a plant?

    Listen

    Phloem is living tissue. It is made of sieve tube elements joined end to end through perforated sieve plates; each element has lost its nucleus and most organelles, and is kept alive by an adjacent companion cell packed with mitochondria. Phloem carries assimilates, chiefly sucrose but also amino acids, from sources to sinks. A source is anywhere assimilates are made or released, such as a photosynthesising leaf or a storage organ mobilising its reserves; a sink is anywhere they are used or stored, such as respiring cells, root and shoot tips, developing fruits and seeds. Sucrose is the transport sugar because it is soluble but metabolically fairly inert. Loading is active: the companion cell uses ATP to pump hydrogen ions out, and sucrose re-enters with those hydrogen ions through a co-transporter protein. A strong answer earns marks for these points. sucrose being actively transported, or co-transported with hydrogen ions, into the phloem or sieve tube. naming the companion or transfer cell as the site at which loading occurs. movement from source to sink, with a named sink such as respiring cells, a storage organ, a root tip or a developing fruit. sucrose being unloaded at the sink by active transport or by facilitated diffusion. Exam tip: Name sucrose every time, and name a specific sink rather than saying the rest of the plant.

    See it

    A vertical flowchart with seven boxes. The first box is the source, a photosynthesising leaf or a storage organ mobilising reserves. An arrow leads to the companion cell, which uses ATP to pump hydrogen ions out. An arrow leads to sucrose re-entering with those hydrogen ions through a co-transporter protein. An arrow leads to sucrose being loaded into a sieve tube element. An arrow leads to transport through sieve tube elements joined end to end by sieve plates. An arrow leads to the sink: respiring cells, root and shoot tips, developing fruits and seeds. A final arrow leads to sucrose being unloaded at the sink by active transport or facilitated diffusion.
    It shows sucrose being actively loaded into the phloem at a source and transported through sieve tube elements to a sink, where it is unloaded.

    Put it in order

    Put these steps in the order they happen.

    1. 1. Sucrose unloaded at the sink by active transport or facilitated diffusion
    2. 2. Sink: respiring cells, root and shoot tips, developing fruits and seeds
    3. 3. Sucrose re-enters with hydrogen ions through a co-transporter protein
    4. 4. Transport through sieve tube elements joined end to end by sieve plates
    5. 5. Source: photosynthesising leaf or storage organ mobilising reserves
    6. 6. Companion cell uses ATP to pump hydrogen ions out
    7. 7. Sucrose loaded into sieve tube element

    Key words

    Say what each word means before you turn the card.

    sieve tube elements and sieve plates

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

    companion cells

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

    sucrose as the transport sugar

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

    source to sink

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

    See every definition
    sieve tube elements and sieve plates
    Living cells joined end to end through perforated plates, each element having lost its nucleus and most organelles.
    companion cells
    Cells adjacent to sieve tube elements, packed with mitochondria, that keep the elements alive and use ATP to pump hydrogen ions out.
    sucrose as the transport sugar
    The sugar carried by phloem because it is soluble but metabolically fairly inert.
    source to sink
    Movement of assimilates from anywhere they are made or released to anywhere they are used or stored.

    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 vertical flowchart with seven boxes. The first box is the source, a photosynthesising leaf or a storage organ mobilising reserves. An arrow leads to the companion cell, which uses ATP to pump hydrogen ions out. An arrow leads to sucrose re-entering with those hydrogen ions through a co-transporter protein. An arrow leads to sucrose being loaded into a sieve tube element. An arrow leads to transport through sieve tube elements joined end to end by sieve plates. An arrow leads to the sink: respiring cells, root and shoot tips, developing fruits and seeds. A final arrow leads to sucrose being unloaded at the sink by active transport or facilitated diffusion.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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