Translocation and the Mass Flow Hypothesis · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.
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How sugary sap travels by pressure

Science · Organisms & Life Processes · ages 17-18
Name ______________________   Date ____________

  1. Which pair is a source and a sink?

    • A sugary leaf and a growing root
    • Two mature leaves side by side
    • Two storage roots feeding each other

  2. Which order of steps describes translocation?

    • Unloading, flow, loading
    • Flow, loading, unloading
    • Loading, flow, unloading

  3. The pressure driven flow of sap through the tubes is a passive step.

    Circle one:   True   False

  4. A ring of bark with the phloem is removed all round a stem. What happens below the ring?

    • Tissues below starve as sugars pile up above
    • Water stops rising in the xylem at once
    • Roots start photosynthesising to compensate

  5. Why does phloem transport need ATP while xylem transport does not?

    • Sap is thicker than xylem water
    • Loading sucrose against its gradient costs energy
    • Sieve tubes are longer than xylem vessels

  6. Aphids pierced into a stem ooze sugary sap without sucking. What does this show?

    • The phloem contents sit under pressure
    • Aphids manufacture sugar inside stems
    • Xylem sap is always sweeter than phloem

  7. Why can phloem sap move either way along a stem while xylem sap only moves up?

    • Sap always flows from source to sink, and sinks sit in different places
    • Phloem tubes pump in both directions at once
    • Xylem vessels are blocked below every leaf

  8. Which observation counts as a difficulty for the mass flow hypothesis?

    • Tracers from a fed leaf turning up in roots
    • Sugars piling above a bark ring
    • Sieve plates that should resist fast flow
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Answer key

For grown-ups. Fold this page away before handing over the rest.

How sugary sap travels by pressure W1-mt_WZ8id0kou4-s1

  1. A sugary leaf and a growing root · Leaves making sugar supply organs that consume or store it.
  2. Loading, flow, unloading · Sap is loaded at the source, flows by pressure, and is unloaded at the sink.
  3. True · Only the loading step burns ATP. The middle flow rides the pressure gradient.
  4. Tissues below starve as sugars pile up above · Ringing cuts the downward sugar route, proving phloem carries it.
  5. Loading sucrose against its gradient costs energy · Active loading burns ATP. Xylem water just rides the transpiration pull.
  6. The phloem contents sit under pressure · Sap welling out on its own means pressure pushes it, as mass flow predicts.
  7. Sap always flows from source to sink, and sinks sit in different places · Each tube runs source to sink, so seasonal sources and sinks set the direction per tube.
  8. Sieve plates that should resist fast flow · Plates across the tubes look like barriers, yet sap moves fast between them.
Worksheet · LightMySky