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Phloem and translocation: pumped at the ends, flowing in the middle questions
The structure of sieve tube elements, sieve plates and companion cells, active loading of sucrose by proton pumping and co-transport, the mass flow hypothesis and the pressure gradient it depends on, sources and sinks, and the evidence for and against mass flow from ringing, aphid stylets and radioactive tracers.
6 original questions · 18 marks · the phloem and translocation: pumped at the ends, flowing in the middle notes · Plant transport and mineral nutrition
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Explain how the loading of sucrose at a source and its removal at a sink together produce the pressure difference that mass flow depends on.
Mark scheme
- B1 ATP drives a pump that moves hydrogen ions out of the companion cell into the cell wall, building up a hydrogen ion gradient
- B1 the hydrogen ions leak back in through a co-transporter that will only carry them with a sucrose molecule, so sucrose is brought in against its own concentration gradient
- B1 sucrose passes on into the sieve tube element through the plasmodesmata, so the water potential of the sap falls sharply
- B1 water enters from the xylem alongside by osmosis, and because the sieve tube has stiff walls and is already full, the hydrostatic pressure inside it rises
- A1 at the sink sucrose is unloaded and converted to something else, so water leaves and the pressure falls there, and sap flows from the high-pressure end to the low-pressure end carrying everything dissolved in it
Ringing a woody stem makes the tissue above the cut swell and grow rich in sugars, and sap flows for hours from the stylet of an aphid whose body has been cut away. Evaluate these two results as support for the mass flow hypothesis.
Mark scheme
- B1 ringing removes the phloem and leaves the xylem intact, so sugar accumulating above the cut and roots starving below it show that organic solutes travel in the phloem
- B1 ringing establishes the route and the direction of flow at that point, but it says nothing about the mechanism moving the sap
- B1 sap continuing to flow from an abandoned stylet shows that the contents of a sieve tube are under positive hydrostatic pressure and that nothing living is needed to push it out
- A1 judgement: the stylet result supports the pressure gradient that mass flow requires, but neither experiment excludes another mechanism producing the same observations, so the evidence supports the hypothesis rather than proving it
Describe how a potato tuber changes from a sink in late summer into a source in the following spring.
Mark scheme
- B1 in late summer sucrose made in the leaves is unloaded into the tuber and stored as starch, so the tuber is a sink
- B1 in spring the stored starch is broken down and sucrose is loaded into the phloem, so the tuber becomes a source
- A1 the new shoot is then the sink, so sap travels up the stem rather than down it; the direction is set by which end of the sieve tube currently holds the higher pressure
A metabolic poison applied to the middle of a stem, far from any loading or unloading, stops translocation. Suggest why this result does not by itself disprove the mass flow hypothesis.
Name the two membrane proteins in a companion cell that together load sucrose into a sieve tube element.
Give one reason why sucrose rather than glucose is the sugar transported in the phloem.
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