Questions on phloem translocation and the mass-flow hypothesis

Question 1

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.

Accepted Answer

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

Question 2

Discuss the sieve plate as evidence against the mass flow hypothesis, using what you know of the structure of a sieve tube.

Accepted Answer

B1 a perforated wall every few hundred micrometres would add resistance to a pressure-driven flow, and the measured pressure gradient is not obviously large enough to overcome that resistance in every species, which is a genuine objection B1 against this, in an undamaged tube the pores are wide and largely clear, and the protein filaments often seen blocking sieve plates in micrographs may be an artefact produced by the cutting and fixing needed to prepare the specimen B1 the sieve plate also has a function unconnected with driving the flow: a punctured tube under high hydrostatic pressure would otherwise empty rapidly, and callose and phloem protein sealing the plate within seconds limits damage to a short length of tube B1 the aphid stylet evidence shows sap flowing under pressure through plates that are working normally, which supports the plates being compatible with mass flow rather than ruling it out A1 taken together the sieve plate is a genuine difficulty for the simplest version of the hypothesis rather than a decisive refutation of it, since it also serves a separate protective function and pressure-driven flow through it has been observed directly

Question 3

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.

Accepted Answer

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, which pushes the sap out without any suction by the aphid 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

Question 4

Describe the structure of a mature sieve tube element, and describe how a companion cell compensates for what the sieve tube element has lost.

Accepted Answer

B1 at maturity a sieve tube element has no nucleus, very few organelles and only a thin layer of cytoplasm around the edge of the cell B1 its end walls become perforated sieve plates, through which the sap passes from one element to the next B1 the companion cell alongside it has a nucleus, dense cytoplasm and many mitochondria B1 it is connected to the sieve tube element by numerous plasmodesmata, through which it supplies the proteins and the ATP the sieve tube element can no longer make for itself

Question 5

Aphid stylets sample sap from the same vascular bundle of one plant at two points: near the leaves the sap contains 0.60 mol dm⁻³ sucrose, and 8 metres further down the stem it contains 0.36 mol dm⁻³ sucrose. Calculate the percentage decrease in sucrose concentration over this distance, and calculate the mean fall in concentration per metre.

Accepted Answer

M1 decrease = 0.60 − 0.36 = 0.24 mol dm⁻³ M1 percentage decrease = 0.24 ÷ 0.60 × 100 A1 = 40% decrease A1 fall per metre = 0.24 ÷ 8 = 0.03 mol dm⁻³ per metre

Question 6

Describe how a potato tuber changes from a sink in late summer into a source in the following spring.

Accepted Answer

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

Question 7

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.

Accepted Answer

B1 mass flow claims that the movement along the tube is driven by a pressure difference generated at the ends, not by anything pumping in the middle B1 a sieve tube element is still a living cell and depends on ATP from its companion cell simply to stay alive and keep its plasma membrane intact A1 poisoning the middle of the route kills those cells and the tube stops working, which is not the same as showing that ATP was being spent to push the sap along

Question 8

Sucrose, rather than starch, is the form of carbohydrate transported in the phloem. Explain why sucrose, and not starch, is suited to being moved rather than stored.

Accepted Answer

B1 sucrose is soluble, so it can dissolve in the sap and be carried in solution along the sieve tube, which an insoluble molecule could not do B1 sucrose is a non-reducing sugar and is not an intermediate in respiration, so it can pass through many cells at high concentration without being drawn into their metabolism or reacting on the way B1 starch is insoluble, so it stays where it is laid down, which suits it to storage rather than movement

Question 9

A young, still-expanding leaf, a fully expanded mature leaf, and a root are compared in early summer. Identify which of the three is a source and which are sinks at this time, giving a reason for each.

Accepted Answer

B1 the mature leaf is a source, because it is photosynthesising in surplus of its own needs; once fully expanded it is never a sink B1 the young leaf is a sink, because it is still expanding and importing sugar to build its own tissues B1 the root is a sink, because it never photosynthesises and always respires, so it almost always imports sucrose rather than exporting it

Question 10

A leaf is supplied with carbon dioxide containing the radioactive isotope carbon-14 for thirty minutes. An hour later, autoradiographs of stem sections taken above and below the leaf both show radioactivity in the phloem, but sections of the root taken at the same time show almost none. Suggest what this pattern shows about the direction sucrose was travelling, and suggest why the root showed so little radioactivity.

Accepted Answer

B1 radioactivity above and below the supplied leaf shows sucrose made in that leaf travelled in both directions in the phloem, up the stem and down it, not just in one direction B1 the leaf is a source with sinks both above it, such as the shoot tip and young leaves, and below it, such as the roots, so sucrose moves from the source towards whichever sinks are drawing on it B1 little activity in the root by this point could mean the root was not yet a strong sink for this leaf's sugar, or that not enough time had passed for the label to travel that far along the route

Question 11

Nitrogen deficiency symptoms appear first in a plant's oldest leaves, while calcium deficiency symptoms appear first in the youngest growing tissues. Explain this difference in terms of phloem mobility.

Accepted Answer

B1 nitrogen-containing compounds such as amino acids are mobile in the phloem, so when nitrogen is short the plant can break down protein in an old leaf and export the amino acids released to younger, growing tissue, which is why the old leaf shows the deficiency B1 calcium has almost no phloem mobility, so once it has been delivered somewhere by the transpiration stream it cannot be redistributed to newer tissue when supply runs short B1 growing points and the youngest leaves therefore run short of calcium first, because they depend on continuing xylem delivery rather than on being resupplied from elsewhere in the plant

Question 12

A gardener removes all the flowers from a tomato plant before any fruit can set, and notices that the leaves near the top of the plant grow unusually large. Suggest, in terms of sources and sinks, why removing the flowers might have this effect.

Accepted Answer

B1 a developing fruit is normally a very strong sink, competing hard for the sugar produced by the leaves B1 with no fruit developing, that large sink is removed, so more sucrose remains available for other sinks, such as the still-growing leaves near the top of the plant B1 with an increased supply of sugar reaching them, those leaves can grow larger than they would while competing with a fruit for the same limited supply

Question 13

Name the two membrane proteins in a companion cell that together load sucrose into a sieve tube element.

Accepted Answer

A1 a hydrogen ion pump, which spends ATP A1 a sucrose and hydrogen ion co-transporter protein

Question 14

Explain why a mechanism such as mass flow, rather than diffusion of sucrose through water, is needed to account for the speed of translocation.

Accepted Answer

B1 phloem sap has been measured moving at roughly 0.15 to 1.5 metres an hour, which is far faster than sucrose could travel by diffusion alone B1 sucrose diffusing through water over the same distance would take decades, so diffusion alone cannot explain the observed speed and a bulk-flow mechanism is required

Question 15

Give one reason why sucrose rather than glucose is the sugar transported in the phloem.

Accepted Answer

B1 sucrose is not an intermediate in respiration, so it is not drawn into the metabolism of every cell it passes; accept that it is a non-reducing sugar and so does not react on the way