Biology › Plant transport and mineral nutrition › Phloem and translocation: pumped at the ends, flowing in the middle

Phloem and translocation: pumped at the ends, flowing in the middle

Phloem carries sugar upwards, downwards and sideways, changes direction with the season, and moves it a hundred times faster than diffusion could. The explanation on the specification is a hypothesis rather than a settled fact, and the questions know it.

Before this Active transport and co-transport at membranes · Water potential and osmosis in plant cells

Before you start

Sugars are actively transported along the phloem from the leaves to the roots. It sounds right, because the phrase 'active transport' does belong in this topic. But look at what would have to do the transporting. A mature sieve tube element has no nucleus, a thin lining of cytoplasm and hardly any organelles; it is barely alive and could not power anything over a metre of stem. Active transport happens at the ends — loading at the source, unloading at the sink — and what happens in between is bulk flow of the whole solution down a pressure gradient, carrying every dissolved thing along with it at the same speed.

What you should be able to do

Describe a sieve tube element and a companion cell, and relate each feature to the job it does.
Explain how sucrose is loaded into a sieve tube at a source, naming the pump and the co-transporter.
Explain how loading and unloading create the pressure gradient that mass flow depends on.
Identify sources and sinks, including an organ that is both at different times of year.
Describe ringing, aphid stylet and tracer experiments and say what each one does and does not establish.
Evaluate the mass flow hypothesis, giving evidence on both sides.

Two cells that only work as a pair

Phloem is a tissue with several cell types in it, and two of them do the transporting. They are best learned together, because neither makes sense alone.

The division of labour is the thing to take from this. One cell has been hollowed out to let sap through; the other keeps its nucleus and its mitochondria and does the living for both.

Sieve tube element: A living cell of the phloem, joined end to end with others into a tube; at maturity it has no nucleus, few organelles and only a thin layer of cytoplasm around the edge.
Sieve plate: The perforated end wall between two sieve tube elements, through which the sap passes.
Companion cell: A cell alongside each sieve tube element with dense cytoplasm, a nucleus and many mitochondria, connected to it by numerous plasmodesmata and responsible for loading it and keeping it alive.
Translocation: The transport of assimilates, mainly sucrose, through the phloem from a source to a sink.

The sieve tube element is a cell that has thrown away most of what a cell normally needs. The nucleus goes, the tonoplast goes, most organelles go, and the end walls become sieve plates with pores wide enough for the contents to move through. What is left is a channel with as little in the way as a living cell can manage. Companion cells supply everything it can no longer make, through plasmodesmata so numerous that the two are sometimes treated as one functional unit.

The sugar carried is sucrose, not glucose, and the choice is deliberate. Sucrose is a non-reducing sugar and is not an intermediate in respiration, so it can travel at high concentration without being drawn into the metabolism of every cell it passes and without reacting on the way. It is also more soluble than starch, which is what plants use when they want sugar to sit still rather than move. Amino acids travel in phloem too, along with potassium, phosphate and the other mobile ions, which is why nitrogen and magnesium shortages show up on a plant's oldest leaves first.

Loading, and the pressure it makes

The mass flow hypothesis explains long-distance movement as bulk flow driven by a difference in hydrostatic pressure between the two ends of the tube. The pressure difference is made osmotically, and it is made by pumping.

Read it as three separate events: something is pumped, water follows it, and the resulting pressure pushes the whole solution along. Only the first and last of those costs the plant anything.

At a source, the companion cell uses ATP to pump hydrogen ions out across its plasma membrane into the cell wall. That builds a concentration gradient of hydrogen ions, and they leak back in through a co-transporter protein that will only carry a hydrogen ion inwards if it carries a sucrose molecule with it. Sucrose is dragged into the companion cell against its own concentration gradient on the back of the hydrogen ion gradient, and passes on into the sieve tube element through the plasmodesmata. The pump spends the ATP; the co-transporter spends none, which is why this is called secondary active transport.

What follows is osmosis and then simple hydraulics. Sucrose in the sieve tube reaches concentrations of roughly 0.3 to 0.9 mol dm⁻³ — ten to thirty per cent sugar by mass — so the water potential of the sap falls sharply. Water enters from the xylem alongside, and because the sieve tube has stiff walls and is already full, the hydrostatic pressure inside it rises. Measurements at a source put it in the region of 800 to 2000 kPa.

At a sink the opposite happens, and it happens because the sink keeps consuming the delivery. Sucrose is unloaded into the surrounding cells and immediately converted into something else — respired, or stored as starch, or built into cellulose — so its concentration in the sieve tube stays low and the unloading gradient never runs out. With less solute, the water potential of the sap rises, water leaves back into the xylem, and the hydrostatic pressure falls. Sap flows from the high-pressure end to the low-pressure end, carrying everything dissolved in it.

The speed is the reason a mechanism is needed at all. Phloem sap moves at roughly 0.15 to 1.5 metres an hour. Sucrose diffusing through water would take decades to travel a metre. Whatever is happening in phloem, diffusion is not it.

Source and sink are jobs, not places

A source is anywhere sucrose enters the phloem; a sink is anywhere it leaves. Neither is a fixed part of the plant, and questions are built on organs that swap roles.

Organ	When it is a source	When it is a sink
A mature leaf	Whenever it is photosynthesising in surplus	Never, once it is fully expanded
A young leaf	Not until it is about a third grown	While it is expanding and importing sugar to build itself
A potato tuber	In spring, feeding the new shoot from its stored starch	In late summer, filling with starch from the leaves
A root	Rarely, and only from stored reserves	Almost always: it never photosynthesises and always respires
A seed or fruit	Never while it is developing	Strongly, and it outcompetes other sinks for the supply

Two consequences follow. Phloem carries material in both directions, up the stem in spring from a tuber and down it in summer from the leaves, so an answer that says phloem transports sugar downwards is only half right. And the direction in any one sieve tube is decided by nothing more than which end of it currently has the higher pressure, which is exactly what a pressure-driven mechanism predicts and a pumped one would not.

This is also where the mineral-mobility point from the first lesson pays off. Because phloem runs from source to sink, a plant that is short of nitrogen can break down protein in an old leaf and export the amino acids to a young one. Calcium has no such route, so it goes where the transpiration stream takes it and stays there.

The evidence, and what it does not settle

Mass flow is a hypothesis, and the specifications say so. Evaluation questions on this topic are worth serious marks, and they want evidence named and interpreted rather than a verdict announced.

Two experiments a century apart. The first shows where the sugar travels; the second shows that something at the far end is under pressure without anything alive being required to push.

Ringing. Cut a complete ring of bark, and with it the phloem, from a woody stem, leaving the xylem intact. Within weeks the stem swells above the cut and the tissue there is rich in sugars, while below the cut the sugar concentration falls and the roots eventually die. That establishes that organic solutes travel in the phloem and that the flow at that point is downwards. It does not establish how.

Aphid stylets. An aphid pushes a fine mouthpart into a single sieve tube and feeds. Anaesthetise it with carbon dioxide and cut the body away, and sap goes on flowing out of the abandoned stylet for hours. Two things follow at once: the contents of a sieve tube really are under positive hydrostatic pressure, and nothing living is needed to keep the sap moving out. Sap sampled this way is what most measurements of phloem sucrose concentration come from.

Radioactive tracers. Supply a leaf with carbon dioxide made with the carbon-14 isotope. The labelled carbon appears in sugars within minutes, and an autoradiograph of a stem section afterwards shows the radioactivity in the phloem and not in the xylem. Repeat over hours and you can watch the label travel to whichever organ is the sink that week.

Reading stylet data as an examiner would

Aphid stylets were used to sample sap from a single tree at two heights. Near the leaves, the sap contained 0.71 mol dm⁻³ sucrose and exuded at 2.3 mm³ h⁻¹. Twelve metres lower down the trunk, sap from stylets in the same vascular bundle contained 0.42 mol dm⁻³ sucrose and exuded at 1.1 mm³ h⁻¹. Explain how these results support the mass flow hypothesis.

Take the concentrations first. Sucrose is more concentrated near the source and less concentrated further from it, which is what continuous unloading along the route and at the sink would produce.

Now the exudation rates. Sap leaves a stylet at a rate set by the pressure behind it, so a rate twice as high near the leaves means the hydrostatic pressure is higher there.

Mass flow requires exactly that: a pressure gradient falling from source to sink, generated by sucrose being loaded at one end and removed at the other. Both measurements go the way the hypothesis predicts, and the two gradients point the same way, which is the part worth saying out loud.

Be careful how far you push it. The data show a gradient consistent with mass flow; they do not rule out another mechanism that would also produce one. 'Support' is the word the question used, and 'support' is the word your answer should use.

Now the other side, because the marks are there too.

Objection	The reply mass flow offers
Sieve plates obstruct the flow. Why would a bulk-flow system evolve sieves in the pipe?	They may hold the tubes open under pressure, and callose and protein can plug them within seconds if the tube is cut, which stops the plant bleeding out
Not all solutes move at the same speed, and mass flow says everything in the sap travels together	Sucrose and amino acids may be loaded and unloaded at different points along the route, which changes their apparent speed without changing the flow
Solutes have been seen moving in opposite directions at the same time	Never convincingly within one sieve tube; adjacent tubes in the same bundle can carry sap in opposite directions
Metabolic poisons stop translocation even in the middle of the route, far from any loading	Sieve tube elements need ATP from their companion cells simply to stay alive and keep their membranes intact, which is not the same as pumping the sap
Measured flow rates in some species are faster than the measured pressure gradient seems able to drive	The measurements are hard: puncturing a sieve tube changes the pressure you are trying to measure

The honest summary is that mass flow accounts for the speed, the direction changes, the pressure in the stylet and the effect of ringing, and that several observations still sit awkwardly with it. That is what a hypothesis looks like while it is still doing work.

TRY IT — Arguing both ways in one answer

'The sieve plate is the strongest argument against mass flow.' Discuss this statement, using what you know about the structure of a sieve tube. (5 marks)

Check your answer

The objection is straightforward. Mass flow describes sap being pushed along a tube by a pressure difference, and a perforated wall every few hundred micrometres adds resistance. A system designed for bulk flow would seem better off without them, and the pressure gradient measured between source and sink is not obviously large enough to overcome that resistance in every species.

Against that, the pores in a sieve plate are wide and, in an undamaged tube, largely clear; the protein filaments seen in electron micrographs may be an artefact of the cut made in preparing the specimen, when callose and phloem protein rush to seal the plate.

The plates also have a job that has nothing to do with flow. A tube under 800 kPa or more that is bitten through would empty itself, and the plant would lose sap and gain an infection route. A plate that seals in seconds limits the damage to a short length of tube.

So the plates are a genuine difficulty for the simplest version of the hypothesis rather than a refutation of it, and the aphid stylet evidence shows sap flowing under pressure through plates that are working normally.

Marks in a question worded like this go to the structure of the argument. State the objection, give the counter, and finish with a judgement that follows from what you have written.

In the exam

Say where the active transport happens. Loading at the source and unloading at the sink are active; the movement along the tube is not.
Name the two proteins. The hydrogen ion pump and the sucrose–hydrogen co-transporter are separate things, and questions that give you a membrane diagram are asking you to tell them apart.
Every step of the loading chain has a consequence: solutes in, water potential down, water in from the xylem, hydrostatic pressure up. Write all four.
Phloem transports in both directions and xylem only up. If a question asks for a difference between the tissues, that is a cleaner answer than any structural one.
For 'evidence for and against', give both. An answer that lists only supporting evidence caps itself, however good the supporting evidence is.
Use the word the question uses. Data that 'support' a hypothesis do not prove it, and examiners mark that distinction.

Check yourself

A plant is supplied with carbon dioxide containing the carbon-14 isotope for one hour in bright light. Sections cut from the stem four hours later are placed against photographic film. The film darkens over the phloem of the vascular bundles and not over the xylem. In a second plant treated identically, a ring of bark had been removed from the stem an hour before the carbon dioxide was supplied; in that plant the film darkens above the ring but not below it. Explain both results.

Answer

The labelled carbon dioxide was fixed in photosynthesis, so the carbon-14 ended up in sugars made in the leaves. Those leaves are sources, and the sugar is loaded into the sieve tubes as sucrose.

The film darkens where the radioactive isotope is, so darkening over the phloem and not the xylem shows that the labelled sucrose travelled in the phloem. This is what separates the two tissues experimentally rather than by assertion.

In the ringed plant, removing the bark removed the phloem while leaving the xylem intact. Sucrose loaded above the ring cannot pass the gap, so it accumulates in the phloem above it and the film darkens there.

Below the ring there is no continuous sieve tube from the leaves, so no labelled sucrose arrives and the film stays clear. The xylem below the ring is undamaged and still carrying water upwards, which is why the plant does not wilt immediately even though its roots are being starved.

One caution if you are asked to evaluate. This shows the route sucrose takes and shows that it needs intact phloem, and it says nothing at all about what drives the movement — mass flow, or anything else, would give the same autoradiograph.

Questions

Written to the command words the boards use. Try them on paper before opening a scheme: the marks go to points made, not to length.

Question 15 marks

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

Question 24 marks

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

Question 33 marks

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

Question 43 marks

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.

Mark scheme

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 52 marks

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

Mark scheme

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

Question 61 mark

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

Mark scheme

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

Worth remembering

Sieve tube elements lose their nucleus and most organelles; companion cells keep both and do the work.
Sucrose travels because it is soluble, non-reducing and not a respiratory intermediate.
Loading is a hydrogen ion pump plus a co-transporter; the flow along the tube is not pumped.
Solute in, water potential down, water in, pressure up — that chain at the source and its reverse at the sink is the whole of mass flow.
Source and sink are roles, and an organ can swap between them with the season.
Ringing shows the route, aphid stylets show the pressure, tracers show the tissue; none of them proves the mechanism.

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