Translocation in Phloem
In spring, sugar flows up to bursting buds; in summer, it flows down to swelling roots and fruit. The same phloem tube can run sugar in opposite directions at different times of year. How does a plant, with no heart and no pump, push thick sugary sap exactly where it is needed? The answer is an elegant pressure-flow system driven entirely by water and osmosis, the translocation theory.
Orient yourself
Predict, scan the goals and key terms, and clear the misconception this lesson exists to fix.
Before You Read
warm-upSqueeze one end of a long, water-filled balloon and the water shoots out the far end, pressure at one end pushes fluid to the other.
Before reading: predict how a plant could create high pressure at the leaf end of a phloem tube without any muscle or pump. What might pull water in to build the pressure?
Learning Intentions
goalsKnow
- What translocation means and what it moves
- The meaning of source and sink
- The steps of the pressure-flow (translocation) theory
Understand
- Why loading sugar at the source draws water in by osmosis
- How a pressure difference drives bulk flow of sap
- Why translocation can run in either direction
Can Do
- Use the translocation theory to explain the movement of nutrients in phloem
- Identify sources and sinks in a plant
- Sequence the steps from sugar loading to unloading
Scan these before reading
vocabMisconceptions To Fix
watch outTwo truths, one lie: Three of these statements about translocation are true. Which one is the lie?
Sources, Sinks and Translocation
What moves, and from where to where
Core Content
Sources, Sinks and Translocation
+5 XPWhat moves, and from where to where
Phloem carries sugar from wherever it is made or stored to wherever it is being used, and these "from" and "to" points can change with the season.
Translocation is the movement of organic nutrients, mainly the sugar sucrose, through phloem from a source (a part that produces or releases sugar, such as a photosynthesising leaf) to a sink (a part that uses or stores sugar, such as a root, fruit or growing tip).
A single organ can switch role: a potato tuber is a sink in summer as it fills with starch, but a source in spring as it releases sugar to fuel new shoots. Because the direction depends only on where the source and sink are, translocation is described as moving sugar from "high concentration source to low concentration sink".
Pause, copy the highlighted definition of translocation, source and sink into your book.
In translocation, a part of the plant that uses or stores sugar (such as a root or fruit) is called a _____.
The Pressure-Flow Mechanism
How osmosis creates the pressure that drives the flow
The Pressure-Flow Mechanism
+5 XPHow osmosis creates the pressure that drives the flow
We just defined source and sink. That raises a question: what actually pushes the sap from one to the other? This card answers it → loading sugar pulls water in, building pressure that drives bulk flow.
When sugar is actively pumped into the phloem at the source, water follows by osmosis, raising the pressure there and forcing the sap to flow toward the lower-pressure sink.
Translocation theory (pressure-flow): (1) at the source, sugar is actively loaded into the phloem using ATP from companion cells; (2) this lowers the water potential, so water enters the phloem by osmosis from the nearby xylem, raising the pressure (turgor) at the source; (3) at the sink, sugar is unloaded and used or stored, water leaves, and pressure falls; (4) the pressure difference between source and sink drives the bulk flow of sap from high pressure (source) to low pressure (sink).
Text description
- Source (leaf), top: sugar is actively loaded into the phloem (using ATP); water follows from the xylem by osmosis; the pressure is high.
- Sink (root or fruit), bottom: sugar is unloaded; water leaves back to the xylem; the pressure is low.
- The pressure difference drives bulk flow of sap down the phloem, from high to low pressure.
Pause, copy the highlighted four-step translocation theory into your book in order.
Loading sugar into the phloem at the source draws water in by osmosis, raising the pressure there.
Translocation can only move sugar downwards because of gravity.
The bulk flow of phloem sap is driven by a pressure difference between source and sink.
Sequence and Explain the Translocation Steps
Do the activity in your book, then check what you have understood.
Activity 1
Sequence and Explain the Translocation Steps
Pattern C, Worked sequence
In your book, write the four steps of the pressure-flow theory in order, from sugar loading at the source to bulk flow toward the sink. Beside each step, write the one word that drives it (active transport, osmosis, pressure, unloading). Then apply it to a real case: explain why sugar flows upward from a leaf to a flower in spring, naming the source and the sink.
Match each step of the pressure-flow theory to what drives it:
- Sugar enters the phloem at the source
- Water follows the sugar into the phloem
- Sap travels along the sieve tube
- Sugar leaves the phloem at the sink
- Osmosis, down a water-potential gradient
- Unloading, into cells that use or store it
- Active transport, using ATP from companion cells
- Bulk flow, down a pressure gradient
Why the Direction Can Reverse
Bidirectional transport, set by the source–sink pattern
Why the Direction Can Reverse
+5 XPBidirectional transport, set by the source–sink pattern
We just built the pressure-flow mechanism. That raises a question: why can phloem run in either direction when xylem cannot? This card answers it → because the source and sink can be anywhere.
Because translocation is driven by wherever sugar is loaded and unloaded, the direction of flow simply follows the changing positions of source and sink.
Translocation is bidirectional: sap flows from wherever sugar is being loaded (the source) to wherever it is being used (the sink). When the leaves are the source, sugar may travel up to fruit or down to roots; when a storage organ becomes the source in spring, sugar travels up to new growth. The flow always runs from high pressure (source) to low pressure (sink), whichever way that points.
This flexibility is why phloem must be living and active: a dead, one-way pipe like xylem could never reverse. The companion cells continually pump sugar to set up the right pressure gradient for the plant's current needs.
Add the highlighted explanation of bidirectional flow to your notes before the check below.
In the pressure-flow theory, the bulk flow of phloem sap moves:
Why aphids tap into phloem, and how scientists use them
Where this biology is doing real work outside the classroom.
Why aphids tap into phloem, and how scientists use them
Aphids feed by pushing a needle-like mouthpart straight into a single phloem sieve tube, and the sap is under such high pressure that it flows into the insect on its own, the aphid barely has to suck. Scientists confirmed the pressure-flow theory by anaesthetising feeding aphids, cutting off their bodies, and collecting the sap that continued to ooze from the mouthparts under the phloem's own pressure. That clever experiment is direct evidence that phloem sap really is under pressure, and it gave researchers a way to sample exactly what a plant is translocating, where, and when.
Use the translocation theory to explain the movement of nutrients through phloem tissue from a source to a sink.
Explain means cause and effect, and the question names the theory you must use. That makes this a sequencing question: each step of pressure-flow must cause the next, and the chain must reach bulk flow.
6 marks. Define source and sink, then run the four steps of pressure-flow in order, making each one the cause of the one after it, and name a real source and sink.
Translocation is the movement of organic nutrients, mainly the sugar sucrose, through the phloem from a source to a sink. A source is any part of the plant that produces or releases sugar, such as a photosynthesising leaf. A sink is any part that uses or stores it, such as a root, a fruit or a growing tip.
At the source, sugar is actively loaded into the phloem sieve tubes. This is active transport against a concentration gradient, and it requires ATP, which is supplied by the companion cell adjacent to each sieve tube.
Loading sugar into the sieve tube raises the solute concentration inside it and so lowers its water potential. Water therefore enters the phloem by osmosis from the neighbouring xylem, and because the sieve tube is enclosed, the incoming water raises the hydrostatic pressure at the source end.
At the sink, sugar is unloaded from the phloem and either used in respiration or converted to starch for storage. Removing solute raises the water potential inside the sieve tube there, so water leaves the phloem, and the pressure at the sink end falls.
A pressure difference now exists along the sieve tube, high at the source and low at the sink, and this difference drives the bulk flow of sap from source to sink, carrying the dissolved sugar with it. For example, in summer a photosynthesising leaf is the source and a potato tuber filling with starch is the sink, so sap flows down the stem to the tuber.
Translocation is the movement of organic nutrients, mainly the sugar sucrose, through the phloem from a source — any part that produces or releases sugar, such as a photosynthesising leaf — to a sink, any part that uses or stores it, such as a root, fruit or growing tip. According to the translocation, or pressure-flow, theory, sugar is first actively loaded into the phloem sieve tubes at the source; this is active transport against a concentration gradient and requires ATP, which the adjacent companion cell supplies. Loading sugar raises the solute concentration inside the sieve tube and so lowers its water potential, so water enters the phloem by osmosis from the neighbouring xylem, and because the sieve tube is enclosed this raises the hydrostatic pressure at the source end. At the sink, sugar is unloaded and either respired or converted to starch for storage; removing solute raises the water potential there, so water leaves the phloem and the pressure at the sink end falls. A pressure difference therefore exists along the sieve tube, high at the source and low at the sink, and this difference drives the bulk flow of sap from source to sink, carrying the dissolved sugar with it. In summer, for instance, a photosynthesising leaf acts as the source and a potato tuber filling with starch as the sink, so sap flows down the stem to the tuber.
Justify the claim that phloem tissue must be composed of living cells, using the translocation theory and evidence from the aphid experiment.
Justify asks you to support a conclusion with argument and evidence. The claim is given to you; your job is to show what makes it necessary, not merely to agree with it.
5 marks. Argue from the theory first — what does translocation require that only a living cell can provide — then bring the experimental evidence, then close by showing what the alternative would fail to do.
Translocation theory requires sugar to be actively loaded into the sieve tube at the source, against its concentration gradient. Active transport consumes ATP, and ATP is produced by respiration, which only a living cell carries out. Since a mature sieve-tube cell has lost its nucleus, that ATP and the associated metabolism are supplied by the companion cell alongside it — a structure that would be pointless if the tissue did not need to be alive.
Translocation is also bidirectional: sap moves from wherever sugar is currently loaded to wherever it is currently used, so the direction reverses as sources and sinks change with the season. A potato tuber is a sink in summer while it fills with starch, and a source in spring when it releases sugar to fuel new shoots. Reversing the flow requires the pressure gradient itself to be rebuilt in the opposite sense, which means loading must be actively controlled — something a dead, passive pipe such as a xylem vessel could never do.
The aphid experiment supplies direct evidence that the mechanism is as the theory describes. Aphids feed by inserting a needle-like mouthpart into a single sieve tube. When feeding aphids are anaesthetised and their bodies cut away, sap continues to flow out of the severed mouthparts on its own for hours, without the insect sucking. This shows that the phloem contents are under positive pressure.
That positive pressure is precisely what the theory predicts, and it can only arise from the active loading of sugar and the osmotic entry of water that follows it — processes requiring living, respiring cells. Had the sap not flowed once the aphid was removed, the pressure would have to have come from the insect, and the pressure-flow mechanism would have lost its central evidence. The claim is therefore justified on two grounds: the theory requires energy and active control that only living cells can provide, and the observed pressure confirms that the plant, not anything external, is generating the flow.
Translocation theory requires sugar to be actively loaded into the sieve tube at the source against its concentration gradient, and active transport consumes ATP, which is produced by respiration in living cells alone; since a mature sieve-tube cell has lost its nucleus, that ATP and metabolism are supplied by the companion cell beside it, a structure that would serve no purpose in a tissue that did not need to be alive. Translocation is also bidirectional, running from wherever sugar is currently loaded to wherever it is used — a potato tuber is a sink in summer as it fills with starch and a source in spring as it releases sugar to new shoots — and reversing the flow requires the pressure gradient to be rebuilt in the opposite sense, which means loading must be actively controlled in a way no dead, passive pipe such as a xylem vessel could manage. The aphid experiment supplies direct evidence for the mechanism: aphids insert a needle-like mouthpart into a single sieve tube, and when feeding insects are anaesthetised and their bodies cut away, sap continues to flow from the severed mouthparts unaided for hours, showing the phloem contents are under positive pressure. That pressure is what the theory predicts, and it can only arise from active sugar loading and the osmotic entry of water behind it, both of which require living cells. Had the sap stopped once the insect was removed, the pressure would have had to come from the aphid and the theory would have lost its central evidence. The claim is therefore justified on two grounds: the theory demands energy and active control that only living cells supply, and the observed pressure confirms that the plant itself generates the flow.
Practise independently
Answer in your own words first. The model answers are for checking, not for copying.
Multiple Choice
+5 XPA fresh set drawn from this lesson's question bank, feedback shown immediately. +5 XP per correct · +25 XP all correct
Pick your answer, then rate your confidence, that tells the system what to drill next.
Short Answer, 3 questions · 13 marks
+5 XPApplyBand 5(5 marks) 1. Use the translocation theory to explain the movement of nutrients through phloem tissue from a source to a sink.
1 mark: sugar actively loaded at the source using ATP · 1 mark: water enters by osmosis raising pressure at source · 1 mark: sugar unloaded at sink, water leaves, pressure falls · 1 mark: pressure difference drives bulk flow source→sink · 1 mark: identifies a valid source and sink
ApplyBand 4(3 marks) 2. A potato tuber fills with starch through summer and then supplies sugar to new shoots in spring. Identify the tuber's role in each season and explain, using translocation theory, why the direction of flow in the phloem reverses between the two.
1 mark: tuber is a sink in summer and a source in spring · 1 mark: explains that sugar is loaded at whichever part is the source, raising pressure there · 1 mark: concludes that flow runs from the high-pressure source to the low-pressure sink, so the direction follows the source-sink pattern rather than gravity
AnalyseBand 5–6(5 marks) 3. Researchers anaesthetise aphids feeding on a stem, cut away the insects' bodies, and find that sap continues to flow out of the severed mouthparts on its own for hours. Analyse how this observation supports the translocation (pressure-flow) theory, and state what a different result would have shown.
1 mark: identifies that the sap flows without the aphid sucking · 1 mark: concludes the phloem contents must be under positive pressure · 1 mark: links that pressure to sugar loading and osmotic water entry at the source · 1 mark: explains that this pressure difference is what the theory says drives bulk flow · 1 mark: states what a contrary result (no flow without the insect) would have implied
Show all answers
Multiple choice
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
Short Answer Model Answer
Q1 (5 marks): According to the translocation (pressure-flow) theory, at the source, for example a photosynthesising leaf, sugar (sucrose) is actively loaded into the phloem sieve tubes using ATP supplied by companion cells. This lowers the water potential inside the phloem, so water enters by osmosis from the neighbouring xylem, raising the turgor pressure at the source end. At the sink, for example a root or fruit, sugar is unloaded and used or stored, water leaves the phloem, and the pressure there falls. The resulting pressure difference between the high-pressure source and the low-pressure sink drives the bulk flow of phloem sap from source to sink, carrying the dissolved nutrients with it. Because the direction depends only on where sugar is loaded and unloaded, translocation can move nutrients up or down the plant.
Q2 (3 marks): In summer the tuber is a sink: the leaves are photosynthesising, so sugar is loaded into the phloem in the leaves and unloaded into the tuber, where it is stored as starch. In spring the tuber is a source: it breaks its starch back down to sugar and loads it into the phloem, while the new shoots, which have no leaves yet, are the sinks. Translocation theory explains the reversal directly. Sugar is actively loaded at whichever part is the source, water follows by osmosis, and the pressure rises there; at the sink the sugar is unloaded, water leaves and the pressure falls. Sap then flows by bulk flow from high pressure to low pressure. Because the high-pressure end is the leaf in summer and the tuber in spring, the direction of flow reverses, which shows the flow is set by the source and sink positions and not by gravity.
Q3 (5 marks): The sap continues to emerge although nothing is sucking it, so the phloem contents must be under positive hydrostatic pressure of their own. That is exactly what the pressure-flow theory predicts: at the source, sucrose is actively loaded into the sieve tube using ATP from companion cells, which lowers the water potential inside; water then enters by osmosis from the neighbouring xylem, and because the sieve tube is a confined space the incoming water raises the turgor pressure. The theory proposes that the resulting pressure difference between the high-pressure source and the low-pressure sink is what drives the bulk flow of sap, and a severed mouthpart is essentially a leak in that pressurised system, which is why it keeps flowing. The observation therefore supports the mechanism rather than merely the destination. Had the sap stopped as soon as the aphid was removed, it would have suggested the insect's own suction was moving it and that the phloem was not pressurised, which would have counted as evidence against pressure-flow.
Retrieve and reflect
Use the spaced-review session, then compare how your thinking has changed since your opening prediction.
Check what actually stuck
A short retrieval session mixing this lesson with earlier ones, so the ideas move into long-term memory instead of fading by the exam.
Lock it in
bossYou can now use the translocation theory to explain how phloem moves sugar from source to sink. Next lesson explains the partner mechanism, how xylem pulls water up by cohesion-tension.
↩ Back to focus-area hubHow did your thinking change?
At the start you guessed how a plant builds pressure without a pump. The answer is osmosis: loading sugar at the source draws water in, raising the pressure there, and that pressure pushes the sap toward the low-pressure sink, exactly like squeezing a water balloon, but with osmosis doing the squeezing.
If you thought sugar only moves downward, revisit the misconception box: translocation runs source-to-sink in either direction, driven by pressure, not gravity.