eduKate Learning Manual
Science | Plant World
Understand → Explain → Apply → Test → Go Deeper

Phloem Transport

How Sugar Can Travel Up or Down a Plant

Big Question: How does a plant distribute carbon compounds from places that can supply them to places that need them, without a central pump and without one fixed direction of travel?

Quick Answer

Phloem is living vascular tissue that transports sugars and many other compounds between sources and sinks. In flowering plants, long-distance transport occurs mainly through sieve-tube elements associated with companion cells.

At a source, transportable sugars such as sucrose enter the phloem through species-dependent loading mechanisms. This lowers the phloem’s solute potential, water enters from nearby tissues including xylem, and hydrostatic pressure rises. At sinks, sugars are unloaded and used or stored, water can leave, and pressure is lower. The resulting pressure gradient drives bulk flow through the sieve-tube network.

source → loading → osmosis → pressure → mass flow → unloading → sink.

What You Will Learn

• Why “phloem carries food downward” is incomplete.
• What sources and sinks are.
• How a young leaf can change from sink to source.
• What sieve-tube elements and companion cells do.
• Why mature sieve elements lose much of their cellular machinery.
• How sugars enter and leave phloem.
• How osmosis creates pressure differences.
• Why long-distance movement is bulk flow rather than molecule-by-molecule diffusion.
• How xylem and phloem exchange water.
• How tracers, aphids, PET and MRI reveal phloem transport.
• Why phloem transport can become a bottleneck for growth and photosynthesis.
• How drought and damage alter carbon allocation.

Part 1 — The Plant Has an Allocation Problem

Photosynthesis happens mainly in illuminated photosynthetic tissues. But a root tip underground cannot photosynthesise. A developing seed inside a fruit cannot simply make all the carbon it needs from light. A flower bud, cambium and young shoot often depend on imported assimilates.

The plant therefore needs a distribution system.

Phloem carries not just carbon for energy. Imported compounds can become:

• respiratory substrates;
• cellulose and other cell-wall materials;
• starch reserves;
• lipids in seeds;
• amino acids and proteins;
• secondary metabolites;
• signals affecting development and defence.

Moving carbon therefore determines what parts of the plant can grow, store, reproduce and recover.

Part 2 — What Is a Source?

A source is tissue that exports more transportable assimilate into the phloem than it imports at that time.

Common sources include mature photosynthetic leaves. But storage organs can become sources too. A tuber, bulb, seed or woody reserve tissue may mobilise stored carbohydrate to support new growth.

“Source” therefore describes a current functional role, not a permanent organ identity.

Part 3 — What Is a Sink?

A sink imports assimilates for use or storage.

• root tips;
• shoot apical meristems;
• young expanding leaves;
• flowers;
• developing fruits;
• seeds;
• storage organs;
• cambial regions making new wood and phloem.

A sink can be strong or weak depending on growth rate, developmental stage and metabolic demand.

Part 4 — A Young Leaf Changes Jobs

A newly forming leaf is often a sink. Its cells are dividing and expanding, but its photosynthetic machinery and surface area are not yet sufficient to pay all of its carbon costs.

As the leaf matures, photosynthetic capacity rises. Eventually it can export more carbon than it imports and become a source.

young leaf: sink → mature leaf: source.

This one transition destroys the idea that “leaf = source” is a fixed rule.

Part 5 — Sieve-Tube Elements: Living Cells With a Strange Design

In flowering plants, sieve-tube elements align end-to-end. Their end walls contain sieve plates with pores that permit sap movement between cells.

As they mature, sieve elements lose the nucleus and much of the machinery found in ordinary living cells. This reduces internal obstruction to flow but leaves them metabolically dependent on neighbouring companion cells.

They remain living cells, but in an unusually reduced state.

Part 6 — Companion Cells: The Metabolic Partners

Companion cells are closely associated with sieve-tube elements and connected to them through plasmodesmata.

They support metabolism, protein turnover, membrane transport and loading or retrieval processes that sieve elements cannot manage independently.

Think of the sieve element–companion cell complex as a partnership:

streamlined transport cell + metabolically capable support cell.

Part 7 — How Does Sugar Enter the Phloem?

There is no single loading strategy used by every plant.

Important strategies include:

• Apoplastic loading: sucrose enters the cell-wall space and is actively taken into the sieve element–companion cell complex using transporters powered indirectly by proton gradients.
• Symplastic loading: sugars move through plasmodesmata from cell to cell.
• Polymer trapping: sucrose moves symplastically into specialised companion cells and is converted into larger oligosaccharides that help maintain directional accumulation.
• Passive loading: some plants maintain sufficiently high sugar concentrations in mesophyll cells that diffusion through symplastic pathways can load phloem without the same active concentrating step.

Different lineages have evolved different combinations. Do not turn one model species into a universal plant.

Part 8 — Osmosis Converts Sugar Loading Into Pressure

When solute concentration rises in source phloem, water potential becomes lower relative to nearby xylem and surrounding tissues. Water enters osmotically.

Because phloem cells have walls, incoming water raises hydrostatic pressure rather than simply causing unlimited expansion.

At sinks, unloading sugars reduces osmotic draw and water may leave the phloem.

The result is a pressure difference along the transport pathway.

Part 9 — Pressure Flow: Bulk Movement Through the Network

Diffusion is too slow to move large quantities of sugar over metres through a tall plant. Long-distance phloem transport therefore relies primarily on bulk flow.

The simplified Münch sequence is:

1. sucrose is loaded at a source;
2. water enters phloem osmotically;
3. source-region turgor pressure rises;
4. sap moves toward lower-pressure regions;
5. sugars are unloaded at sinks;
6. water can return to xylem or surrounding tissues;
7. the source–sink pressure difference is maintained while loading and unloading continue.

This does not mean every sieve tube is a perfectly passive pipe. Living tissues regulate loading, unloading, retrieval, signalling and responses to damage along the route.

Part 10 — Xylem and Phloem Are Coupled

Water links the two vascular systems.

At source regions, water moving into sugar-rich phloem may come from xylem or nearby tissues. At sink regions, water can leave phloem and re-enter the xylem stream.

xylem moves a water-dominated stream under tension; phloem moves a solute-rich stream under positive pressure.

This contrast is useful, but real local pressures and flows vary across tissues and conditions.

Part 11 — Why Sugar Can Move Up and Down at the Same Time

Different sieve tubes can connect different source–sink pairs.

A mature leaf in the middle of a plant may export sugar upward toward a growing shoot and downward toward roots at the same time through different pathways.

The same individual sieve tube does not normally carry two opposite bulk streams at once. Bidirectional transport at plant scale emerges because the vascular network contains many conduits serving different routes.

Part 12 — Sink Strength: Not Every Destination Pulls Equally

A rapidly developing fruit can become a strong sink because it unloads and metabolises or stores large quantities of carbon. A growing meristem can compete with roots or storage tissues. After flowering, allocation patterns can shift dramatically.

Sink strength depends on both sink size and sink activity.

Hormones, metabolism, development, temperature and stress can alter these relationships.

Part 13 — Storage Can Reverse the Route Later

A potato tuber or other storage organ may receive sugars during one season and later release stored carbohydrate to support sprouts.

A seed is a sink while it fills, then becomes a reserve that fuels a new seedling before the seedling becomes photosynthetically independent.

today’s sink can become tomorrow’s source.

Part 14 — Phloem Carries More Than Sugar

Phloem sap can contain amino acids, inorganic ions, hormones, proteins, peptides and RNA molecules as well as sugars.

Some of these substances participate in long-distance signalling between organs.

This means phloem is simultaneously a nutrient-allocation system and an information route. But the presence of a molecule in phloem does not automatically prove it has a long-distance signalling function; function requires evidence.

Part 15 — Damage: Why Sieve Tubes Need Fast Sealing

Phloem operates under positive pressure. If a sieve tube is cut, sap could be lost rapidly.

Plants have mechanisms that help seal wounded sieve elements. Calcium-associated responses, P-proteins in many species and callose deposition can reduce leakage and isolate damaged regions.

The system therefore faces another tradeoff: remain open enough for transport but able to close quickly after injury.

Part 16 — Aphids Accidentally Reveal Phloem Pressure

Aphids insert extremely fine stylets into sieve elements to feed on phloem sap.

Because the phloem can be under positive pressure, sap may enter an aphid’s feeding pathway without the insect having to suck continuously with the same effort required to draw xylem sap under tension.

Researchers have even used severed aphid stylets as tiny access points for collecting phloem sap and estimating pressure or composition.

A feeding insect thereby becomes an experimental window into a tissue that is otherwise very difficult to sample without triggering wound responses.

Part 17 — Carbon Tracers Make the Invisible Route Visible

Scientists can feed a leaf carbon dioxide containing a detectable isotope and then follow where newly fixed carbon travels.

Radioactive carbon-14 has long been used to create autoradiographs showing where assimilated carbon accumulates. Short-lived carbon-11 can be followed dynamically with radiation detectors and positron-emission techniques.

These experiments answer a simple but powerful question:

which leaf supplied which sink, and how quickly did the carbon arrive?

Part 18 — MRI Can Watch Flow Without Cutting the Plant

Nuclear magnetic resonance imaging can measure water movement in vascular tissues non-destructively. Special plant MRI systems have allowed researchers to observe xylem and phloem flow in intact plants over time.

This matters because cutting phloem changes pressure and can trigger rapid sealing. A measurement method that leaves the pathway intact can reveal behaviour that destructive sampling might disturb.

Explore phloem imaging methods in a new tab →

Part 19 — The Girdling Experiment: What Happens If Phloem Is Interrupted?

If a ring of bark containing phloem is removed from a woody stem while the central xylem remains intact, downward transport of assimilates can be disrupted.

Sugars may accumulate above the girdle, while tissues below eventually become carbon-starved. Roots can decline even though water initially continues to move upward through xylem.

This separates two transport jobs:

xylem can still move water while phloem carbon delivery has been interrupted.

Do not perform destructive girdling on living trees for a classroom demonstration. Use existing diagrams, cut plant material or published evidence instead.

Part 20 — What Drought Does to Phloem

Drought reduces photosynthesis through stomatal closure and metabolic stress, lowering carbon supply. At the same time, phloem transport depends on water relations because hydrostatic pressure and sap viscosity matter.

As phloem sap becomes more concentrated, viscosity can increase. Sink metabolism can slow. Growth may stop before carbon reserves are exhausted.

Phloem and xylem stress therefore interact. A plant cannot allocate carbon independently of its hydraulic state.

Part 21 — Transport Capacity Can Limit Photosynthesis

If a source leaf produces assimilates faster than they can be exported or consumed, carbohydrates can accumulate locally.

That accumulation can feed back on metabolism and gene regulation, reducing photosynthetic activity.

This reverses the usual school logic. Photosynthesis does not simply determine how much carbon the plant has. The plant’s ability to move and use carbon can also influence how much photosynthesis continues.

source capacity ↔ transport capacity ↔ sink demand.

Follow One Carbon Atom

1. A carbon atom enters a leaf as carbon dioxide.
2. Photosynthesis incorporates it into an organic molecule.
3. Carbon enters sucrose in a source leaf.
4. Sucrose is loaded into phloem.
5. Water enters and helps create pressure.
6. The sucrose molecule moves with bulk phloem sap.
7. It reaches a growing root tip.
8. Sucrose is unloaded.
9. The carbon may be respired, stored, built into a cell wall or incorporated into another molecule.

The wider route continues across the living and non-living world:

eduKate Learning Manual: One Carbon Atom | Air → Leaf → Animal → Reef → Atmosphere →

A Text Diagram You Can Draw Anywhere

SOURCE LEAF
photosynthesis
     ↓
sucrose loading
     ↓
water enters from xylem
     ↓
HIGHER PHLOEM PRESSURE
     ↓
==========================
 SIEVE TUBE MASS FLOW
==========================
     ↓
LOWER PHLOEM PRESSURE
     ↓
sucrose unloading
     ↓
SINK
root / fruit / seed / meristem / storage organ

Direction is set by source → sink,
not by gravity alone.

How Scientists Study Phloem

• Radioisotope tracers: follow newly fixed carbon.
• PET imaging: tracks short-lived carbon-11 through intact plants.
• MRI/NMR flow imaging: measures vascular flow non-invasively.
• Aphid stylets: access pressurised phloem sap with minimal tissue disruption.
• Fluorescent tracers: reveal symplastic connectivity and transport.
• Girdling experiments: separate xylem and phloem consequences.
• Pressure probes: estimate turgor in suitable phloem cells.
• Genetic perturbations: alter transporters, plasmodesmata or sieve-element development.
• Metabolomics: measures transported sugars, amino acids and other solutes.

The strongest explanation is supported by multiple methods that disturb the system in different ways.

Observation vs Inference

After removing several developing fruits from a plant, sugars increase in nearby leaves.

• Observation: leaf carbohydrate concentration increased after fruit removal.
• Inference: removing a strong sink may have reduced carbon export demand.
• Alternative: fruit removal could also change hormones, water relations or source metabolism.
• Better test: combine carbohydrate measurements with phloem transport tracers, gas exchange and hormonal measurements.

Common Misconceptions and Better Models

Misconception	Better model
Phloem carries food downward.	Phloem transports between sources and sinks; plant-scale movement can be upward or downward.
Every leaf is always a source.	Young leaves are commonly sinks before becoming sources.
Roots are always sinks.	Roots often import carbon, but stored reserves can later be mobilised and exported.
Sugar diffuses all the way through the stem.	Long-distance transport is mainly pressure-driven bulk flow through sieve tubes.
Phloem cells are dead like xylem vessels.	Sieve elements are living, highly specialised cells supported by companion cells.
Pressure flow means no energy is needed.	Bulk flow may follow a pressure gradient, but maintaining sources, sinks, loading and unloading can require metabolic energy.
Only sugar travels in phloem.	Phloem also transports amino acids, ions, hormones and other molecules.
More photosynthesis always means more growth.	Transport capacity, sink demand, nutrients, water and development can all constrain growth.

Primary Science / PSLE Bridge

At Primary level, the essential model can stay simple:

• leaves make food through photosynthesis;
• plants need to move manufactured food to other parts;
• phloem transports manufactured food;
• roots, fruits and growing parts can receive this food;
• transport links plant organs into one system.

When the child asks “Does it always move down?”, that is the moment to introduce source and sink.

Go Beyond Primary Science

School idea	Higher-resolution model
Phloem carries food	Living sieve-element/companion-cell complexes distribute assimilates and signals.
Food moves from leaf to root	Dynamic source–sink networks determine allocation direction.
Sugar enters phloem	Loading can be apoplastic, symplastic, polymer-trapping or passive depending on species.
Food moves through tubes	Osmotically generated pressure gradients drive bulk flow through a resistive network.
Fruit needs food	Sink strength, unloading capacity and metabolism influence allocation.
Photosynthesis supplies sugar	Source activity, phloem transport capacity and sink demand form a coupled control system.

Deep Science Window — Pressure Flow Is Powerful but Not Finished Science

The Münch pressure-flow model explains a great deal, but researchers continue to investigate how pressure gradients behave in very tall plants, how sieve-tube resistance changes, how radial exchange along transport paths affects flow and how loading strategies alter pressure generation.

A useful scientific model can therefore be both strongly supported and still incomplete in its details.

Deep Science Window — The Phloem Network Is Not Just Plumbing

Phloem connects carbon status, growth, defence, hormonal state and developmental programmes. Sugar itself can act as a signal. RNAs and proteins may move long distances. Pathogens can exploit the network, and insects tap it directly.

The transport system therefore participates in coordination across the plant body.

Evidence Boundaries

• Source ≠ permanent organ identity. Functional roles change with development.
• Sink ≠ simply “lower part.” A sink can be above or below a source.
• Pressure flow ≠ active pumping along every centimetre. Loading and unloading maintain gradients; bulk sap follows pressure differences.
• One loading mechanism ≠ all plants. Species differ substantially.
• Presence in phloem ≠ proven signal. Functional signalling requires experimental evidence.
• Carbon accumulation ≠ proof of transport limitation. Source metabolism and sink demand must also be measured.
• Girdling ≠ phloem-only perturbation forever. Long-term tissue damage changes water relations, hormones and root function too.

Explore Elsewhere

• Wikipedia — Phloem → anatomy, loading and translocation.
• Wikipedia — Pressure Flow Hypothesis → the Münch framework.
• National Geographic — Photosynthesis → revisit where many source sugars begin.

Checkpoint Questions

1. What is a source?
2. What is a sink?
3. Why can a young leaf be a sink?
4. What is a sieve-tube element?
5. Why does it need a companion cell?
6. What happens to water when sugar is loaded into source phloem?
7. What generates the pressure gradient?
8. Why is diffusion alone too slow for long-distance transport?
9. How can sugar move up and down the same plant?
10. Why can a storage organ switch from sink to source?
11. How are xylem and phloem hydraulically connected?
12. Why are aphids useful to phloem researchers?
13. How can carbon isotopes reveal source–sink routes?
14. Why can sink removal affect photosynthesis?
15. What makes “phloem transports food” correct but incomplete?

Can You Explain WHY?

• Why does adding sugar to source phloem lead to pressure?
• Why can one leaf supply both roots and a growing shoot?
• Why might a plant reduce photosynthesis if export capacity is saturated?
• Why does phloem damage threaten roots even if xylem still carries water?
• Why can drought disrupt carbon transport even before the plant runs out of stored carbohydrate?

Manual Summary

Phloem solves the plant’s carbon-allocation problem. Sources load assimilates. Water enters and raises pressure. Sap moves by bulk flow through sieve tubes. Sinks unload carbon for growth, metabolism or storage. Source and sink identities change with development, so transport direction is dynamic rather than fixed by gravity.

make → load → pressurise → move → unload → build.

Where to Go Next

• eduKate Learning Manual: Photosynthesis
• eduKate Learning Manual: Water Transport
• eduKate Learning Manual: One Carbon Atom
• How a Plant Builds and Runs Itself

Research Sources and Further Reading

• Plant Physiology (2025) — Source–Sink Transport as a Constraint on Photosynthesis and a Driver of Ecophysiological Patterns
• Journal of Experimental Botany — Phloem Transport: A Review of Mechanisms and Controls
• Plant Physiology — Radial–Axial Transport Coordination Enhances Sugar Translocation
• Journal of Experimental Botany — Phloem Imaging
• Journal of Experimental Botany — Phloem Transport and Drought
• Plant Physiology — Phloem Loading Strategies and Water Relations
• Wikipedia — Phloem

---

eduKate Learning Manuals aim to replace isolated facts with causal models that still work when the question changes.