Biology · Plant Structure, Transport & Nutrition

Phloem transport, sources, sinks and carbon allocation

A developing leaf can need imported carbon before it produces enough to export. Later, the same leaf can supply roots or fruits. Phloem connects these changing roles, and the direction of transport follows the connected sources and sinks rather than a fixed rule of downward movement.

Cells, Membranes & Organisation introduces selective membranes, osmosis and connections between cells. Energy, Enzymes & Metabolism introduces respiration, carbon fixation and ATP coupling. Their necessary ideas are restated here. Plant Diversity & Evolution provides the wider evolutionary context; on a first reading, remember that vascular tissues belong to particular plant lineages. These lessons use selected vascular land plants rather than a universal plant body. Chemistry's concentration and energy chapters and Physics's fluid dynamics chapter offer optional return paths.

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By the end, you can

Explain source-to-sink transport through loading, pressure-driven bulk flow and unloading, then account for carbon allocation.

A role depends on the current carbon account

A source is a region with net export of transported organic material; a sink has net import. A mature photosynthetic leaf often acts as a source. A growing root, young leaf or developing seed can act as a sink because it uses carbon for respiration, construction or storage.

An organ containing sugar is not automatically a source. It may still import more than it exports. A storage organ can receive carbon in one developmental phase and later release stored material to support new growth. The same anatomy can therefore participate in opposite net transfer roles at different times.

Transported carbon commonly travels as sucrose, although other sugars and sugar alcohols contribute in some species. Phloem sap also contains other substances and signals. The carbon account below counts carbon atoms in equivalent units rather than treating every sap molecule as sugar.

Several sinks can be connected to a source. Allocation depends on transport connections, supply, sink metabolism and developmental conditions. The closest organ is not automatically the only recipient, and a source does not consciously decide which partner deserves carbon.

Loading can create a pressure difference

In a selected source region, sugar loading makes the solute contribution to phloem water potential more negative. Water can enter through membranes from neighbouring tissues. As the constrained living system takes up water, its hydrostatic pressure rises. At the receiving region, unloading and metabolism can help maintain conditions for delivery.

The liquid in a connected sieve tube can then move by bulk flow along a pressure gradient. Sugar travels with that liquid; it does not have to diffuse independently along the entire distance. Sieve plates contain pores between angiosperm sieve-tube elements, so geometry and resistance matter.

The constructed source has a pressure contribution of +0.8 MPa and the sink +0.2 MPa, an axial difference of 0.6 MPa. Their assigned solute contributions are −1.2 and −0.6 MPa, respectively. Both totals are −0.4 MPa. Equal local total water potentials in this simplified picture do not erase the pressure difference driving axial bulk flow. Membrane osmosis and axial flow through a conduit are different comparisons.

Loading and unloading pathways vary. Some involve energy-dependent membrane transport, such as sugar transport coupled to a proton gradient maintained using ATP. Other loading routes can be passive or involve different metabolic arrangements. Energy use in establishing or regulating transport conditions does not mean a motor mechanically pumps every parcel along the sieve tube.

In different tubes, bulk flows can run in different directions at the same time. Our one source-to-sink tube has one assigned net direction. The model omits gravity and complex local exchange; it does not establish a universal pressure profile or a real flow rate.

A pressure gradient along a living sieve tube

The selected pressures illustrate an axial driving difference. They are not measured values or a claim that all species use energy-dependent loading.

Read the diagram in words

The source has Ψs −1.2 and Ψp +0.8 MPa; the sink has −0.6 and +0.2 MPa. Both local totals are −0.4 MPa, while source pressure exceeds sink pressure by 0.6 MPa. A connected tube with porous plates carries the assigned source-to-sink bulk flow. Loading and unloading are separate steps; the water-entry relation describes establishment rather than a quantified ongoing lateral flux at the displayed totals.

MPa
Description	Drawing x coordinate	Drawing y coordinate	Solute contribution	Pressure contribution	Local total water potential
Selected source region	1	1	-1.2	0.8	-0.4
Selected sink region	9	1	-0.6	0.2	-0.4

Connected sieve-tube route
Displayed structure	Drawing x coordinate	Drawing y coordinate	Width	Height
Connected sieve-tube route · 1	2	0.5	6	1

Sieve-plate pores
Displayed structure	Drawing x coordinate	Sieve-pore y positions
1	4	0.7, 1, 1.3
2	6	0.7, 1, 1.3

Associated companion cell in this angiosperm model
Displayed structure	Drawing x coordinate	Drawing y coordinate	Width	Height
Associated companion cell in this angiosperm model · 1	2	2.2	1	1
Associated companion cell in this angiosperm model · 2	7	2.2	1	1

A pressure gradient along a living sieve tube
Process	From	To	Axial pressure difference (MPa)
Assigned net axial flow source → sink	Selected source region	Selected sink region	0.6

A pressure gradient along a living sieve tube
Process	From	To
Sugar loading at source	(1, -1)	(1, 0.5)
Delivery and use or storage at sink	(9, 1.5)	(9, 3)
Water entry can raise turgor during establishment; no lateral flux assigned at displayed totals	(1, 3)	(1, 1.5)

A pressure gradient along a living sieve tube
Process	From	To
Some loading routes use energy coupling	(3.5, 3.5)	(2.5, 2.2)

Pressure contribution

Solute contribution

Local total water potential

Axial pressure difference: 0.6 MPa

Sieve-plate pores

Local osmosis and axial bulk flow are different comparisons

Constructed instantaneous equal-height phloem state, not a full steady-state physiological solution. No rate assigned. Axial gravity, complex loading modes and other exchange terms omitted.

Export is only one destination for fixed carbon

Our constructed leaf fixes 100 carbon-account units during an interval. It releases 20 in carbon dioxide through respiration, retains 30 in its own organic pools and exports 50. The exported amount is allocated as 20 to a root, 15 to a growing shoot and 15 to a developing fruit.

Each recipient has its own account. The root retains 12 and respires 8; the shoot retains 10 and respires 5; the fruit retains 13 and respires 2. Across the entire defined plant boundary, 65 units remain in organic pools and 35 leave as respiratory carbon dioxide. The internal exports cancel from the whole-plant account.

Retained carbon is not automatically permanent storage, harvestable yield or reproductive success. It can later be consumed or transferred. The model has no next interval, no other carbon inputs and no measured response; it isolates the meaning of gross fixation, respiration, export and retention.

Count an internal export once

Carbon conserved across the whole-plant boundary is 100 = 65 + 35. Retained pools can have later fates outside this interval.

Read the diagram in words

The leaf's input of 100 divides into 30 retained, 20 respired and 50 exported: 20 to root, 15 to shoot and 15 to fruit. Root outcomes are 12 retained and 8 respired; shoot 10 and 5; fruit 13 and 2. Whole-plant retention is 65 and respiratory output 35. Internal export is not an additional external input.

Carbon-account units
Description	Drawing x coordinate	Drawing y coordinate	Amount in the stated account
Atmospheric carbon fixed	0	3	100
Source leaf	2	3	—
Leaf retained	4	0	30
Leaf respiratory CO2	4	1	20
Root receives 20	4	3	—
Shoot receives 15	4	5	—
Fruit receives 15	4	7	—
Root retained	7	2.5	12
Root respiratory CO2	7	3.5	8
Shoot retained	7	4.5	10
Shoot respiratory CO2	7	5.5	5
Fruit retained	7	6.5	13
Fruit respiratory CO2	7	7.5	2

Carbon-account units
From	To	Amount in the stated account
Atmospheric carbon fixed	Source leaf	100
Source leaf	Leaf retained	30
Source leaf	Leaf respiratory CO2	20
Source leaf	Root receives 20	20
Source leaf	Shoot receives 15	15
Source leaf	Fruit receives 15	15
Root receives 20	Root retained	12
Root receives 20	Root respiratory CO2	8
Shoot receives 15	Shoot retained	10
Shoot receives 15	Shoot respiratory CO2	5
Fruit receives 15	Fruit retained	13
Fruit receives 15	Fruit respiratory CO2	2

Carbon-account units
Description	Amount in the stated account
Internal transfer	50
Retained organic carbon	65
Carbon leaving as CO2	35

A later storage-organ interval; separate from the network above
Storage-organ import	Storage-organ export	Net export
2	9	7

External input

Internal transfer

Retained organic carbon

Carbon leaving as CO2

Whole-plant boundary excludes internal arrows from external inputs

Positions are schematic; amounts appear on every edge

Constructed single-interval carbon account with no other inputs or outputs. Carbon-account units represent equal amounts of carbon atoms; no ATP yield or permanent biomass fraction inferred.

Worked example

Change the boundary before adding the arrows

For the leaf alone, 100 = 20 + 30 + 50. For the root alone, its incoming 20 = 8 + 12. For the whole plant, source-to-root export is internal, so adding it as a second external input would count the same carbon twice.

Sum the retained pools: 30 + 12 + 10 + 13 = 65. Sum respiratory losses: 20 + 8 + 5 + 2 = 35. Together they equal the original fixation of 100. This is a material account, not an ATP-yield calculation.

Now imagine a later phase in which a storage organ exports 9 units to a new shoot while importing 2. Its net export is 7, so it is a source over that interval. Calling it a source describes the net balance, even though material crossed its boundary in both directions.

Check your understanding

A young leaf imports 12 carbon units and exports 4 during an interval. Is it a source because it exports some carbon?
Show reasoning
It is a net sink: import minus export is 8 units. Source and sink roles describe the net transfer account over stated conditions and time, not the mere presence of sugar or a single export arrow.
A source and sink have equal total water potentials in the simplified local model. Must pressure-driven movement along their connected sieve tube be zero?
Show reasoning
No. Their pressure contributions can differ while their solute contributions offset that difference in the totals. The model has a 0.6 MPa axial pressure difference. Local membrane water exchange and axial bulk-flow driving forces must be distinguished; a rate also needs conduit properties and other relevant terms.
Why is whole-plant retained carbon 65 rather than 115 after adding the exported 50?
Show reasoning
The exported 50 is already included in the recipient accounts. It crosses internal plant boundaries, not the external whole-plant boundary. Adding it again double-counts carbon fixed in the leaf.

What to carry forward

Name the boundary and current source or sink role, then distinguish energy-coupled loading from pressure-driven transport.

References

Resources supporting this lesson’s explanations and examples.