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If xylem is the great passive river of plant water transport, then phloem is its active, metabolic counterpart — the living tissue that carries the products of photosynthesis from the leaves where they are made to every other part of the plant that consumes or stores them. Unlike xylem, which is essentially a one-way upward flow of water driven by evaporative tension, phloem moves dissolved organic solutes — predominantly sucrose, but also amino acids, hormones, mineral ions and even mobile signalling RNAs — in any direction the plant requires, from sources of supply to sinks of demand. The mechanism, the mass-flow hypothesis proposed by Ernst Münch in 1930 (paraphrased throughout), is one of the most elegant applications of osmosis at the whole-organ scale: active loading at the source creates an osmotically generated hydrostatic pressure that drives bulk flow through living sieve-tube elements to sinks where the solutes are actively unloaded, water leaves by osmosis, and the pressure falls. This lesson examines phloem structure, the mass-flow framework, the experimental evidence that supports it, the relationship between source and sink, and the comparison with xylem that completes the AQA mass-transport-in-plants pair (Sections 3.3.4 — plants component).
This lesson maps to AQA 7402 Section 3.3.4 — Mass transport in plants (phloem and translocation) (refer to the official AQA specification document for exact wording). The specification requires that students understand the structure of phloem in flowering plants, the mass-flow hypothesis of translocation, the relationship between sources and sinks, and the experimental evidence supporting (and limiting) the mass-flow model.
Phloem is composed of several cell types, of which two are central: sieve-tube elements (the conducting cells) and companion cells (associated metabolic support cells). Unlike xylem, all phloem cells are alive at functional maturity — the conducting tissue is metabolically dependent, which is why translocation is reversibly disrupted by metabolic inhibitors but xylem flow is not.
Sieve-tube elements are elongated cells (a few hundred micrometres long, ~30 μm in diameter) joined end-to-end to form sieve tubes. The end walls between adjacent elements are perforated with large pores forming a sieve plate — the structural signature of phloem. Sieve plates are visible under light microscopy as transverse striations.
Sieve-tube elements undergo a remarkable process of partial cellular dismantling during maturation: they lose their nucleus, ribosomes, vacuole, Golgi apparatus and most other organelles, retaining only a thin layer of cytoplasm pressed against the cell wall, plus the plasma membrane and a few endoplasmic reticulum strands and modified mitochondria. The lumen is thereby cleared for efficient bulk flow — a structural compromise that resembles the dead xylem vessel except that the plasma membrane is preserved (so the cell remains osmotically active) and the lumen contents are at high positive pressure rather than under tension.
Sieve-tube elements lack the machinery for protein synthesis and cannot maintain themselves. They depend entirely on associated companion cells.
Each sieve-tube element is connected to one or more companion cells through abundant plasmodesmata in the lateral walls. Companion cells are fully living, nucleated, metabolically very active: they have dense cytoplasm, many mitochondria (providing ATP), an active rough endoplasmic reticulum, and an active Golgi apparatus. They control the metabolism of the sieve-tube element, supply it with proteins synthesised in their own ribosomes (transferred through the plasmodesmatal connections), and — crucially — drive the active loading of sucrose into the sieve tube at sources. A sieve-tube element and its companion cell(s) together form a functional unit sometimes called the sieve element / companion cell complex (SE/CC complex).
Phloem also contains phloem parenchyma (storage and lateral transport) and phloem fibres / sclereids (structural support). These cells matter for the tissue's stability but are not directly involved in translocation.
flowchart TB
SOURCE[SOURCE TISSUE<br/>e.g. mature leaf]
SOURCE -->|sucrose produced<br/>by photosynthesis| CC1[Companion cell at source<br/>H+ ATPase pumps H+ out<br/>sucrose enters via H+/sucrose symporter<br/>active loading]
CC1 -->|plasmodesmata| SE1[Sieve-tube element at source<br/>sucrose concentration high<br/>water potential low<br/>water enters by osmosis<br/>HYDROSTATIC PRESSURE RISES]
SE1 -->|bulk flow<br/>through sieve plates<br/>50-100 cm/h| SE2[Sieve-tube element at sink<br/>sucrose concentration low<br/>water potential rises<br/>water leaves by osmosis<br/>HYDROSTATIC PRESSURE FALLS]
SE2 -->|plasmodesmata| CC2[Companion cell at sink<br/>active unloading of sucrose]
CC2 --> SINK[SINK TISSUE<br/>e.g. growing meristem,<br/>developing fruit,<br/>root storage organ]
The defining property of translocation is its bidirectionality. Phloem can move solutes in either direction along its length, and the direction depends on the local source-sink relationship, not on a fixed physiological "up" or "down" as in xylem.
A source is a tissue that exports photosynthate (or releases stored carbohydrate). The textbook source is a mature photosynthesising leaf, which exports the sucrose it produces. But other sources exist: a storage root in early spring, when stored starch is mobilised to sucrose for export to growing buds; a senescing leaf in autumn, exporting amino acids back to perennial tissues before leaf-fall.
A sink is a tissue that imports photosynthate for use (in growth, respiration, defence, reproduction) or for storage. Common sinks include: apical and lateral meristems (growing shoot and root tips); developing fruits and seeds; young leaves before they become net exporters; storage organs (tubers, taproots, bulbs) accumulating reserves; and the roots generally, which depend on shoot-derived sugars.
The source-sink balance shifts over the plant's lifecycle and over the day. A leaf may be a sink while it is expanding (importing sugars for growth), become a source after it reaches a defined leaf-area threshold, remain a source for most of its life, and become a source again of amino acids during senescence. Phloem must accommodate all of this, which is why bidirectionality is structurally essential. Within a single sieve tube, only one direction of flow occurs at any moment — but adjacent sieve tubes may carry flow in opposite directions when source-sink relationships differ at the local scale.
The mass-flow hypothesis was proposed by Ernst Münch in 1930 (paraphrased throughout — no verbatim quotation). It explains bulk flow through phloem as a pressure-driven movement of solution from source to sink, with the pressure generated by osmotic loading and the bulk flow carrying solutes along passively.
In the leaf, photosynthetic cells (mesophyll) produce sucrose, which diffuses through plasmodesmata or apoplastically to reach the companion cells at the minor veins. Active loading then occurs at the companion cell plasma membrane.
The mechanism is proton-sucrose co-transport (symport). A plasma-membrane H⁺-ATPase uses ATP to pump H⁺ out of the companion cell into the apoplast, establishing a steep proton gradient (the apoplast becomes acidic; the cytoplasm remains near pH 7). A specific sucrose-H⁺ symporter (SUT/SUC family) then allows sucrose to enter the companion cell down the proton gradient: each sucrose molecule comes in with one or two H⁺ ions, harnessing the energy of the proton gradient to transport sucrose against its concentration gradient. Sucrose then passes through plasmodesmata into the sieve-tube element, where it accumulates to very high concentrations (~0.5 M, equivalent to a treacly syrup).
The high solute concentration in the sieve-tube element lowers its water potential. Water enters by osmosis from adjacent cells — particularly from the xylem in the same vascular bundle, where water potential is comparatively high. The volume of water entering the sieve tube cannot easily escape (the sieve plate offers some resistance; the lateral cell walls are non-extensible), so hydrostatic pressure rises to several MPa — the highest steady-state pressures recorded in plants.
The high pressure at the source drives bulk flow of solution through the sieve tubes toward sinks, where pressure is lower. Flow rates of 50–100 cm h⁻¹ are typical — far faster than diffusion could ever achieve over the distances involved (diffusion of sucrose over a metre would take years; bulk flow does it in an hour). The flow carries sucrose and all other phloem solutes passively along with the water.
At sink tissues, sucrose is unloaded — by reversal of the loading mechanism, by hydrolysis to glucose and fructose (which depletes free sucrose at the unloading site and maintains the gradient), or by symplastic flow through plasmodesmata directly into sink cells. Unloaded sucrose is then used in respiration (synoptic link to Section 3.5.2), incorporated into cellulose or starch, or accumulated in storage parenchyma. Active unloading depletes sieve-tube solute concentration.
With solute depleted at the sink, the sieve-tube element's water potential rises and water leaves by osmosis into surrounding cells, and ultimately back into the xylem. Hydrostatic pressure at the sink end falls; the pressure gradient from source to sink is maintained; bulk flow continues.
The whole cycle is therefore an osmotically generated hydrostatic pressure gradient driving bulk flow. The energy expenditure is at the source loading step (and the sink unloading step in some systems), not in the bulk flow itself — bulk flow through living sieve tubes is passive once the pressure gradient is established.
Multiple independent lines of evidence support the mass-flow hypothesis. Each tells a different part of the story.
If a complete ring of bark (containing the phloem in dicot stems — the cortex with phloem is external to the cambium) is removed from a stem, the phloem is interrupted while xylem remains intact. After several days, sugars and other organic solutes accumulate immediately above the ring, the bark above swells, and the tissue below the ring dies of carbohydrate starvation. The classical experiment, sometimes attributed to early plant physiologists including Stephen Hales (paraphrasing the framework) and to Marcello Malpighi (17th century, paraphrased), demonstrates that organic solutes move downward in the bark (phloem), not in the wood (xylem). Water transport is unaffected — the leaves above the ring continue to transpire normally — confirming that water moves in the xylem.
Aphids feed by inserting their stylets (mouthparts) directly into sieve-tube elements. The high hydrostatic pressure of the phloem sap forces it into the aphid's gut — visible as honeydew droplets exuded from the aphid's anus. If an aphid is anaesthetised (with CO₂) and the body excised, leaving the stylet in place, phloem sap continues to exude under pressure from the cut stylet. The sap can be collected in tiny capillaries and analysed. The technique demonstrates: (1) phloem sap is under high positive pressure (consistent with mass flow); (2) it is rich in sucrose (~10–25 % w/w, far more than any other solute); (3) loading rates at sources are quantifiable. This is one of the most elegant experimental techniques in plant physiology and is examinable on AQA papers.
Radioactive ¹⁴CO₂ is supplied to a single illuminated leaf. The leaf incorporates the ¹⁴C into sucrose by photosynthesis. Autoradiography of the rest of the plant at intervals shows the labelled sucrose moving away from the leaf in the phloem, reaching growing meristems, developing fruits and roots over hours. The pattern of movement directly demonstrates source-sink relationships and provides quantitative flow rates of 50–100 cm h⁻¹, consistent with bulk flow.
Treating the phloem with metabolic inhibitors (cyanide blocks ATP synthesis; respiratory uncouplers dissipate proton gradients) stops translocation, but only after the local supply of ATP and proton gradient is depleted. Specifically, blocking the source-loading H⁺-ATPase stops translocation; blocking the sink-unloading step also slows translocation. This demonstrates that translocation depends on active processes at source and sink but that the bulk flow itself is passive (not a direct ATP-driven pumping along the sieve tube).
The measured translocation rate of 50–100 cm h⁻¹ is roughly 10⁶ times faster than diffusion would allow over distances of even a few centimetres. Translocation must therefore be bulk flow, not diffusion — a quantitative argument that any A* candidate can make.
The mass-flow hypothesis is the dominant model and is the AQA-required framework. However, A-Level depth requires awareness that alternative or supplementary hypotheses have been proposed and that the precise mechanism of flow through the sieve plates remains an area of active research. Two alternatives merit mention.
Cytoplasmic streaming — observed in many plant cells as a circulation of cytoplasm — was historically proposed as a mechanism for translocation. Each sieve-tube element would actively circulate its contents, and substances would be passed from element to element via the sieve pores. The hypothesis is now largely discounted as the principal mechanism because cytoplasmic streaming is too slow to account for measured translocation rates, and because sieve-tube elements lose most of their cytoplasm during maturation. Streaming may contribute to local mixing within the element but not to long-distance flow.
Electro-osmosis — bulk water movement driven by an electrochemical gradient across the sieve plate — has been proposed as a supplement to pressure flow, particularly to explain how flow continues through sieve plates with apparently substantial frictional resistance. Modern measurements suggest electro-osmosis is at most a minor contributor.
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