Phloem Transport: Mass Flow Hypothesis Revision Notes | A Level AQA Biology

1. Why Phloem Transport Matters

A leaf in full sunlight can produce far more glucose than it can use on its own. Meanwhile, a growing root tip in the dark has no access to photosynthesis at all. The plant needs a distribution system — and that system is the phloem.

Phloem is the vascular tissue responsible for transporting organic substances — primarily sucrose and amino acids — around the plant. This movement is called translocation, and it is fundamentally different from water transport in xylem. Where xylem carries water and mineral ions upward in a largely passive process driven by transpiration, phloem carries dissolved organic solutes in any direction: up, down, or laterally, depending on where they are needed.

Understanding translocation is not only essential for your exam — it also underpins how plants respond to damage, how crops are treated with systemic pesticides, and why girdling a tree kills it. The mechanism behind all of this is the mass flow hypothesis.

2. The Structure of Phloem

Before you can understand how translocation works, you need a clear picture of the tissue doing the work. Phloem is composed of several cell types, but the two most important for translocation are sieve tube elements and companion cells.

• Sieve tube elements: elongated cells joined end-to-end to form continuous sieve tubes. Their end walls are perforated to form sieve plates, allowing solution to flow between cells. At maturity, sieve tube elements have no nucleus, few organelles, and a thin layer of cytoplasm lining the cell wall.
• Companion cells: closely associated with each sieve tube element via numerous plasmodesmata (cytoplasmic connections). They retain a nucleus and many mitochondria, providing the metabolic support that the sieve tube element lacks.
• Phloem parenchyma: provides additional support and storage functions within the phloem tissue.
• Phloem fibres: provide structural support but are not directly involved in translocation.

The relationship between sieve tube elements and companion cells is critical. Because sieve tube elements have no nucleus, they cannot regulate their own gene expression or produce their own proteins. Companion cells carry out these functions on their behalf, passing materials through the plasmodesmata. This metabolic dependence means that active loading of solutes into the phloem requires energy — a point that becomes central to evaluating the mass flow hypothesis.

3. The Mass Flow Hypothesis: The Core Mechanism

The mass flow hypothesis, proposed by Ernst Munch in 1930, explains translocation as a pressure-driven bulk flow of solution through sieve tubes. The key insight is that differences in solute concentration between source and sink generate a pressure gradient, and it is this gradient that moves the solution — not individual molecular diffusion.

Think of it like a hosepipe connected to a water tank. If you increase the pressure at one end, water flows toward the lower-pressure end. In phloem, the "pressure" is osmotic in origin: high solute concentration at the source draws water in by osmosis, raising hydrostatic pressure; low solute concentration at the sink means water leaves, lowering pressure. The resulting pressure gradient drives flow from source to sink.

Worked example: Tracing the Mass Flow Process from Source to Sink

Describe, in sequence, how sucrose moves from a mesophyll cell in a leaf to a root cell, according to the mass flow hypothesis.

1. Mesophyll cells produce sucrose by photosynthesis. Sucrose is actively loaded into companion cells and then into sieve tube elements, often via a proton-sucrose co-transporter. This requires ATP from companion cell mitochondria. — Active loading raises the solute concentration inside the sieve tube at the source end. This is an energy-requiring step — a key piece of evidence that mass flow is not entirely passive.
2. The high solute concentration in the sieve tube lowers the water potential inside relative to the surrounding xylem and mesophyll cells. Water moves in by osmosis down its water potential gradient. — Water entry increases the volume of solution inside the sieve tube, raising hydrostatic pressure at the source end.
3. High hydrostatic pressure at the source end, low hydrostatic pressure at the sink end. The pressure difference drives bulk flow of solution through the sieve plates toward the sink. — This is the 'mass flow' — the entire solution moves as a bulk fluid, not by individual molecular diffusion. All dissolved solutes move together in the same direction.
4. At the sink (e.g. a root cell or developing fruit), sucrose is removed from the sieve tube — either by active transport or by conversion to starch or other insoluble compounds. This lowers the solute concentration at the sink end. — Removing sucrose at the sink maintains the concentration gradient between source and sink, sustaining the pressure gradient and keeping the flow going.
5. As solute concentration falls at the sink end, water potential rises. Water moves out of the sieve tube by osmosis, often into the xylem to be recycled upward by transpiration. — Water loss at the sink reduces hydrostatic pressure there, reinforcing the pressure gradient that drives flow from source to sink.

Answer: Sucrose is actively loaded at the source, raising solute concentration and drawing water in by osmosis. The resulting high hydrostatic pressure drives bulk flow of solution through sieve tubes toward the sink, where sucrose is unloaded and water exits, maintaining the pressure gradient.

Notice that the direction of flow is always from high hydrostatic pressure (source) to low hydrostatic pressure (sink). This means phloem can transport solutes in any direction — upward to a developing fruit, downward to a root, or even laterally — depending on where the source and sink are located at any given time. This bidirectional capability is something xylem cannot achieve.

4. Evidence: Tracers and Ringing Experiments

A hypothesis is only as good as the evidence supporting it. Two classic experimental approaches have been used to investigate translocation in plants: the use of radioactive tracers and ringing (or girdling) experiments. Each provides a different type of evidence, and together they form the basis of exam questions on this topic.

4a. Radioactive Tracer Experiments

Radioactive tracers exploit the fact that a radioactive isotope of an element behaves chemically identically to its non-radioactive counterpart, but can be detected by the radiation it emits. In translocation studies, plants are supplied with carbon dioxide containing the radioactive isotope carbon-14 ($^{14}$C). The plant incorporates this into sucrose during photosynthesis.

By exposing sections of the plant to photographic film (autoradiography) or using a Geiger counter, researchers can track where the labelled sucrose travels. The results consistently show that the radioactive sucrose moves through the phloem, not the xylem, and that it moves from photosynthetically active leaves (sources) toward growing regions and roots (sinks). This directly supports the source-to-sink direction predicted by the mass flow hypothesis.

Aphid stylet experiments provide an elegant refinement. Aphids insert their mouthparts (stylets) directly into phloem sieve tubes to feed on the sugar-rich sap. If an aphid is anaesthetised and its body removed, the stylet remains in place and acts as a micro-sampling tube. Sap continues to exude from the cut stylet under pressure — direct evidence that phloem contents are under positive hydrostatic pressure, as the mass flow hypothesis predicts.

4b. Ringing (Girdling) Experiments

A ringing experiment involves removing a complete ring of bark from a woody stem. The bark contains the phloem; the wood beneath contains the xylem. Removing the ring of bark therefore removes the phloem at that point while leaving the xylem intact.

The results are striking. Above the ring, organic solutes accumulate — the bark swells as sucrose builds up because it can no longer pass downward through the phloem. Below the ring, the roots are starved of organic solutes and eventually die, even though water continues to move upward through the intact xylem. This demonstrates two things: that organic solutes travel in the phloem (not the xylem), and that downward transport of assimilates to roots depends on an intact phloem pathway.

Together, tracer and ringing experiments confirm that phloem is the tissue responsible for translocating organic substances, and that this transport is directional and pressure-driven — consistent with the mass flow hypothesis.

5. Evaluating the Mass Flow Hypothesis

The mass flow hypothesis is the best-supported model for translocation, but it is not without limitations. AQA examiners frequently ask you to evaluate it — meaning you must present evidence both for and against, then reach a reasoned conclusion. Here is how to structure that thinking.

When writing an evaluation answer, avoid listing points. The examiner wants you to weigh the evidence. A strong conclusion might read: 'The mass flow hypothesis is well supported by tracer and aphid stylet evidence, and correctly predicts the direction and pressure-driven nature of translocation. However, observations of differential solute velocities and bidirectional flow in adjacent sieve tubes suggest the mechanism may be more complex than a simple bulk flow model, and the hypothesis remains incompletely proven.'

6. Comparing Phloem and Xylem Transport

Examiners often ask you to compare translocation in phloem with water transport in xylem. The two systems share the same vascular tissue bundle but differ fundamentally in what they carry, how they carry it, and the energy requirements involved.

7. Exam Technique and Common Mistakes

Questions on phloem transport and the mass flow hypothesis appear regularly in AQA papers, often as extended-response questions worth 6 marks or as data-interpretation questions using tracer or ringing experiment results. The following guidance will help you convert your understanding into marks.

Phloem transport and the mass flow hypothesis sit at the intersection of structure, function, and experimental evidence — exactly the kind of multi-layered topic AQA rewards when you write with precision. Secure your understanding of the source-to-sink pressure gradient, know your experimental evidence and what each piece actually demonstrates, and practise structuring evaluation answers with a clear conclusion.