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This lesson covers phloem transport and translocation as required by the Edexcel A-Level Biology specification (9BI0). You need to understand the structure of phloem tissue, the mechanism of translocation (the mass flow hypothesis), the concepts of sources and sinks, and the evidence supporting the mass flow hypothesis.
Phloem is a living transport tissue that carries dissolved organic solutes — primarily sucrose and amino acids — from where they are produced (sources) to where they are needed (sinks).
Sieve tube elements are the main conducting cells in phloem. They are arranged end to end to form sieve tubes.
| Feature | Description | Functional significance |
|---|---|---|
| Living cells | Contain a thin layer of cytoplasm and a cell membrane but are alive throughout | Required for active transport of sucrose at the source |
| No nucleus | The nucleus degenerates during development | Reduces obstruction to flow |
| Very few organelles | Most organelles (including ribosomes, tonoplast, and most of the ER) degenerate | Maximises the open channel for sap flow |
| Sieve plates | Perforated end walls between adjacent sieve tube elements; contain many sieve pores | Allow the mass flow of phloem sap from one element to the next |
| Plasmodesmata | Cytoplasmic connections between sieve tube elements and companion cells | Allow transfer of substances and signals between the two cell types |
Companion cells are specialised parenchyma cells closely associated with sieve tube elements. Each companion cell is connected to its adjacent sieve tube element by numerous plasmodesmata.
| Feature | Description | Functional significance |
|---|---|---|
| Dense cytoplasm | Packed with organelles, especially mitochondria | High metabolic rate; provides ATP for active loading of sucrose into sieve tubes |
| Large nucleus | Retained (unlike sieve tube elements) | Controls the metabolic activity of both the companion cell and the adjacent sieve tube element |
| Many mitochondria | Very high density of mitochondria | Generate the ATP needed for active transport (loading sucrose into the sieve tube) |
| Plasmodesmata connections to sieve tube | Multiple connections through the shared cell wall | Allow direct transfer of sucrose and other materials into the sieve tube element |
Key Definition: Translocation — the transport of dissolved organic solutes (mainly sucrose and amino acids) through the phloem from sources (e.g. photosynthesising leaves) to sinks (e.g. roots, growing tips, storage organs).
| Term | Definition | Examples |
|---|---|---|
| Source | A part of the plant that produces or releases organic solutes (net exporter) | Photosynthesising leaves, storage organs that are releasing stored reserves (e.g. a potato tuber in spring) |
| Sink | A part of the plant that consumes or stores organic solutes (net importer) | Roots, growing meristems (shoot and root tips), developing fruits and seeds, storage organs that are accumulating reserves |
Important: A single organ can switch between being a source and a sink depending on the time of year and the plant's stage of development. For example, a potato tuber is a sink when it is growing and storing starch, but becomes a source when it is sprouting and mobilising stored starch as sucrose.
The mass flow hypothesis, proposed by Ernst Münch in 1930, is the most widely accepted explanation for translocation in phloem.
Active loading at the source: Sucrose is actively loaded into the sieve tube elements at the source (e.g. a photosynthesising leaf) by companion cells. This involves:
Lowered water potential at the source: The high concentration of sucrose inside the sieve tube lowers the water potential (makes it more negative). Water enters the sieve tube from the adjacent xylem by osmosis.
Hydrostatic pressure at the source: The influx of water creates a high hydrostatic pressure (turgor pressure) in the sieve tube at the source.
Unloading at the sink: At the sink (e.g. a root cell), sucrose is actively or passively removed from the sieve tube. Sucrose may be:
Raised water potential at the sink: Removal of sucrose raises the water potential in the sieve tube at the sink. Water leaves the sieve tube by osmosis, reducing the hydrostatic pressure.
Pressure gradient: The difference in hydrostatic pressure between the source (high) and the sink (low) drives the mass flow of phloem sap from source to sink.
| Location | Sucrose | Water potential | Hydrostatic pressure | Water movement |
|---|---|---|---|---|
| Source (e.g. leaf) | High (actively loaded in) | Low (more negative) | High | Water enters sieve tube from xylem |
| Sink (e.g. root) | Low (removed for use) | Higher (less negative) | Low | Water leaves sieve tube |
| Net result | — | — | Pressure gradient drives mass flow from source to sink | — |
The following diagram summarises the mass flow of sucrose from source to sink through the phloem:
graph LR
A["Source<br/>(leaf)"] -->|"Sucrose loaded<br/>(active transport)"| B["Sieve Tube<br/>(high pressure)"]
B -->|"Mass flow<br/>(pressure gradient)"| C["Sieve Tube<br/>(low pressure)"]
C -->|"Sucrose unloaded"| D["Sink<br/>(root/fruit)"]
Exam Tip: When describing translocation, always mention: (1) active loading of sucrose at the source using companion cells and co-transport, (2) osmosis of water into the sieve tube lowering water potential, (3) high hydrostatic pressure driving mass flow towards the sink, and (4) sucrose removal at the sink.
| Evidence | Explanation |
|---|---|
| Aphid stylet experiments | Aphids insert their stylets into individual sieve tubes to feed. If the aphid is anaesthetised and removed, phloem sap exudes from the cut stylet under pressure, confirming that sieve tubes are under positive pressure. Analysis of the sap shows it is rich in sucrose. |
| Radioactive tracer studies | If a source leaf is supplied with ¹⁴CO₂, radioactively labelled sucrose (¹⁴C-sucrose) can be detected in the phloem sap, moving towards sinks. Autoradiographs confirm that labelled sucrose is confined to the phloem. |
| Ringing experiments | Removing a ring of bark (which contains phloem but not xylem) from a tree trunk causes sugars to accumulate above the ring, confirming that phloem transports organic solutes downward. |
| Callose formation | Sieve plates can be blocked by callose (a polysaccharide) when the phloem is damaged. This suggests that sap normally flows through the sieve pores under pressure — blocking them stops the flow. |
| Companion cells are metabolically active | Companion cells have many mitochondria and are closely connected to sieve tubes. Treating companion cells with metabolic inhibitors (e.g. cyanide) stops translocation, confirming that active processes are required. |
The mass flow hypothesis is not universally accepted without reservations:
| Criticism | Counter-argument |
|---|---|
| Sieve plates would impede mass flow | Some researchers argue that the resistance caused by sieve plates is too great for mass flow alone. However, mature sieve pores are relatively large, and callose deposits seen in early microscopy may have been artefacts of damage during preparation. |
| Bidirectional transport | Some studies show that different solutes can move in opposite directions within the same vascular bundle. However, this may occur in different sieve tubes within the same bundle. |
| Speed of flow in some species | In some plants, the observed rate of translocation is faster than the mass flow hypothesis would predict for the measured pressure gradients. |
| Requires metabolic energy | The mass flow hypothesis predicts that flow itself is passive (driven by pressure), but loading and unloading require ATP. This is consistent with the observation that metabolic inhibitors stop translocation. |
Exam Tip: The specification requires you to understand the mass flow hypothesis and the evidence that supports it. You should also be able to describe its limitations. In an essay, present the evidence in a balanced way and conclude by stating that the mass flow hypothesis remains the best-supported model.
| Feature | Xylem | Phloem |
|---|---|---|
| Substance transported | Water and mineral ions | Sucrose and amino acids |
| Direction | Upward only (roots → leaves) | Bidirectional (source → sink) |
| Cell state | Dead at maturity | Living (sieve tube elements + companion cells) |
| Cell wall | Lignified | Cellulose (not lignified) |
| End walls | Absent (broken down) | Present as sieve plates (perforated) |
| Mechanism | Passive — cohesion-tension, driven by transpiration | Active loading; passive mass flow driven by pressure gradient |
| Energy requirement | No metabolic energy (passive) | Requires ATP for loading at the source |
| Flow rate | Fast (~several metres per hour) | Slower (~1 metre per hour) |
Sucrose can reach the companion cells from the mesophyll by two routes:
The apoplast route with active co-transport is the main mechanism in many plant species and allows loading against the concentration gradient (phloem sap can contain 10–30% sucrose, much higher than in mesophyll cells).
| Feature | Detail |
|---|---|
| Phloem structure | Living sieve tube elements (sieve plates, few organelles) + companion cells (many mitochondria, nucleus) |
| Translocation | Transport of sucrose and amino acids from source to sink |
| Mass flow hypothesis | Active loading at source → low ψ → water enters → high pressure → mass flow to sink |
| Loading mechanism | H⁺-ATPase + sucrose/H⁺ co-transport (active) |
| Evidence | Aphid stylets, radioactive tracers, ringing experiments, metabolic inhibitors |
| Source vs sink | Source = net exporter (e.g. leaf); sink = net importer (e.g. root, fruit) |
Translocation and the mass flow hypothesis are frequently examined — make sure you can describe the mechanism in full, explain the role of companion cells, and evaluate the evidence.
This material sits in Edexcel 9BI0 Topic 7 (Run for your life — Exchange and Transport) at the plant-transport pole, paired with xylem (lesson 8) and root water uptake (lesson 10). Candidates must (i) describe sieve-tube and companion-cell structure (sieve-tube elements are living but lack nuclei, ribosomes and most organelles; sieve plates with perforated pores; companion cells densely packed with mitochondria and rough ER, linked to the sieve tube via plasmodesmata), (ii) define and apply the source / sink concept as a functional rather than fixed designation, (iii) explain the mass-flow (pressure-flow) hypothesis end-to-end (active loading at source → low Psi → osmotic water entry from xylem → high turgor pressure → bulk flow along pressure gradient → unloading at sink → water re-enters xylem), (iv) describe the apoplast loading route with H+/sucrose co-transport (proton-pumping ATPase + secondary active symport) and contrast with the symplast route via plasmodesmata, and (v) evaluate the supporting / problematic evidence (aphid-stylet sap, radioactive 14C-CO2 pulse-chase, girdling, metabolic inhibitors). Synoptic with lesson 8 (xylem and phloem run side-by-side in vascular bundles; xylem flow is sun-powered and passive, phloem flow is ATP-powered at loading), lesson 10 (root mineral uptake and the apoplast/symplast architecture re-used at the endodermis), Topic 1 (sucrose as the disaccharide of translocation; α-1,2-glycosidic bond between glucose and fructose; non-reducing sugar), Topic 5 (light-independent reactions produce triose phosphate → sucrose for export from photosynthesising leaves; seasonal mobilisation of starch reserves at the source), and Topic 5 (active transport energetics — H+-ATPase generates the proton gradient that powers symport). Refer to the official Pearson Edexcel 9BI0 specification for exact wording.
Question (8 marks):
A student investigates phloem transport in Phaseolus vulgaris (runner bean) using the following procedure. A young growing leaf and a mature fully-expanded leaf are each fed 14C-labelled CO2 for 30 minutes in light, and autoradiography of the plant is performed 4 hours later.
The young leaf retains nearly all of the 14C signal within itself; very little label is exported. The mature leaf shows strong 14C signal in the roots and in developing fruits, but weak signal in itself.
(a) Explain these observations using the source / sink concept. (3)
(b) Outline the mechanism by which 14C-labelled sucrose moves from the mature leaf to the developing fruits. (4)
(c) State one limitation of using 14C autoradiography as evidence for the mass-flow hypothesis. (1)
Solution with mark scheme:
(a) M1 (AO1) — A source is a tissue that is a net exporter of sucrose (typically a photosynthesising mature leaf); a sink is a net importer where sucrose is used in respiration or stored as starch.
M1 (AO2) — The mature leaf is a source: it photosynthesises in excess of its own metabolic needs and exports sucrose (and therefore 14C label) via the phloem to roots and fruits.
A1 (AO2) — The young leaf is itself a sink (still expanding, importing carbon for cell-wall synthesis and respiration); it retains 14C from its own photosynthesis and imports more from elsewhere rather than exporting. Source / sink is functional, not fixed by tissue identity.
(b) M1 (AO1) — At the source, an H+-ATPase in the companion-cell membrane pumps H+ out using ATP, setting up an electrochemical proton gradient.
M1 (AO1) — H+ re-enters via a sucrose / H+ symporter, dragging sucrose into the sieve tube against its concentration gradient (secondary active transport).
M1 (AO2) — Loading lowers the sieve-tube Psi; water enters by osmosis from the adjacent xylem, raising turgor pressure at the source.
A1 (AO2) — At the sink (fruit), sucrose is unloaded (respired / stored as starch), Psi rises, water leaves to the xylem; the pressure gradient drives mass flow, carrying 14C-sucrose to the fruit.
(c) M1 (AO3) — The label shows where sucrose ends up, but does not directly prove that flow is by bulk pressure-driven mass flow rather than e.g. cytoplasmic streaming or active sieve-tube pumping. (Alternative acceptable answer: cannot resolve flow direction within a single sieve tube on autoradiography timescales.)
Total: 8 marks (M5 A2 + M1 AO3).
Question (6 marks): Explain how the structure of phloem tissue is adapted to the mass flow of organic solutes from sources to sinks.
Mark scheme decomposition by AO:
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