Transport in Plants — Translocation

This lesson covers the role of phloem in transporting dissolved sugars (and other organic substances) around a plant — a process called translocation — as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand the differences between xylem and phloem, what is meant by source and sink, and how translocation works.

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What Is Translocation?

Translocation is the movement of dissolved sugars (mainly sucrose) and amino acids through the phloem from where they are made or stored (sources) to where they are needed (sinks).

Unlike the transpiration stream in xylem, translocation can move substances in both directions — upward or downward — depending on where the source and sink are located.

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Phloem Structure

Phloem tissue is made up of two main cell types:

Cell type	Structure	Function
Sieve tube elements	Living cells with very little cytoplasm; end walls have pores forming sieve plates	Form the tube through which dissolved sugars flow
Companion cells	Small cells with a large nucleus, dense cytoplasm and many mitochondria	Provide energy (ATP) for active loading of sucrose into the sieve tubes

Key points:

• Sieve tube elements are living (unlike xylem vessels, which are dead).
• The sieve plates (perforated end walls) allow the sugar solution to flow from one cell to the next.
• Companion cells are essential because sieve tube elements lack most organelles; the companion cell carries out metabolic functions for both cells.

graph TD
    A["Companion cell"] -->|"Provides ATP via active transport"| B["Sucrose loaded into sieve tube"]
    B --> C["Sieve tube element"]
    C -->|"Flow through sieve plates"| D["Next sieve tube element"]
    D --> E["Sugar delivered to sink (e.g. root, fruit)"]

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Source and Sink

The direction of flow in the phloem depends on where sugars are produced (or released from storage) and where they are used (or stored).

Term	Definition	Examples
Source	An organ that produces or releases sugars	Leaves (photosynthesising), storage organs releasing stored starch
Sink	An organ that uses or stores sugars	Roots, growing tips (meristems), developing fruits, storage organs building up starch

A single organ can switch between being a source and a sink:

• In spring, a potato tuber acts as a source — it breaks down stored starch into sucrose to fuel new growth.
• In autumn, a potato tuber acts as a sink — it receives sucrose from the leaves and converts it back into starch for storage.

Exam Tip: Sources and sinks are not fixed. A storage organ (like a tuber or bulb) can be a source at one time of year and a sink at another. Always explain the direction of sugar movement with reference to where sucrose is being produced/released and where it is being used/stored.

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How Translocation Works

The mechanism of translocation is sometimes called the pressure-flow model (also known as the mass-flow hypothesis). At GCSE level, the key steps are:

1. Sucrose is actively loaded into the sieve tubes at the source (this requires energy from companion cells).
2. The high concentration of sucrose inside the sieve tube lowers the water potential, so water enters by osmosis from nearby xylem vessels.
3. This creates a high pressure at the source end of the phloem.
4. At the sink, sucrose is removed from the sieve tubes (used for respiration, growth, or converted to starch for storage).
5. Water follows the sucrose out of the sieve tube by osmosis, lowering the pressure at the sink end.
6. The pressure difference between source (high pressure) and sink (low pressure) drives the flow of the sugar solution along the phloem.

graph LR
    A["Source (e.g. leaf)"] -->|"Sucrose loaded actively"| B["High sucrose → water enters by osmosis → high pressure"]
    B -->|"Mass flow of sucrose solution"| C["Phloem sieve tubes"]
    C -->|"Sucrose removed at sink"| D["Sink (e.g. root)"]
    D -->|"Low sucrose → water leaves → low pressure"| E["Pressure gradient drives flow"]

Exam Tip: Translocation is an active process — it requires energy (ATP). This is different from transpiration in xylem, which is mainly a passive, physical process driven by evaporation.

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Comparing Xylem and Phloem

Feature	Xylem	Phloem
Substance transported	Water and mineral ions	Dissolved sugars (sucrose) and amino acids
Direction of flow	One-way: roots → leaves (upward)	Two-way: source → sink (up or down)
Cell status	Dead cells (no cytoplasm)	Living cells (sieve tube elements + companion cells)
End walls	Broken down completely	Perforated sieve plates
Wall thickening	Lignin (for support)	No lignin
Mechanism	Transpiration pull (passive)	Pressure-flow / mass flow (active — requires ATP)
Energy required?	No (passive process)	Yes (active loading of sucrose)

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Evidence for Translocation

Scientists have gathered evidence to support the pressure-flow model:

• Aphid experiments: Aphids insert their mouthparts (stylets) into individual sieve tubes. When the aphid is anaesthetised and removed, sugar solution continues to flow from the cut stylet, showing that the phloem contents are under pressure.
• Radioactive tracers: If a leaf is supplied with radioactive ¹⁴CO₂, radioactive sucrose later appears in the phloem, confirming that sugars produced by photosynthesis are transported through the phloem.
• Ringing experiments: Removing a ring of bark (which contains the phloem) from a tree trunk causes sugars to accumulate above the ring, showing that sugars travel downward through the phloem.

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Summary

• Translocation is the transport of dissolved sugars and amino acids through the phloem.
• Phloem consists of sieve tube elements (living, with sieve plates) and companion cells (provide energy).
• Sugars move from sources (where they are made/released) to sinks (where they are used/stored).
• The pressure-flow model explains translocation: active loading creates high pressure at the source, low pressure at the sink, driving mass flow.
• Translocation is an active process requiring ATP.
• Xylem and phloem differ in structure, the substances they carry, direction of flow and whether the process is active or passive.

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