Translocation: Phloem
While the xylem transports water and minerals upward from the roots, the phloem transports dissolved organic substances — primarily sucrose and amino acids — around the plant. This process is called translocation. This lesson covers the structure of phloem, the mechanism of translocation, and the key differences between xylem and phloem, as required by Edexcel GCSE Biology (1BI0) Topic 6.
What is Translocation?
Translocation is the transport of dissolved sugars (mainly sucrose) and other organic molecules such as amino acids through the phloem tissue of a plant.
Why Sucrose?
Plants convert glucose (made by photosynthesis) into sucrose for transport because:
- Sucrose is more chemically stable than glucose — it is less reactive and less likely to be broken down during transport
- Sucrose is a disaccharide (made of glucose + fructose) — it is soluble in water, making it easy to transport as a dissolved solution
- At the destination, sucrose is converted back to glucose (or other molecules) for use by cells
Exam Tip: In exams, be specific — phloem transports sucrose (not "sugar" or "glucose"). The mark scheme usually requires the word "sucrose".
Sources and Sinks
Translocation moves substances from sources to sinks:
Sources
A source is any part of the plant that produces or releases sugars:
- Leaves — where photosynthesis produces glucose, which is converted to sucrose
- Storage organs releasing stored food — e.g., a tuber (potato) in spring as it uses its starch reserves to fuel new growth
Sinks
A sink is any part of the plant that uses or stores sugars:
- Roots — need glucose for respiration and growth; may convert to starch for storage
- Growing tips (meristems) — rapidly dividing cells need glucose for respiration and cellulose production
- Developing fruits and seeds — need energy and building materials
- Storage organs accumulating food — e.g., a tuber growing larger in summer
| Source | Sink |
|---|
| Photosynthesising leaves | Roots |
| Storage organs releasing food | Growing tips |
| Developing fruits and seeds |
| Storage organs accumulating food |
Exam Tip: Translocation moves substances in both directions (up and down), unlike xylem which only moves water upward. The direction depends on where the sources and sinks are.
Structure of Phloem
Phloem is a living tissue made up of two main cell types:
1. Sieve Tube Elements
- Living cells (but with reduced cell contents — no nucleus, very few organelles)
- Arranged end-to-end to form long tubes
- The end walls between adjacent sieve tube elements have sieve plates — these are perforated walls with small holes (pores) that allow the sap to flow from one cell to the next
- The reduced cell contents (no nucleus, fewer organelles) leaves more space for the sap to flow through
2. Companion Cells
- Found alongside each sieve tube element
- Living cells with a nucleus, many mitochondria and dense cytoplasm
- Connected to the sieve tube elements by plasmodesmata (tiny channels through the cell wall)
- Their main role is to provide energy (from aerobic respiration in their many mitochondria) to drive the active loading of sucrose into the sieve tubes
- They effectively "manage" the sieve tube element because it has no nucleus of its own
| Feature | Sieve tube elements | Companion cells |
|---|
| Nucleus | No | Yes |
| Mitochondria | Very few | Many |
| End walls | Sieve plates (perforated) | Normal cell walls |
| Main role | Transport of sucrose | Provide energy for active loading |
| Living/dead | Living (but reduced contents) | Living (fully functional) |
Exam Tip: Do not say phloem cells are "dead" — that is xylem. Sieve tube elements are alive but have no nucleus. Companion cells are fully alive with a nucleus and many mitochondria. This distinction is important.
The Mechanism of Translocation
Translocation is an active process — it requires energy (ATP). This is a critical difference from water transport in the xylem, which is passive (driven by transpiration).
How Translocation Works (Simplified for GCSE)
- Sucrose is actively loaded into the sieve tube elements at the source (e.g., a leaf). This is done by companion cells, which use energy from respiration to pump sucrose into the phloem.
- The loading of sucrose lowers the water potential inside the sieve tube at the source end.
- Water enters the phloem by osmosis from the surrounding xylem, increasing the hydrostatic pressure at the source end.
- At the sink (e.g., a root), sucrose is removed from the phloem and used by cells. This raises the water potential in the sieve tube at the sink end.
- Water leaves the phloem by osmosis at the sink end, lowering the pressure.
- The pressure difference between the source (high pressure) and the sink (low pressure) causes the phloem sap to flow from source to sink.
This is known as the pressure flow hypothesis (or mass flow hypothesis).
Exam Tip: The key point is that translocation requires energy (ATP) because sucrose is actively loaded into the phloem. If you are asked "Is translocation active or passive?", the answer is active. The energy comes from the companion cells.
Evidence for Translocation
Scientists have gathered evidence to support our understanding of translocation:
1. Ringing Experiments
- A ring of bark (which contains the phloem) is removed from a tree trunk, leaving the xylem intact
- Over time, the area above the ring swells — sugars accumulate because they cannot pass the gap in the phloem
- The area below the ring eventually dies — it is starved of sugars
- This shows that phloem transports sugars downward from the leaves
2. Radioactive Tracers
- A leaf is supplied with radioactive ¹⁴CO₂ (carbon-14 labelled carbon dioxide)
- The plant photosynthesises and incorporates the radioactive carbon into glucose and then sucrose
- Autoradiography (exposing photographic film to the plant) shows that the radioactive sucrose moves through the phloem
- This provides direct evidence that phloem is the tissue responsible for sugar transport
3. Aphid Stylets
- Aphids (plant lice) feed by inserting their needle-like mouthparts (stylets) into phloem sieve tubes
- Scientists anaesthetise the aphid and remove its body, leaving the stylet in the phloem
- Phloem sap continues to flow out of the stylet under pressure
- The sap can be collected and analysed — it contains mainly sucrose and amino acids
- This confirms both the contents of phloem sap and that it flows under pressure
Xylem vs Phloem: Comparison Table
This table is essential for the exam — learn it thoroughly: