Sugars are transported in phloem through a process driven by pressure differences, moving from areas where they are produced (sources) to areas where they are needed or stored (sinks).
The primary sugar transported is sucrose. This transport occurs within specialized cells called sieve elements, which are part of the phloem tissue in plants.
The Process of Phloem Transport: A Pressure-Flow Mechanism
Phloem transport is best explained by the pressure-flow hypothesis. This mechanism relies on differences in turgor pressure between source and sink regions.
Here’s a breakdown of the key steps:
- Loading at the Source: Sugars (primarily sucrose) produced during photosynthesis in leaves (the source) are actively or passively loaded into the sieve elements of the phloem. This increases the concentration of solutes within these cells.
- Water Enters Sieve Elements: As the solute concentration rises in the sieve elements at the source, water follows the sugar molecules into the sieve elements through osmosis. This occurs because water passively diffuses from areas of lower solute concentration (like the nearby xylem) to areas of higher solute concentration (the sieve elements).
- Building Turgor Pressure: The influx of water causes the volume of the sieve element contents to increase, creating high turgor pressure at the source end of the phloem tube.
- Movement Towards the Sink: This high pressure at the source pushes the sugary sap (phloem sap) through the sieve tubes towards regions of lower pressure – the sinks. This bulk flow movement is similar to how water flows through a garden hose when the faucet is turned on.
- Unloading at the Sink: At the sink tissues (e.g., roots, fruits, growing points), sugars are unloaded from the sieve elements. This unloading can be active or passive depending on the plant and tissue type.
- Reducing Turgor Pressure: As sugars are removed from the sieve elements at the sink, the solute concentration decreases. Consequently, water moves out of the sieve elements by osmosis, typically returning to the xylem. This reduces the turgor pressure at the sink end, maintaining the pressure gradient that drives the flow from source to sink.
Key Components Involved
- Sieve Elements: The conducting cells of the phloem.
- Companion Cells: Associated with sieve elements, providing metabolic support and aiding in sugar loading/unloading.
- Source: Tissues that produce or store sugars (e.g., mature leaves).
- Sink: Tissues that utilize or store sugars (e.g., roots, fruits, flowers, growing shoots).
- Sucrose: The primary sugar transported.
- Water: Moves osmotically, creating the pressure gradient.
Summarizing the Flow
| Component | Location | Role in Transport |
|---|---|---|
| Sugars | Phloem (Sap) | Solute transported |
| Water | Phloem (Sap) | Follows sugar by osmosis, creates pressure |
| Source | e.g., Leaves | Loads sugars, high pressure area |
| Sink | e.g., Roots | Unloads sugars, low pressure area |
| Pressure | Phloem (Turgor) | Drives bulk flow from source to sink |
This pressure-driven bulk flow allows plants to efficiently distribute energy and building blocks (in the form of sugars) to all parts of the organism, supporting growth, storage, and reproduction.
