Proton Pumping: Companion cells use ATP to actively pump hydrogen ions () out of the cell into the surrounding leaf tissue (apoplast).
Co-transport: A concentration gradient of is established; as these ions diffuse back into the companion cell through a co-transporter protein, they carry sucrose molecules with them against their concentration gradient.
Sieve Tube Entry: Once inside the companion cell, sucrose moves into the sieve tube elements via plasmodesmata (symplastic pathway) or through the cell wall (apoplastic pathway).
This mechanism allows plants to concentrate sucrose in the phloem to levels much higher than in the surrounding mesophyll cells.
| Feature | Xylem (Transpiration) | Phloem (Translocation) |
|---|---|---|
| Driving Force | Negative pressure (tension/pull) | Positive pressure (hydrostatic/push) |
| Energy Requirement | Passive (solar-driven evaporation) | Active (ATP for loading/unloading) |
| Direction | Unidirectional (roots to leaves) | Bidirectional (source to sink) |
| Cell Viability | Dead cells (vessels/tracheids) | Living cells (sieve tube elements) |
While both involve mass flow, the phloem requires living cells because the maintenance of a pressure gradient depends on active metabolic processes like membrane transport.
Xylem flow is primarily regulated by environmental factors (humidity, light), whereas phloem flow is regulated by the plant's metabolic demands at various sinks.
Identify the Source/Sink: Always check the context of the question; a 'source' is not always a leaf. For example, in early spring, a storage root is the source and the new buds are the sinks.
Explain the Water Potential Link: When describing mass flow, you must explicitly state that loading sucrose lowers water potential, which causes osmosis, which increases hydrostatic pressure. Missing any step in this chain often loses marks.
Living vs. Dead: Remember that metabolic inhibitors (like cyanide) will stop phloem transport but will not immediately stop xylem transport, as phloem requires ATP for loading.
Directionality: Phloem sap can move up and down the plant simultaneously in different sieve tubes, but within a single sieve tube, the flow is always from high to low pressure.
Diffusion vs. Mass Flow: A common error is stating that sucrose moves by diffusion. While loading involves diffusion/active transport, the movement through the tube itself is mass flow, which is much faster than diffusion.
Gravity: Students often assume phloem only flows 'down' to the roots. In reality, it flows to wherever the demand (sink) is, including upward to developing flowers or fruits.
Water Source: Do not forget the role of the xylem; without the adjacent xylem providing water for osmosis, the hydrostatic pressure in the phloem could not increase.
