Step 1: Phloem Loading: Sucrose is actively transported from source cells (like leaf mesophyll) into the companion cells and then into the sieve tube elements. This process often requires energy in the form of ATP to move solutes against their concentration gradient.
Step 2: Water Uptake: The resulting high solute concentration in the sieve tube lowers the water potential (), drawing water in from the xylem. This creates the high hydrostatic pressure necessary for flow.
Step 3: Bulk Flow: The sap moves down the pressure gradient from the source region toward the sink region. The sieve plates act as structural supports to prevent the tubes from collapsing under this high internal pressure.
Step 4: Phloem Unloading: At the sink (roots, fruits, or growing tips), sucrose is removed from the phloem. As solute concentration drops, water potential increases, and water exits the phloem, returning to the xylem or entering surrounding tissues, which maintains the low-pressure environment.
| Feature | Xylem | Phloem |
|---|---|---|
| Substance Transported | Water and dissolved minerals | Organic solutes (sucrose, amino acids) |
| Direction of Flow | Unidirectional (Roots to Leaves) | Multidirectional (Source to Sink) |
| Cell Viability | Dead at functional maturity | Living at functional maturity |
| Driving Force | Transpiration pull (negative pressure) | Pressure-flow (positive pressure) |
| Energy Requirement | Passive (Solar driven) | Active (ATP required for loading) |
Identify Source and Sink: Always remember that 'source' and 'sink' are functional definitions, not anatomical ones. A storage root can be a sink in the summer (storing sugar) but a source in the spring (providing sugar for new growth).
Energy Requirements: Be prepared to explain why phloem transport is considered an 'active' process. Even though the flow itself is physical, the establishment of the pressure gradient requires ATP for sugar loading.
Directionality: Unlike the xylem, which only moves upward, phloem sap can move up or down depending on where the sink is located relative to the source. Always check the relative positions of leaves and growing organs in a problem.
Cell Structure Logic: If asked why sieve tubes lack nuclei, explain that it reduces internal resistance to mass flow, allowing the sap to move more freely through the conduit.
Gravity Misconception: Students often mistakenly believe that phloem sap moves downward simply due to gravity. In reality, the pressure gradient is strong enough to move sap upward to developing fruits or flowers at the top of a tree.
Living vs. Dead: A common error is assuming all vascular tissue is dead. While xylem vessels are dead, sieve tube elements must be living to maintain the semi-permeable membrane required for osmosis and pressure generation.
Diffusion vs. Mass Flow: Do not confuse translocation with diffusion. Diffusion is the movement of individual molecules; mass flow is the movement of the entire fluid volume driven by pressure.
