Understanding:
• High concentrations of solutes in the phloem at the source lead to water uptake by osmosis
• Incompressibility of water allows transport along hydrostatic pressure gradients
At the Source
- The active transport of solutes (such as sucrose) into the phloem by companion cells makes the sap solution hypertonic
- This causes water to be drawn from the xylem via osmosis (water moves towards higher solute concentrations)
- Due to the incompressibility of water, this build up of water in the phloem causes the hydrostatic pressure to increase
- This increase in hydrostatic pressure forces the phloem sap to move towards areas of lower pressure (mass flow)
- Hence, the phloem transports solutes away from the source (and consequently towards the sink)
Active Translocation via Mass Flow
Understanding:
• Raised hydrostatic pressure causes the contents of the phloem to flow towards sinks
At the Sink
- The solutes within the phloem are unloaded by companion cells and transported into sinks (roots, fruits, seeds, etc.)
- This causes the sap solution at the sink to become increasingly hypotonic (lower solute concentration)
- Consequently, water is drawn out of the phloem and back into the xylem by osmosis
- This ensures that the hydrostatic pressure at the sink is always lower than the hydrostatic pressure at the source
- Hence, phloem sap will always move from the source towards the sink
- When organic molecules are transported into the sink, they are either metabolised or stored within the tonoplast of vacuoles
Mechanisms of Phloem Unloading
