Mass Flow

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•  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

active translocation

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•  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

phloem unloading