Mass Flow

• Mass flow


  The transport of solutes and water in the phloem driven by hydrostatic pressure gradients
• 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
• Active translocation via mass flow

  1. Active transport of solutes (such as sucrose) into the phloem by companion cells makes the sap solution hypertonic

  2. This causes water to be drawn from the xylem via osmosis

  3. The build up of water in the phloem causes the hydrostatic pressure to increase

  4. The increase in hydrostatic pressure forces the phloem sap to move towards areas of lower pressure
• Phloem unloading at the sink

  1. Solutes within the phloem are unloaded by companion cells and transported into sinks (roots, fruits, seeds, etc.)

  2. This causes the sap solution at the sink to become increasingly hypotonic

  3. Consequently, water is drawn out of the phloem and back into the xylem by osmosis

  4. This ensures that the hydrostatic pressure at the sink is always lower than the hydrostatic pressure at the source

  5. Organic molecules transported into the sink are either metabolised or stored within the tonoplast of vacuoles