mass transport (plants)

• xylem

  tissue that transports water and mineral ions from roots → leaves

  • has a tube-like structure
• cohesion-tension theory

  • water is lost by evaporation/transpiration
  • lowering the water potential
  • causing tension to pull water up the stem
  • water molecules are cohesive and stick together forming hydrogen bonds
  • form a continuous column and travel from the roots to the leaves
• phloem

  tissue that transports organic substances

  • has a sieve tube elements
  • companion cells (provides ATP for active transport)
• brief summary of mass flow hypothesis:

  movement of solutes / assimilated from source cell to the sink cell
• source cell movement:

  solutes lower the water potential

  • water enters via osmosis from companion cell

  increases the hydrostatic pressure 

  • causing solutes to be pushed along sieve tubes towards the sink cell
• sink cell movement:

  solutes are used up (by respiring cells)

  • increasing the water potential

  hydrostatic pressure decreases
• use of tracers

  tracks radioactive labelled carbon

  • plants are isolated with CO2 (photosynthesis)
  • sugars are created - consists of radioactive carbon

  placed on X-ray film 

  • shows sugars being transported in black
• ringing experiment

  ring of bark and phloem is removed

  • part above would start swelling due to sugars
  • shows evidence that sugars can’t be transported without a phloem
• Use your understanding of the mass flow hypothesis to explain how pressure is generated inside this phloem tube.

  • Sucrose actively transported (into phloem);
  • Lowering water potential
  • Water moves (into phloem) by osmosis (from xylem)
• Describe the mass flow hypothesis for the mechanism of translocation in plants.

  • In source sugars actively transported into phloem;
  • By companion cells;
  • Lowers water potential of sieve cell & water enters by osmosis;
  • Increase in pressure causes mass movement towards sink
  • Sugars converted in root for respiration for storage