Transport of Organic Molecules in the Phloem

• What two key cells does the phloem contain?

  1. Sieve tube elements
  2. Companion cells
• Describe sieve tube elements.

  • Perforated end walls
  • Living
  • No nucleus
  • Few organelles
• What is the role of companion cells?

  Provide the ATP required for active transport of organic substances.
• What is the mass flow hypothesis?

  Mass flow from the source of production (leaves) to the sink, where organic substances are used up in respiring tissues.
• How does the mass flow hypothesis work?

  1. Sucrose lowers the water potential of source cell.
  2. Water enters by osmosis.
  3. Respiring (sink) cell is using up sucrose so has a higher water potential.
  4. Water leaves the sink cell by osmosis.
  5. The source cell has an increased hydrostatic pressure while the sink cell has a reduced hydrostatic pressure.
  6. The solution is forced towards the sink cell via the phloem.
• What is the first stage of translocation?

  1. Photosynthesis in the chloroplasts of the leaves creates organic substances such as sucrose.
  2. Sucrose is actively transported into the sieve tube elements using the companion cells.
• What is the second stage of transpiration?

  1. The increase of sucrose in the sieve tube element lowers the water potential.
  2. Water enters from surrounding xylem vessels via osmosis.
  3. The volume of water is increased, increasing the hydrostatic pressure. This causes liquid to be forced towards the sink.
• What is the third stage of translocation?

  1. Sucrose is used in respiration at the sink, or stored as insoluble starch.
  2. More sucrose is actively transported into the sink cell, decreasing the water potential.
  3. Water moves from the sieve tube elements into the sink cell by osmosis.
  4. The removal of water decreases the volume in the STE, decreasing the hydrostatic pressure.
• What does translocation depend on?

  The difference in hydrostatic pressure between the source and sink end of sieve tube elements.
• The process of translocation.

  A) Decreasing pressure gradient

  B) Companion cells

  
• What are two methods of investigating translocation?

  1. Tracers
  2. Ringing
• Explain the use of tracers.

  • Radioactively labelled carbon
  • Used in photosynthesis to produce radioactively labelled sugars
  • Thin stem slices are placed on x-ray films, which turn black when exposed to radiation
  • This highlights the phloem and proves sugars are transported there.
• Explain the process of ringing.

  • A ring of bark and phloem is removed.
  • The trunk swells above the removed section.
  • The liquid from the swelling contains sugars.
  • Proves that when there is no phloem, sugars cannot be transported.