Phloem

• Describe the mass flow hypothesis for the mechanism of translocation in plants

  • sucrose in source is actively transported into phloem by companion cells
  • lowers water potential of sieve cell and water enters by osmosis
  • increases pressure causing mass movement towards sink (sucrose forced to sink)
  • sucrose used by sink for respiration
• How do enzymes maintain a concentration gradient from source to sink?

  They change the solutes at the sink. They hydrolyse them into something else. This ensures a lower concentration at the sink
• How are tracers used to investigate transport of organic substances in phloem?

  • tracing radioactively labelled carbon

  • isolate plant and only provide radioactively labelled carbon dioxide

  • absorbed in plant over time as used in photosynthesis

  • creates sugars containing radioactive carbon

  • thin slices from stem cut and placed on x-ray film

  • turns black if exposed to radioactive material

  • section of stem containing sugar turns black

  • shows where phloem are and shows that sugars are transported in phloem
• How does the ringing experiment prove that phloem transports sugars?

  • ring of bark and phloem removed off tree trunk

  • causes trunk to swell above removed section

  • analysis of liquid in swelling shows sugars are not transported when phloem is gone

  • proves phloem transports sugars