7.8 - Phloem, transport of organic substances

• What’s translocation ?

  Movement of organic molecules and mineral ions around a plant
• Name of areas where photosynthesis occurs and sugars are produced ?

  Sources
• Name of areas that need substances found in sources ?

  Sinks
• What 2 cells are phloems made up of ?

  Sieve tube elements and companion cells
• What’s the mass flow hypothesis ?

  Theory which states that mass flow of solutes takes place in the phloem
• What’s active loading ?

  Where sucrose is actively transported into sieve tube elements
• Describe what occurs in active loading ?

  • Sucrose is produced from glucose in photosynthesising palisade cells
  • it moves by facilitate diffusion down a conc gradient into companion cells
  • H+ ions are actively transported into cell wall spaces using ATP
  • sucrose is co-transported with H+ ions into the sieve tube elements via co-transport proteins
• Explain how mass flow of sucrose occurs ?

  • Sucrose is actively transported into sieve tube elements (active loading)
  • this reduces the water potential in the sieve tube elements
  • xylem has higher wp so water moves into sieve tube elements via osmosis, down a wp grad
  • this increases the hydrostatic pressure within them
  • solutes move from higher pressure at the source to low pressure at respiring cells (sinks)
  • they actively transport sucrose in from the sieve tubes, increasing wp at the sink
  • water moves out of the phloem via osmosis to maintain hydrostatic pressure gradient