Phloem & Translocation

• Complete the following about translocation:

  • Occurs in phloem
  • explained by mass flow hypothesis
  • transport of organic substances through plant
• Complete the following about the sieve tube elements.

  • Living cells
  • contain no nucleus
  • few organelles
  • this makes cell hollow
  • allowing reduced resistance to flow of sugars
• Complete the following about companion cells.

  • Provide ATP required for active transport of organic substances
  • contains many mitochondria
• Describe the mass flow hypothesis.

  • Organic substances, sucrose, move in solution from leaves (after photosynthesis) to respiring cells
  • source -> sink direction
• How is the pressure for translocation generated?

  • Photosynthesising cells produce glucose which diffuses into companion cell
  • companion cell actively transports glucose into phloem
  • this lowers water potential of phloem so water moves in from xylem via osmosis
  • hydrostatic pressure gradient generated
• What happens to sucrose after translocation?

  • Used in respiration at the sink
  • stored as insoluble starch
• How do organic substances move from source to sink?

  • Sucrose lowers the water potential of the source cell, which causes water to enter by osmosis.
  • This increases the hydrostatic pressure in the source cell
  • The respiring cell has a more positive water potential, so water leaves the sink cell by osmosis.
  • This decreases the hydrostatic pressure of the sink cell
  • This results in the source cell having a higher hydrostatic pressure than the sink cell, so the solution is forced towards the sink cell via the phloem