Phloem

• Phloem cells are living and have few organelles/cytoplasm which reduces resistance
• Sieve tube elements

  • not proper cells, have no nucleus or ribosomes
  • lined up end to end to form a tube
  • tube has cross walls which has pores to allow sap to flow through - sieve plates
• Companion cells

  • small cells in between sieve tubes
  • have large nucleus, dense cytoplasm and lots of mitochondria
  • have small vacuoles
  • cytoplasm linked to the sieve tube elements by plasmodesmata
• A source is where organic substances are made
• A sink is where organic substances are used
• Translocation can occur in both directions
• Mass flow is the accepted theory of phloem transport because the rate of transport is too fast for diffusion
• Movement of sucrose through the phloem via mass transport

  • sucrose loaded by co-transport
  • water moves via osmosis from the xylem to the phloem because of the water potential gradient
  • sucrose moves via mass flow through the phloem through hydrostatic pressure
  • various active transport mechanisms via companion cells into sink cells
• Evidence supporting mass flow theory

  • pressure within sieve tubes shown by sap being released
  • concentration of sucrose higher in leaves than roots
  • downward flow of phloem occurs in daylight not at night
  • increases in sucrose levels in leaf followed by similar increases in phloem
  • companion cells possess many mitochondria and readily produce ATP
• Evidence against mass flow theory

  • function of sieve plates is unclear as they would seem to hinder mass flow
  • not all solutes move at the same speed
  • sucrose is delivered at same rate to all regions rather than more quickly to areas with lower sucrose concentrations