Mechanism of translocation

• Sink cell (respiring cell):

  • Respiring cell is using up sucrose, and therefore it has a more positive water potential
  • Water leaves the sink cell by osmosis
  • This decreases the hydrostatic pressure in the sink cell
• Source cell (photosynthesising leaf cell):

  • Sucrose lowers water potential of source cell
  • Water enters by osmosis
  • This increases the hydrostatic pressure in the source cell
• Source to sink explanation:

  • The source cell has a higher hydrostatic pressure than the sink cell, so the solution is forced towards the sink cell via the phloem
• Translocation (1): How sucrose transports from the source to the sieve tube element

  • Photosynthesis occurring in the chloroplasts of leaves creates organic substances, eg sucrose
  • Sucrose is actively transported into the sieve tube element, using the companion cell
• Translocation (1): Transfer of sucrose into sieve elements from photosynthesising tissue

  • Sucrose manufactured from products of photosynthesis in cells with chloroplasts
  • Sucrose diffuses down a concentration gradient by facilitated diffusion from photosynthesising cells into companion cells
  • Hydrogen ions actively transported from companion cells into spaces within cell walls using ATP
  • These hydrogen ions then diffuse down a concentration gradient through carrier proteins into the sieve tube elements
  • Sucrose molecules are transported along with the hydrogen ions in co-transport
• Translocation (2): Mass flow of sucrose through sieve tube elements

  • The increase of sucrose in the sieve tube element lowers the water potential
  • Water enters the sieve tube elements from the surrounding xylem vessels via osmosis
  • The increase in water volume in the sieve tube element increases the hydrostatic pressure causing the liquid to be forced towards the sink
• Translocation (2): Mass flow of sucrose through sieve tube elements

  • The sucrose produced by photosynthesising cells (source) is actively transported into the sieve tubes
  • This causes the sieve tubes to have a low water potential
  • As the xylem has a much high water potential, water moves from the xylem into the sieve tubes by osmosis - creating a high hydrostatic pressure within them
• Translocation (3): Transport of sucrose to the sink (respiring cells)

  • Sucrose is used in respiration at the sink, or stored as insoluble starch
  • More sucrose is actively transported into the sink cell, which causes the water potential to decrease
  • This results in osmosis of water from the sieve tube element into the sink (some water also returns to the xylem)
  • The removal of water decreases the volume in the sieve tube element and therefore hydrostatic pressure decreases
  • Movement is due to difference in hydrostatic pressure between the source and sink end of the sieve tube element
• Translocation (3): Transport of sucrose to the sink

  • The sucrose is actively transported by companion cells, out of the sieve tubes and into the sink cells