Translocation

• Xylem:

  -long cell with no end wall to allow continuous flow of water

  -no organelles to obstruct the flow of water

  -thick walls with lignin to withstand tension and waterproof the vessel

  -pits in walls allow lateral movement to get around blocked vessels
• Phloem:

  -made of sieve tube elements which are long, living cells arranged end to end

  -their end walls are perforated with pores to form sieve plates

  -companion cells carry out metabolic functions for the sieve tube elements

  -they actively transport sucrose into sieve tube
• Translocation:

  Transport of organic molecules e.g amino acids, sucrose, from one part of a plant to another part. This occurs in phloem vessels.

  Sugars produced during photosynthesis are transferred from the sites of production (sources), to the site of use/storage (sinks). Sinks can be found anywhere in a plant, either below or above the sources. So translocation has to occur in either direction.
• Loading at the source:

  -high concentration of sucrose into sieve tubes, due to active transport by companion cells

  -lowers water potential

  -water moves from xylem to sieve tubes via osmosis

  -high hydrostatic pressure at source
• Transfer to the sink:

  -sucrose actively transported into respiring cells for use

  -water potential of respiring cells decreases

  -water moves into respiring cells via osmosis

  -hydrostatic pressure at sink decreases

  -mass flow down pressure gradient from sources to sink
• In the source, sucrose is actively transported into phloem by companion cells. This lowers water potential of sieve tube. Water enters by osmosis. The increase in volume increases the hydrostatic pressure. This causes mass movement towards respiring cells. Sucrose is then removed from phloem at sink by active transport.
• where are water and ions transported in the xylem ?

  from roots to leaves
• where are solutes transported in the phloem?

  from source to sink (up and down)