Translocation

• Phloem vessels have sieve-tube elements which are stacked end to end to form long vessels. Each sieve tube element has a companion cell closely associated with it.
• Sieve tube element has few organelles, and only a thin layer of cytoplasm at the edge. This redces resistance to flow of sap in the phloem vessel.
• The end plates of each sieve tube element are perforated to allow the passage of cell sap.
• The companion cells are connected with plasmodesmata (lateral sieve area) to allow the passage of large molecules to the sieve tube elements as a result of the loss of major organelles.
• Sucrose is transported through the phloem. It comes from glucose that is produced in photosynthesis.
• Sucrose is a disaccharide formed via a condensation reaction between glucose and fructose, forming a glycosidic bond.
• Sources are the sites where photosynthesis has occured, producing sucrose.
• Sinks are sites that use sucrose. This will be for respiration, to produce ATP.
• H+/Sucrose Co-transporter

  The H+ is diffusing from high to low concentration via a carrier protein. The sucrose molecule also binds to the carrier protein so that it is released into the cell with the H+.

  The concentration gradient for the H+ is maintained by actively transporting H+ out of the cell.
• Mass Flow Theory 1

  As photosynthesis maintains a high concentration of sucrose in the source cell, the sucrose moves down the concentration gradient into the companion cell via facilitated diffusion.
• Mass Flow Theory 2

  H+ are actively transported from the companion cell into the spaced between the cell walls (of the companion cell and sieve cell).

  The H+ then move down a concentration gradient into the sieve cell via co-transport with sucrose.

  Sucrose has now moved from the source into the phloem.
• Mass Flow Theory 3

  As sucrose moves into the sieve cells, it decreases the water potential.

  This causes water to move from the xylem into the sieve cell via osmosis.

  This causes hydrostatic pressure to increase in the sieve cell that is closest to the source.
• Mass Flow Theory 4

  Due to being used in respiration, there is a low concentration of sucrose at the sink cells.

  Sucrose is actively transported into the companion cells from the sieve cells, before moving into the sink cells via facilitated diffusion.

  As sucrose enters the sink cells. water potential is decreased, causing water to move into the sink cell from the sieve cell.

  This decreases hydrostatic pressure in the sieve cells that are closest to the sink cells.
• Mass Flow Theory 5

  As there is a high hydrostatic pressure near the source cells, and a low hydrostatic pressure near the sink cells, water will move down the hydrostatic pressure gradient, moving the sucrose with it. This is known as the mass flow theory.
• A respiratory inhibitor will stop/slow respiration, meaning it will stop ATP production. This prevents the active transport of H+, so there is no concentration gradient for the H+. Therefore, no co-transport of sucrose (and H+) will take place, so no pressure gradient is created. So, the process of translocation no longer takes place.