Translocation

• • Assimilates- substances made by the plant, using substances absorbed from the environment.
  • These include sugars (mainly transported as sucrose) and amino acids.
  • Source: a part of the plant that loads materials into the transport system
  • for example, the leaves photosynthesise and the sugars made are moved to other parts of the plant.
  • Sink: a part of the plant where those materials are removed from the transport system
  • for example, the roots receive sugars and store them as starch and convert them back when needed.
  • Translocation: the transport of assimilates throughout a plant.
• Phloem loading from the leaf to the companion cell into the sieve tube element: 


  • Sucrose is loaded into the sieve tube by an active process. This involves the use of energy from ATP in the companion cells. The energy is used to actively transport hydrogen ions (H+) out of the companion cells.
  • This increases their concentration outside the cells and decreases their concentration inside the companion cells.
  • As a result, a concentration gradient is created.
• Phloem loading from the leaf to the companion cell into the sieve tube element: (2)

  • The hydrogen ions diffuse back into the companion cells through special cotransporter proteins. These proteins only allow the movement of the hydrogen ions into the cell if they are accompanied by sucrose molecules. This is known as cotransport.
  • It is also called secondary active transport, as it results from the active transport of the hydrogen ions out of the cell and moves the sucrose against its concentration gradient.
• Phloem loading from the leaf to the companion cell into the sieve tube element: (3)

  • As a result of the increased concentration of sucrose in the companion cell and sieve tube element, water also moves in by osmosis.
  • This leads to a build up of turgor pressure due to the rigid cell walls. The water carrying the assimilates moves into the tubes of the sieve elements, reducing the pressure in the companion cells, and moves up or down the plant by mass flow to areas of lower pressure (the sinks).

  
• Phloem unloading from the sieve tube to the sinks

  • Sucrose entering the sieve-tube element makes the water potential inside the sieve tube more negative (lower).
  • As a result, water molecules move into the sieve-tube element by osmosis from the surrounding tissues. This increases the hydrostatic pressure in the sieve tube at the source.

  
• Phloem unloading from the sieve tube to the sinks (2)

  • Where sucrose is being used it can diffuse out of the sieve tube via the plasmodesmata.
  • It may also be removed by active transport. The removal of sucrose from the sap makes the water potential less negative (higher), so that water moves out of the sieve tube into the surrounding cells by osmosis. This reduces the hydrostatic pressure in the phloem at the sink. Some of the water that carried the solute to the sink is drawn into the transpiration stream.
  • Therefore a pressure gradient is set up along the sieve tube in either direction.