7. Translocation

= The transport of assimilates throughout a plant.

Assimilates = substances that have become a part of the plant.

Source = A part of the plant that loads assimilates into the phloem sieve tubes.

Sink = A part of the plant that removes assimilates from the phloem sieve tubes.

Example) source - photosynthesising leaves

                sink- actively transporting roots.

Sucrose is loaded into the phloem sieve tubes by an active process, involving ATP produced in the companion cell. The energy is used to actively transport hydrogen ions out of the companion cells. This leaves a low concentration in the companion cell, so the hydrogen ions diffuse back through special cotransporter proteins.

These proteins only allow the hydrogen ions to diffuse back through if they're accompanied by sucrose molecules. This is called cotransport. It can also be called secondary active transport, as it moves the sucrose molecules against its concentration gradient.

1) Many mitochondria in the companion cell, produce ATP.

2) This ATP is then used in the active hydrogen pumps. They pump the hydrogen ions out of the companion cell and into the source cell.

3) Sucrose is produced in the source cell.

4) When the hydrogen actively pumps into the source cell, it lowers the concentration in the companion cell so must transport back. It can only do this via the cotransporter proteins which require sucrose to accompany the hydrogen.

5) It moves the hydrogen by facilitated diffusion carrying the sucrose with it.

6) This increases the sucrose concentration in the companion cell, so the sucrose diffuses through the plasmodesmata inot the sieve tube elements.

7) During this whole process the sucrose has been lowering the water potential, so water has been following by osmosis.

8) High hydrostatic pressure is created at the top of the sieve tube element and low hydrostatic pressure at the bottom of the sieve tube element. The high hydrostatic pressure is due to the many water and sucrose molecules.

9) This means that the solution of sucrise and water (sap) moves by mass flow down the hydrostatic pressure gradient.

10) Sucrose moves by facilitated diffusion through the plasmodesmata into the companion cell, through to the sink. Water follows by osmosis.

11) sucrose is being used by the sink cell (i.e. for active transport)

• An aphid feeding on a plant stem can be used to show that the mouthparts are taking food from the phloem.
• When a tree is ringed and the phloem is removed, sugars collect above the ring.
• If you radioactively label carbon dioxide it soon appears in the phloem.
• The PH of companion cells is higher than surrounding cells. The concentration of sucrose is higher in the source than in the sink.

• additional mitochondria in companion cells.
• translocation stopped if inhibitors used to stop ATP formation.
• mass flow so high energy is required.

• not all the solutes in the phloem sap move at the same rate.
• sucrose is moved to all parts of the plant at the same rate, rather than moving quicker to areas of a lower concentration.
• the role of the sieve plates is unclear.