Translocation

• Translocation is the TRANSPORT of ASSIMILATES throughout the plant, in the PHLOEM TISSUE. 
• A SOURCE RELEASES SUCROSE INTO the phloem.
• A SINK REMOVES SUCROSE from the phloem. 

Sucrose is loaded INTO the phloem by an active process. ATP is used by the COMPANION CELLS to ACTIVELY TRANSPORT HYDROGEN ions out of their CYTOPLASM and into surrounding TISSUE. 

• This sets up a DIFFUSION GRADIENT and the HYDROGEN ions DIFFUSE BACK into the companion cells. 
• This diffusion occurs through special TRANSPORTER PROTEINS. 
• These proteins enable hydrogen ions to bring sucrose into the companion cells. 
• As the concentration of sucrose builds up inside the companion cells, they DIFFUSE into the SIEVE TUBE ELEMENTS through the PLASMODESMATA. 

At the SOURCE:

• Sucrose entering sieve tube elements REDUCES WATER POTENTIAL inside sieve tube
• Causes WATER to move into SIEVE TUBE ELEMENT by OSMOSIS from tissues.

• This INCREASES HYDROSTATIC PRESSURE in the sieve tube at the SOURCE.

Movement at the SINK:

• Sucrose is used in the cells surrounding the phloem.
• Sucrose may be CONVERTED into STARCH for storage or may be used in a metabolic process e.g. RESPIRATION. 
• This REDUCES sucrose CONCENTRATION in these cells.
• Sucrose molecules move by DIFFUSION or ACTIVE TRANSPORT from sieve tube element into surrounding tissue. 
• This INCREASES WATER POTENTIAL in sieve tube element, so water molecules move into surrounding cells by OSMOSIS. 
• This REDUCES HYDROSTATIC PRESSURE in the phloem at the SINK. 

Movement along the phloem:

• Water entering the phloem at the SOURCE, moving DOWN the hydrostatic pressure GRADIENT and leaving the phloem at the SINK, produces a FLOW OF WATER along the phloem. 
• Flow carries SUCROSE+assimilates along the phloem (MASS FLOW).

• Mass flow can occur in EITHER DIRECTION - UP OR DOWN the plant depending on WHERE the SUGARS are needed.
• Mass flow may occur up or down the plant in the SAME PHLOEM TUBE at DIFFERENT times. 
• It may be moving ASSIMILATES UP the plant in some tubes and DOWN the plant in other tubes at the SAME TIME.

Most obvious SOURCE is a LEAF from summer to winter. Act as a SINK in spring.

ROOTS are a SOURCE in some times of the year and a SINK in others.