Translocation

What is translocation

The transportation of water, it requires energy and can occur in any direction

The process of Translocation (Part 1)

A source where sucrose is added or loaded into the phloem (usually a photosynthesizing leaf)

A sink where sucrose is removed from the phloem for storage or use (usually a respiring/growing part of the plant)

Glucose is produced in the leaves

The process of Translocation (Part 2)

H+ is actively pumped out of the companion cells by symport via a co-transporter protein.

Sucrose is moving against a concentration gradient because there is sucrose in the companion cell and phloem 

Sucrose moves into the sieve tubes via plasmodesmata 

The process of Translocation (Part 3)

Because of the higher solute potential in the phloem, water moves into phloem by osmosis

The water pushes the sucrose up and down the phloem

Sucrose diffuses into the respiring cell/ sink and is used in respiration

The process of Translocation (Part 4)

The water potential of the fluid in the phloem becomes less negative than the water in the xylem

As a result, water leaves the phloem and re-enters the xylem since the xylem has a more negative water potential than the phloem

Water continues to rise up the xylem because of root pressure, capillarity and the transpiration pull

Evidence proving the theory of Translocation

There is a basic pH in the phloem since H+ ions are pumped out the companion cells

There is a concentration gradient for the source and sink due to sucrose being transported in plants

There is an electrochemical gradient across the membrane of the companion cells since H+ ions are being transported

Criticisms disproving the theory of Translocation

Different substances move at different rates in the phloem even though they are expected to move uniformly