Translocation Mechanisms
Translocation
Occurs in the phloem
Movement of assimilates
Source – part of the plant that loads assimilates into the phloem sieve tubes
Sink – part of the plant which unloads assimilate from the phloem sieve tubes
A part of a plant can be both a source and a sink since the flow goes two ways
Active Loading
Sucrose loading into sieve tube elements is an active process
Uses energy from ATP in companion cells
Hydrogen ions are actively transported out of companion cells
H+ concentration increases outside the cells and decreases inside the cells creating a
concentration gradient
H+ ions diffuse back into companion cells along this gradient through special
cotransporter proteins which only allow the movement of H+ ions back into the cell if
they are accompanied by sucrose molecules – this is cotransport
Movement of Sucrose
A solution (known as sap) consisting of dissolved sugars such as sucrose, amino acids
and other assimilates can be made to move up or down the plant as required in a
process called mass flow
Caused by a difference in hydrostatic pressure between two ends of the phloem which
produces a pressure gradient
Sap enters at as source and exits at a sink
Source:
o Any part of the plant that loads assimilates into the sieve tubes
o Could be roots where starch is converted to sucrose
o Could be leaves where sugars are made in photosynthesis
Sink:
o Any part of the plant which unloads assimilates from the sieve tubes
o Could be used for respiration and growth in the meristem
o Could be converted to starch for storage in the roots
Translocation
Occurs in the phloem
Movement of assimilates
Source – part of the plant that loads assimilates into the phloem sieve tubes
Sink – part of the plant which unloads assimilate from the phloem sieve tubes
A part of a plant can be both a source and a sink since the flow goes two ways
Active Loading
Sucrose loading into sieve tube elements is an active process
Uses energy from ATP in companion cells
Hydrogen ions are actively transported out of companion cells
H+ concentration increases outside the cells and decreases inside the cells creating a
concentration gradient
H+ ions diffuse back into companion cells along this gradient through special
cotransporter proteins which only allow the movement of H+ ions back into the cell if
they are accompanied by sucrose molecules – this is cotransport
Movement of Sucrose
A solution (known as sap) consisting of dissolved sugars such as sucrose, amino acids
and other assimilates can be made to move up or down the plant as required in a
process called mass flow
Caused by a difference in hydrostatic pressure between two ends of the phloem which
produces a pressure gradient
Sap enters at as source and exits at a sink
Source:
o Any part of the plant that loads assimilates into the sieve tubes
o Could be roots where starch is converted to sucrose
o Could be leaves where sugars are made in photosynthesis
Sink:
o Any part of the plant which unloads assimilates from the sieve tubes
o Could be used for respiration and growth in the meristem
o Could be converted to starch for storage in the roots
