Part 4: Water and assimilate movement
4. Pressure flow
Translocation (moving assimilates moving from sources to sinks) is facilitated by pressure flow.
The following example is for photoassimilates – sugars made in photosynthesis.
Stage 1
At the sources, sugar molecules are moved into the phloem cells which increases the solute concentration of those cells.
Stage 2
Water then follows the sugars into the cells by osmosis. The increase in amount of water in the cells creates turgor pressure.
Stage 3
This pressure forces the water and sugars along the phloem tubes towards the sinks.
Stage 4
At the sinks the sugars are actively removed from the phloem cells.
Stage 5
The water follows the sugars by osmosis as the solute concentration outside the phloem cells becomes higher than inside.
Stage 6
As the water leaves, an area of lower pressure is created which ‘sucks’ more sugars and water along the phloem.
Watch this video for an overview of sugar sources, sinks, and pressure flow.
Sugar Transport: Pressure Flow Hypothesis (YouTube, 3:58)
Draw your own diagram of pressure flow in your notes – don’t just copy the example below, think of a way of representing the information in your own way.
Example:
