Translocation in the phloem
Translocation
- Translocation moves sugars (assimilates) through the phloem.
- It carries them from where they are made to where they are needed.
- It is driven by pressure, set up by active loading.
Source and sink
- A source is where the assimilate is made or released — e.g. a photosynthesising leaf.
- A sink is where it is used or stored — e.g. a growing root.
- Sucrose and amino acids flow from source to sink.
Practice
In translocation, a "source" is:
A source makes/releases the assimilate (e.g. a leaf); a sink uses or stores it (e.g. a growing root).
Loading at the source
- Companion cells load sucrose into the sieve tube against its gradient — active transport:
- proton pumps pump hydrogen ions out,
- cotransporter proteins then bring sucrose back in with those ions.
- Loading sucrose lowers the water potential in the sieve tube, so water enters by osmosis — raising the hydrostatic pressure.
Practice
Sucrose is loaded into the sieve tube by:
Companion cells pump H⁺ out (proton pumps), then cotransporters bring sucrose in with the ions — active transport.
Practice
Loading sucrose at the source raises the pressure in the sieve tube because it:
More sucrose lowers the water potential; water follows by osmosis, raising hydrostatic pressure.
Mass flow
- At the sink, sucrose is removed, so water potential rises, water leaves, and pressure falls.
- This makes a pressure difference: high at the source, low at the sink.
- Sap flows from high to low pressure — mass flow.
Practice
Mass flow occurs because sap flows:
Removing sucrose at the sink lowers its pressure; sap flows down the pressure gradient from source to sink.
You've got it
Key idea
- translocation = moving assimilates (sucrose, amino acids) in phloem, source → sink
- source = made/released (leaf); sink = used/stored (root)
- loading: active transport (proton pump + cotransport) → lowers water potential → water in → pressure ↑
- mass flow: sap flows from high pressure (source) to low pressure (sink)