What is translocation?
Translocation is the transport of organic food substances — mainly sucrose, together with some amino acids — through the phloem from where they are made or stored to where they are needed. Because these substances can move in either direction (up or down the plant), translocation is described as bidirectional.
Source and sink
Translocation always runs from a source to a sink:
- A source is any part that makes or releases sugar, for example a photosynthesising leaf or a storage organ that is mobilising its reserves.
- A sink is any part that uses or stores the sugar, for example growing roots, developing fruits, or storage tubers.
The same organ can be a source at one time and a sink at another — a potato tuber stores sugar (sink) while it forms, then releases it (source) when the plant sprouts.
How translocation is powered
Translocation is an active process that needs energy. At the source, sucrose is actively loaded into the sieve tubes using ATP supplied by the companion cells. This lowers the water potential in the sieve tube, so water enters from the nearby xylem by osmosis and raises the pressure. The high pressure pushes the sap along to the sink, where sucrose is unloaded — this is the pressure-flow (mass-flow) mechanism.
Key idea
Translocation moves food from source (where sugar is made) to sink (where it is used or stored), through living phloem, and it requires energy. Xylem transport, by contrast, is passive and one-way.
Example
If a complete ring of bark (which contains the phloem) is removed from a tree trunk, sugars cannot pass the gap. Sugar builds up and the bark swells above the ring, showing that food is carried downward in the phloem.
Remember
Xylem carries water and minerals up, passively. Phloem carries food from source to sink, actively and in both directions. Do not mix them up.