Translocation is the movement of sugar, mainly sucrose, in the phloem. It moves from a source, such as a photosynthesising leaf, to a sink, such as a root, a fruit or a growing tip.
This lesson is part of transport in plants. You will use it again in comparing xylem and phloem structure and function.
What are a source and a sink?
A source makes or releases sugar, and a sink uses or stores it. The phloem connects them.
| Term | Meaning | Example |
|---|---|---|
| Source | Organ that makes sugar | Leaf in daylight |
| Sink | Organ that uses or stores sugar | Root, fruit, growing shoot |
| Phloem | Tissue that carries the sugar | Sieve tubes |
The direction depends on the plant’s needs. A leaf sends sugar to a growing shoot in spring and to a root store later.
How does the sugar travel?
Sugar made in a leaf is loaded into the phloem. Water follows by osmosis, which raises the pressure at the source.
The pressure pushes the sugary sap along the sieve tubes to the sink. There the sugar is removed for use or stored, and the pressure falls.
Worked example: a sweet potato plant
A sweet potato plant makes sugar in its leaves all day. At night the leaf stops photosynthesis, but the root is still storing starch.
- By day, the leaf is the source. Sugar enters the phloem.
- The sugar travels down the stem to the root, where it is converted to starch and stored.
- At night, sugar already in the phloem keeps moving to the root.
Answer: “Sucrose is transported by translocation from the leaf, the source, to the root, the sink, through the phloem.”
Translocation versus transpiration
The two words differ in tissue, substance and direction. A table removes the confusion.
| Feature | Translocation | Transpiration |
|---|---|---|
| Substance | Sucrose and other organic substances | Water and dissolved minerals |
| Tissue | Phloem | Xylem |
| Direction | Up or down | Upwards |
| Energy | Needs energy | Driven by evaporation |
The mistake that loses marks
The usual slip is to write “translocation moves water up the plant”. That describes transpiration, not translocation.
A second slip is to say sugar always moves downwards. Name both source and sink to show the direction is set by where the sugar is needed.
Check yourself
A ring of bark is removed from the stem of a healthy plant. After a few days, the stem swells above the ring and the roots below grow poorly. Explain both observations.
Answer
Removing the bark removed the phloem, so sugar cannot move past the ring. Sugar collects above the ring and the stem swells.
Roots are the sink but no longer receive sugar, so they grow poorly.
What to study next
Put both tissues together in comparing xylem and phloem structure and function. Then test yourself with the transport in plants practice set.
If a teacher could quiz you on the two processes until they no longer swap, see online one-to-one Biology tuition.