8.4 Translocation
- Syllabus
- 0610–2026–2027
- Topic
- 8.4
- Level
- —
Learning objectives
8.4.1Translocation as the movement• Describe translocation as the movement of sucrose and amino acids in phloem from sources to sinks8.4.2Describe: (a) sources as the parts• Describe: (a) sources as the parts of plants that release sucrose or amino acids (b) sinks as the parts of plants that use or store sucrose or amino acids8.4.3Some parts of a plant may act• Explain why some parts of a plant may act as a source and a sink at different times
Define translocation in phloem
Translocation is the movement of sucrose and amino acids in phloem from sources to sinks.
| Part of the definition | Required idea |
|---|---|
| substances | sucrose and amino acids |
| tissue | phloem |
| direction | from a source to a sink |
Movement may be upwards or downwards in the plant because the location of a source relative to a sink can differ.
Translocation is not transpiration. Phloem carries sucrose and amino acids from sources to sinks; xylem carries water and mineral ions.
Distinguish sources from sinks
A source releases sucrose or amino acids into phloem; a sink removes them from phloem to use or store them.
| Role | What it does | Common examples |
|---|---|---|
| source | releases transported nutrients | mature photosynthesising leaves; a storage organ releasing reserves |
| sink | uses or stores transported nutrients | growing shoot or root tips, flowers, fruits, seeds and storage organs filling with reserves |
Phloem therefore links producing or releasing regions to regions of growth, respiration, reproduction or storage.
A source is defined by release and a sink by use or storage. The terms do not mean 'top' and 'bottom' of the plant, so movement is not always downward.
Explain why organs can switch between source and sink
Source and sink are changing roles, not permanent labels for particular organs.
| Organ or stage | Role | Reason |
|---|---|---|
| young leaf or growing shoot | sink | needs imported sucrose and amino acids for growth |
| mature photosynthesising leaf | source | produces and releases sugars |
| storage organ while reserves accumulate | sink | receives and stores transported nutrients |
| storage organ during sprouting or flowering | source | releases reserves to support new growth |
For example, a potato tuber can be a source in spring while a new shoot grows, then become a sink later when mature leaves send sucrose back for storage.
An organ changes role when its balance of production or reserve release versus use or storage changes; its physical position does not determine the role.