Topic 8.4 · Supplement
Translocation
Everything on this page is Supplement content, and everything on it happens in phloem — a completely different tissue, carrying completely different substances, from the xylem water story earlier in this chapter. Keep that boundary firm and the rest of the topic is mostly a matter of applying one definition consistently.
What is translocation?
Translocation is the movement of sucrose and amino acids through phloem from sources to sinks. That definition packs in three separate ideas worth pulling apart: the substances being moved are specifically sucrose and amino acids, the tissue carrying them is phloem, and the direction of movement is always described in terms of source and sink rather than up or down.
Sources
A source is a part of the plant that releases sucrose or amino acids into the phloem for transport. A mature, actively photosynthesising leaf is the most familiar example — it produces sugars through photosynthesis, converts some of that carbohydrate to sucrose, and releases it for transport elsewhere in the plant. A storage organ can act as a source too, once its stored reserves are converted into transportable sucrose and released — so “source” doesn’t mean “leaf” specifically, it means whichever organ is currently releasing these substances.
Sinks
A sink is a part of the plant that uses or stores sucrose or amino acids. Growing roots, growing shoots, developing flowers, fruits and seeds, and storage organs that are currently accumulating reserves all count as sinks. Some of these use the imported material directly, for growth and respiration; others store it for release later, once they themselves become sources.
A plant part can be a source and a sink at different times
Source and sink describe a part’s current transport role, not a fixed identity, and that distinction is the part of this topic most worth being able to apply to an unfamiliar example. A storage organ can act as a sink while sugars are actively being transported into it and stored, then later act as a source once those reserves are broken down and released to supply developing shoots elsewhere. A young, still-developing leaf typically behaves as a sink, because it needs imported material to grow — but once it matures and is photosynthesising strongly in its own right, it can become a source instead.
Because sources and sinks can sit above or below each other in different parts of the plant at different times, phloem transport isn’t reliably upward or downward the way xylem transport is. The only reliable rule is source → sink: the direction follows wherever the current sources and sinks happen to be located, not a fixed direction built into the tissue itself.
Xylem transport and translocation
| Xylem transport | Translocation |
|---|---|
| Occurs in xylem | Occurs in phloem |
| Carries water and mineral ions | Carries sucrose and amino acids |
| Closely tied to transpiration from the leaves | Direction depends on the current positions of sources and sinks |
| Water is drawn upward through the plant | Material can move toward a sink in any direction |
The most reliable way to keep the two systems apart in an answer is to ask, for whatever substance the question mentions, which tissue is carrying it and why it’s moving — rather than trying to memorise the vocabulary of each system in isolation. This current syllabus point doesn’t require the detailed structure of sieve tubes and companion cells, and it doesn’t require a full pressure-flow account of how loading and unloading actually happens at the cellular level — the source-to-sink model above is the required depth.
Cross-links
Translocation is the phloem half of this chapter’s transport story; the xylem half — what it carries and where to identify it in a section — is covered on xylem and phloem. The water movement that translocation is so often confused with, along with the reason that movement is generally upward rather than source-to-sink, is explained on the transpiration page.