Lesson 12 of 12 / Nutrition and transport in plants
Translocation: moving sugars in plants
How can sugars travel down to roots and up to growing shoots?
I can describe translocation in phloem and interpret evidence from ringing and tracer studies.
I can describe phloem translocation using experimental evidence.Syllabus
Syllabus K325, Topic 10(n). Describe phloem translocation using experimental evidence.
About 6 min
Make a guess. You are not marked.
A leaf makes sugar. A fruit growing above that leaf needs it. Can sugar move upwards in a plant?
Show the answer
Yes. Phloem carries it from source to sink.
Sugar moves from where it is made to where it is used or stored, upwards or downwards.
The key idea
Translocation moves food, mainly sucrose, through phloem from sources to sinks. Its direction depends on where food is supplied and used or stored.
Mature leaf = source. Growing shoot and roots = sinks. Sucrose can move upwards and downwards in different phloem pathways; height does not define source or sink.
Gold: phloem. Blue: xylem. Green arrows: source-to-sink food movement. Purple dots in the tracer view: labelled material. Original schematic; one drawn stem line represents several transport pathways.
Explanation
A source supplies sugars to the transport system, such as a mature photosynthesising leaf. A sink receives sugars for use or storage, such as a growing root, fruit or young shoot. Translocation carries organic food, mainly sucrose, through phloem sieve tubes between these regions.
Food can move upwards or downwards in different phloem pathways. A mature leaf may supply roots below it and growing shoots above it. A storage organ can act as a sink while storing food and later as a source when stored material is mobilised for new growth.
In a ringing study, a ring of bark containing phloem is removed while xylem remains intact. For a shoot with photosynthesising leaves above the ring and sinks below it, sugars can accumulate above the interruption and the lower sinks receive less. Water transport may initially continue through intact xylem. This supports the role of phloem in food transport.
Tracer studies provide another line of evidence. A leaf is supplied with labelled carbon dioxide, which becomes incorporated into sugars during photosynthesis. Detecting labelled material along phloem and later in sinks supports movement of newly made food. Interpret a study from its stated source, sink and tissue evidence rather than assuming all sugar always moves downwards.
Step by step
- 1
Locate source and sink
Use the plant's current growth and storage activity, not only height.
- 2
Choose phloem and sucrose
Distinguish food transport from water transport in xylem.
- 3
Interpret the evidence
Link an interruption or labelled material to the route of movement and its limits.
Worked example
A ring below the leaves
Phloem is removed in a ring below mature leaves, but xylem remains intact. Sugars accumulate above the ring. Explain the observation.
One way to explain it
Sugars produced in the leaves are normally translocated through phloem to sinks below. Removing the phloem interrupts this route, so food accumulates above the ring while lower sinks receive less. Intact xylem can initially continue carrying water.
Why this answer works
- Locate the source above the interruption.
- Name phloem as the interrupted food pathway.
- Avoid claiming that all water transport stops immediately.
Watch out for this
A student says: "Phloem always carries food downwards and xylem carries the same food upwards." What is wrong with this?
Show the answer
Phloem carries food from sources to sinks, which may be above or below. Xylem chiefly carries water and mineral ions from roots towards leaves.
Check your understanding
A mature leaf supplies a developing fruit above it. Which description is correct?
Show the answer
Sucrose is translocated upwards in phloem from the leaf source to the fruit sink.
Source-to-sink transport can be upwards or downwards in different pathways.
Quick recall
How can sugars travel down to roots and up to growing shoots?