Summary
The pressure-flow movement of sucrose through phloem from sources to sinks.
In depth
Pressure-flow hypothesis (Münch): sugars are actively loaded into phloem at sources, lowering water potential so water enters by osmosis, raising pressure; sap flows to sinks where sugar is unloaded and water leaves.
Evidence: ringing experiments show sugar accumulation above a cut.
Sugars are loaded either symplastically or apoplastically. Pressure builds from osmosis at the source; unloading at sinks lowers pressure. Flow speed is about 1 m per hour.
Key idea: roots can be sinks in summer and sources when mobilising stored starch in spring.
Münch proposed the pressure-flow hypothesis in 1930. Loading uses an apoplastic route with sucrose-proton co-transport into companion cells, or a symplastic route in some plants, and requires ATP to run proton pumps. At the sink, sugars are removed for growth or storage, which keeps the gradient. Hydrostatic pressures in sieve tubes are about 1 to 3 MPa.
Sources include mature leaves and storage organs in spring, and sinks include roots, fruits, young leaves and tubers. A storage organ can change its role from sink to source (a bulb in spring). Experiments with aphid stylets and tracers support the model, while questions remain for very tall trees. Translocation is affected by metabolic inhibitors since loading is active.
Example: in spring, sugars move from roots up to buds in maple trees, the basis of maple syrup tapping.
Frequently asked questions
What is Translocation?
The pressure-flow movement of sucrose through phloem from sources to sinks.
Which area of biology covers Translocation, and how advanced is it?
Translocation is one of the 56 topics in Plant Biology. It is rated Intermediate, so it suits learners who already know the core vocabulary. Related topics in this area include Phloem, Xylem, Auxin and Phototropism.
Can you give an example of Translocation?
Sources are leaves; sinks are roots, fruits and growing tips.
What should I learn before Translocation?
Work through the Beginner topics in Plant Biology first, such as Plant Kingdom, Photosynthesis, Chlorophyll, then return to Translocation. Intermediate topics assume the Beginner vocabulary.
What topics are related to Translocation?
The closest topics to Translocation are Phloem, Xylem, Auxin and Phototropism, Cohesion-Tension Theory. Comparing them side by side is a good way to see how the ideas connect.
How can I check that I understand Translocation?
Read the summary and the in-depth section, then use the quick check on the interactive lesson for Translocation. To practise the whole of Plant Biology, use the Plant Biology quiz.
How is Translocation different from Phloem?
Translocation: The pressure-flow movement of sucrose through phloem from sources to sinks. Phloem: Vascular tissue made of living cells that transports sugars from sources to sinks. Read both side by side and note what each one explains.
How should I study Translocation step by step?
First, read the summary and write the definition in your own words. Second, read the in-depth section and note any example. Third, compare it with Phloem and explain the difference aloud. Finally, take the Plant Biology quiz and come back to this page for any question you miss.