Trace assimilates from sources to sinks
Sucrose and amino acids move dissolved in water through phloem sieve tubes from exporting sources to importing sinks.
AS Level · Cambridge International Biology 9700, 2028–2030. Written practice is self-checked; identifying tissues from real slides, drawing biological sections, supervised practical competence and human subject-teacher review remain necessary.
Before you begin
- Distinguish plant cell walls from cell membranes and recognise that vascular tissues form pathways through roots, stems and leaves.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What defines a source in phloem translocation?
What you will learn
- State that assimilates such as sucrose and amino acids move in phloem sieve tubes
- Identify sources and sinks from their current roles rather than fixed organ names
Assign source and sink by what an organ is doing
A source releases assimilates to phloem; a sink removes them for growth, respiration, storage or conversion. A mature photosynthesising leaf can act as a source. A growing root, fruit or developing leaf can act as a sink when importing assimilates.
An organ can change role with its activity and developmental stage. A storage organ that accumulates carbohydrate is a sink; when stored carbohydrate is mobilised and exported to new growth, it acts as a source.
Phloem carries the aqueous assimilate solution
Sucrose and amino acids are examples of assimilates dissolved in water and transported in phloem sieve tubes. The route is source to sink; it is not defined as always upwards or always downwards.
Different sieve tubes can serve different source–sink pairs. A whole plant can transport in more than one overall direction at the same time without requiring opposing flow in a single sieve tube.
Pause and explain
Which pair can be transported as assimilates in phloem?
Worked example
A mature leaf exports sucrose to a developing fruit. Identify the source, sink and tissue.
Show the worked solution
- The exporting mature leaf is the source.
- The importing fruit is the sink; phloem sieve tubes carry the assimilate.
Answer The mature leaf is the source because it exports sucrose. The developing fruit is the sink because it imports and uses or stores assimilate. Sucrose moves dissolved in water through phloem sieve tubes.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A mature leaf supplies sucrose to a growing root. Identify the source, sink, transported substance and route.
Give me a hint
Use current export/import roles.
Compare my reasoning
- The mature exporting leaf is the source; the importing root is the sink.
- Sucrose dissolved in water travels in phloem sieve tubes.
The mature leaf is the source because it exports sucrose; the growing root is the sink because it imports assimilate. Sucrose dissolved in water moves through phloem sieve tubes.
Look for these in your work
- Source correct
- Sink correct
- Substance and tissue correct
A storage root imports carbohydrate in one season and later supplies a developing shoot. Explain how its role changes.
Give me a hint
Track whether the organ is importing or exporting.
Compare my reasoning
- During accumulation it imports and is a sink.
- During later mobilisation it exports and is a source.
While it accumulates carbohydrate, the storage root imports assimilates and acts as a sink. When stored carbohydrate is mobilised and exported to a growing shoot, it acts as a source.
Look for these in your work
- First role linked to import
- Second role linked to export
- Role follows current activity
At the same time, a source leaf supplies a root below and a fruit above. Does that prove that one sieve tube carries two opposite streams? Explain.
Give me a hint
Separate whole-plant destinations from a single transport pathway.
Compare my reasoning
- Different source–sink pairs can use different sieve tubes.
- Overall transport in multiple directions does not mean opposite flow within one tube.
No. Separate phloem pathways can connect the leaf to the root and the fruit. The plant can have overall transport in different directions, while each sieve tube carries flow along its own source-to-sink pathway.
Look for these in your work
- Multiple pathways explained
- Single-tube direction distinguished
Common mistakes
- Defining sources and sinks by organ name alone
- Saying phloem transport is always downward
- Confusing translocation of assimilates with transpiration of water vapour
What can you explain now?
What defines a source in phloem translocation?
Compare with the explanation
A region that releases assimilates to the phloem
Source status depends on export activity.
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Make it stick
How do you identify a source and a sink, and which assimilates travel through phloem?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.