Identify sources and sinks from organ activity
Use evidence of release, growth, use and storage rather than the organ’s name alone.
Extended only (Supplement); Biology 0610, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked
Before you begin
- Complete the translocation definition lesson.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Why can a growing organ need sucrose?
What you will learn
- Infer source/sink roles from stated activity
- Distinguish current function from organ identity
Look for release or use
A source releases organic nutrients into phloem; a sink uses or stores what arrives. A mature illuminated leaf exporting surplus sucrose is a source. A young developing leaf importing nutrients can be a sink, even though both organs are called leaves.
Growing tissues need sucrose for respiration and for building other molecules. Amino acids support protein synthesis. Storage organs can receive nutrients without rapidly growing, so “sink” includes storage as well as immediate use.
Ask what happens now
A storage root accumulating carbohydrate is a sink at that time. If it later releases stored carbohydrate for transport to shoots, it can become a source. The role is determined by net release or receipt in the stated situation.
Do not classify an organ from its position above or below the soil. Evidence of the actual movement and organ activity is stronger than a remembered label. This lesson identifies roles; it does not require the detailed AS & A Level pressure-flow mechanism.
Pause and explain
Which evidence identifies a source?
Worked example
A mature leaf exports and a young leaf imports sucrose. Why do they have different roles?
Show the worked solution
- Exporting organic nutrient makes the mature leaf a source.
- Import for growth/use makes the young leaf a sink.
Answer Source: exporting mature leaf. Sink: importing developing leaf; the role depends on activity, not the shared organ name.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
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.
Classify an exporting mature leaf and a fruit storing incoming sucrose.
Give me a hint
Use release and storage.
Compare my reasoning
- The leaf releases for transport, so it is a source.
- The fruit stores incoming nutrients, so it is a sink.
Leaf: source. Storing fruit: sink.
Look for these in your work
- Assign both roles
- Link the assignments to activities
Why can a young green leaf still be a sink?
Give me a hint
Photosynthesis need not yet meet all its growth needs.
Compare my reasoning
- The supplied young leaf imports organic nutrients.
- Use of incoming nutrients for development gives it a sink role.
It can need imported nutrients for growth before exporting surplus; its current importing/use role makes it a sink.
Look for these in your work
- Use the stated import evidence
- Avoid a colour-only source rule
A record says only “root”. What extra evidence is needed to identify its role?
Give me a hint
Ask about the direction and purpose of nutrient movement.
Compare my reasoning
- Determine whether it releases stored nutrients or receives them.
- Identify current use/storage or export.
Find whether the root is releasing organic nutrients or receiving them for use/storage; its name alone is insufficient.
Look for these in your work
- Request current movement evidence
- State the two possible roles
Common mistakes
- Assuming every green leaf is currently a source.
- Leaving storage out of the sink definition.
What can you explain now?
Which evidence identifies a source?
Compare with the explanation
Net release of organic nutrients for transport
Release defines the source role.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Give two situations in which leaves can have different source/sink roles.
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.