Explain how source and sink roles change over time
Trace storage and later remobilisation, and a developing leaf’s change from import to export.
Exam scope and assessment notes
Mapped to the named 0973 section using teaching from the corresponding single-science course. A 0973-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade. This qualification reports two grades on the 9–1 scale; confirm the entry route for your zone and exam series.
Before you begin
- Recall source/sink definitions and carbohydrate storage.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What identifies a sink during storage?
What you will learn
- Explain seasonal changes in a storage organ
- Explain a developing leaf’s source/sink transition
Storage now, release later
During a period of storage, a root or other storage organ receives organic nutrients and acts as a sink. Later, stored carbohydrate can be mobilised and released for transport to growing shoots; that same organ then acts as a source. The organ name stays the same while its activity changes.
In the supplied seasonal example, leaves supply a storage root during the storage period. At the start of shoot growth, the storage root supplies developing shoots. Trace each arrow separately rather than trying to use one permanent role label.
Leaves can also change role
A young leaf can import nutrients while it develops, acting as a sink. Once its photosynthesis produces enough surplus for export, it can act as a source. Being green or being called a leaf does not by itself establish when that transition has occurred.
Role change depends on current release/use/storage and conditions, not the calendar alone. Avoid asserting that every storage root switches on the same date or that the whole plant has only one source and one sink. Several organs can contribute at once.
Sources for this lesson
Pause and explain
When can a developing leaf become a source?
Worked example
A root stores arriving carbohydrate in one period and releases it to growing shoots later. Identify both roles.
Show the worked solution
- Receiving for storage makes it a sink in the first period.
- Releasing for transport makes it a source later.
Answer The storage root changes from sink to source as its current activity changes.
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 the root during carbohydrate accumulation and during later release to shoots.
Give me a hint
Use the action in each period.
Compare my reasoning
- Accumulation of incoming carbohydrate is storage: sink.
- Release to other parts is export: source.
During accumulation: sink. During release: source.
Look for these in your work
- Assign both periods
- Justify receipt versus release
Explain a young leaf’s change from importing to exporting sucrose.
Give me a hint
Connect development and surplus production with roles.
Compare my reasoning
- During growth it imports and uses organic nutrients, making it a sink.
- Later surplus production/export gives a source role.
The leaf can change from a developing sink to an exporting source as its current nutrient balance changes.
Look for these in your work
- Explain the initial import
- Explain later release of surplus
A table labels “all roots are sinks, all leaves are sources”. Rewrite it to accommodate seasonal evidence.
Give me a hint
Use conditional roles rather than fixed organ names.
Compare my reasoning
- Roots can receive/store or release nutrients at different times.
- Leaves can import while developing and export when mature and producing surplus.
Roles depend on current release, use and storage: roots and leaves can each change source/sink status over time.
Look for these in your work
- Cover both organ types
- Use current activity as the rule
Common mistakes
- Assigning permanent source/sink labels to organ names.
- Using calendar date without evidence of nutrient movement.
What can you explain now?
When can a developing leaf become a source?
Compare with the explanation
When it releases surplus organic nutrients for transport
Its current exporting activity defines the role.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Draw two time panels for a storage organ and reverse its role only when the stated activity changes.
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.