Predict balanced growth and a changed light direction
Distinguish straight elongation from a lack of growth.
Exam scope and assessment notes
Extended only (Supplement) · Biology 0970, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked; human subject-teacher review, supervised practical competence and cumulative assessment remain pending. Cambridge states that IGCSE 0970 has the same subject content as IGCSE 0610, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Identify the root and shoot of a seedling; recall growth, cell structure and the variables in a controlled investigation.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Can a shoot elongate without curving sideways?
What you will learn
- Predict similar elongation under a balanced model
- Limit predictions to subsequent growth after a changed stimulus
Predict balanced growth and a changed light direction
Equal model increments, 20 µm on each side, permit straight elongation. Reversing a light source changes predictions for subsequent growth, not instant reversal of old tissue. Original teaching model.
| Model and side | Initial / µm | Final / µm |
|---|---|---|
| Balanced · Left | 20 µm | 40 µm |
| Balanced · Right | 20 µm | 40 µm |
| Feature | Meaning |
|---|---|
| Balanced record | Both sides 20→40 µm; increments 20 µm |
| Prediction | Straight elongation in the simplified model |
| Changed source | New shaded side can elongate more |
| Limit | No instant reversal or invented curvature angle |
Predict similar elongation under a balanced model
In a simplified balanced shoot model, auxin reaches responsive tissue on both sides in similar amounts and stimulates similar cell elongation. The shoot can become longer while remaining straight. A lack of lateral curvature therefore does not prove that no growth occurred, that no auxin was made or that the tip was inactive. Measure length change separately from directional bending.
An invented matching-cell record starts at 20 µm on both sides and ends at 40 µm on both sides. Each side elongates by 20 µm, so this record has equal elongation increments. It supports straight growth in the balanced model rather than a direction of curvature. This is a supplied comparison, not a claim that all cells in every plant grow equally under every light condition.
Limit predictions to subsequent growth after a changed stimulus
If a one-sided source is moved to the opposite side in a separate controlled trial, the newly shaded side changes. The shoot model predicts greater elongation on that shaded side and subsequent growth towards the new source. The curve can develop over time; a previously grown segment need not instantly reverse its shape. Use timed observations to distinguish earlier and later growth.
An actual shoot exposed to both light and gravity can have responses to both stimuli. The balanced model isolates side-to-side elongation for reasoning, rather than proving that all external influences are absent. State which conditions have been controlled and which measurements are supplied. Keep predictions qualitative unless a measured angle or a suitable geometric model is also provided.
Pause and explain
A source changes sides. Which prediction is appropriately limited?
Worked example
Both matching sides grow from 20 to 40 µm. What follows?
Show the worked solution
- Each side adds 20 µm.
- Equal increments support straight elongation in the balanced model.
Answer Both sides elongate equally by 20 µm; the model predicts straight growth, not absence of growth.
Try a topic test
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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.
Calculate both elongation increments for matching cells 20→40 µm on each side.
Give me a hint
Subtract the start from the end on each side.
Compare my reasoning
- Left adds 20 µm.
- Right adds 20 µm.
Both sides elongate by 20 µm. The balanced model predicts straight growth.
Look for these in your work
- Calculate both increments
- Distinguish growth from bending
A straight shoot becomes longer. Why does this not disprove auxin action?
Give me a hint
Equal elongation can occur with auxin on both sides.
Compare my reasoning
- Auxin can stimulate both sides similarly.
- Similar growth does not produce a lateral curve.
Auxin can stimulate similar elongation on both sides, producing a longer straight shoot. A lack of curvature does not show a lack of auxin action.
Look for these in your work
- Allow auxin-supported straight growth
- Separate length and curve evidence
A source moves from left to right. Predict new growth and state a limit about the old curved region.
Give me a hint
The shaded side becomes the left.
Compare my reasoning
- The left side can elongate more, bending new growth right.
- Previously grown tissue need not instantly reverse.
The newly shaded left side can elongate more and new growth can bend right towards the source. The old curved region need not instantly reverse.
Look for these in your work
- Predict the new shaded side and bend
- Limit the prediction to subsequent growth
Common mistakes
- Treating no curve as no growth.
- Expecting a changed source to instantly reverse old tissue.
What can you explain now?
A source changes sides. Which prediction is appropriately limited?
Compare with the explanation
Subsequent shoot growth can bend towards the new source
The response develops over time.
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Make it stick
Explain straight growth with auxin and predict new growth after a source changes sides.
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.