Explain tropic shoot growth as chemical control
Connect a directional stimulus with unequal growth through auxin.
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
Which signal belongs to this shoot-growth account?
What you will learn
- Identify a chemical plant-growth signal
- Link unequal growth with shoot curvature
Explain tropic shoot growth as chemical control
| Feature | Meaning |
|---|---|
| Signal | Auxin, a chemical growth regulator |
| Growth | Unequal elongation on opposite shoot sides |
| Geometry | Longer side outside; bend towards the shorter side |
| Scope | Shoot explanation; not an unchanged root-growth rule |
Identify a chemical plant-growth signal
Phototropism and gravitropism of a shoot are examples of the chemical control of plant growth. The signal in this syllabus account is auxin. A directional stimulus affects the distribution of this chemical, and different sides of a growing region elongate by different amounts. The resulting curve changes the direction of new shoot growth while the plant remains anchored.
Auxin is a plant growth regulator, not an electrical impulse travelling along an animal neurone. Plants have neither the pancreatic blood route nor the sensory–relay–motor reflex arc taught for animals. Use the plant-specific sequence: auxin is made in the shoot tip, moves from the tip through plant tissue, becomes unevenly distributed under a directional stimulus and stimulates shoot cell elongation.
Link unequal growth with shoot curvature
When one side of the shoot elongates more than the other, the longer side forms the outside of a curve. The shoot bends towards the side that has elongated less. In the one-sided light model, the shaded side grows more, so the shoot bends towards light. In a horizontal shoot gravity model, the lower side grows more, so the shoot bends upwards, away from gravity.
Describe the cause and the visible response as different links. Saying only that a shoot follows light names a response but does not explain chemical control. Saying only that auxin is present leaves out uneven distribution and unequal elongation. The required Supplement explanation concerns shoots; the same stimulatory effect must not be transferred unchanged to root cells.
Pause and explain
Towards which side does an unequally elongating shoot bend?
Worked example
Explain why a shoot curves when one side elongates more.
Show the worked solution
- Auxin distribution can cause unequal shoot cell elongation.
- The longer side forms the outside and the shoot bends towards the shorter side.
Answer Chemical control produces unequal cell elongation; the shoot bends towards the side that elongates less.
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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.
Explain why shoot phototropism is described as chemical control.
Give me a hint
Include signal distribution and growth.
Compare my reasoning
- A light stimulus changes auxin distribution.
- Unequal auxin causes unequal shoot cell elongation and curvature.
Light changes auxin distribution; unequal auxin causes unequal shoot cell elongation and a curve. This is chemical control of growth.
Look for these in your work
- Name auxin
- Link unequal distribution to unequal elongation
Compare the side growing more in the shoot-light and horizontal shoot-gravity models.
Give me a hint
Both concern shoots, but the stimulus references differ.
Compare my reasoning
- In one-sided light the shaded side elongates more.
- In a horizontal shoot the lower side elongates more.
Shoot-light model: shaded side elongates more. Shoot-gravity model: lower side elongates more.
Look for these in your work
- Use the light reference
- Use the gravity reference
Correct: auxin travels in plant blood and contracts shoot muscles to pull the tip towards its fastest-growing side.
Give me a hint
Replace the animal route and mechanical action.
Compare my reasoning
- Auxin moves through plant tissue and stimulates cell elongation.
- Unequal elongation bends the shoot towards its less-elongating side.
Auxin moves through plant tissue, not blood, and stimulates shoot cell elongation rather than muscle contraction. Unequal growth bends the shoot towards the side that elongates less.
Look for these in your work
- Correct signal and growth mechanism
- Correct the bending direction
Common mistakes
- Using an animal blood route for plant auxin.
- Saying the shoot bends towards its faster-growing side.
What can you explain now?
Towards which side does an unequally elongating shoot bend?
Compare with the explanation
The side that elongates less
The longer side forms the outside of the curve.
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Make it stick
Explain chemical control with a complete stimulus-to-curvature chain and the correct bending geometry.
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