Explain shoot phototropism through auxin distribution
Trace the full shaded-side elongation chain towards light.
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
Light is on the left. Which side has more auxin in the taught model?
What you will learn
- Identify auxin on the shaded side of a shoot
- Connect differential elongation with positive phototropism
Explain shoot phototropism through auxin distribution
| Feature | Meaning |
|---|---|
| Source and movement | Shoot tip; diffusion through the plant from the tip |
| Distribution | More auxin on shaded side |
| Growth | Shaded shoot cells elongate more |
| Result | Bend towards the lit side and source; positive phototropism |
Identify auxin on the shaded side of a shoot
In the taught one-sided light model, auxin is made in the shoot tip and diffuses through the plant from the tip to growing tissue. Directional illumination produces an unequal distribution, with more auxin on the shaded side of the shoot. First locate the source and then identify the shaded side; if light comes from the left, the right side is shaded.
Auxin stimulates shoot cell elongation. More auxin on the shaded side produces greater elongation there in this model, while the lit side elongates less. The shaded side becomes the longer outside of the curve. The shoot bends towards its shorter, lit side, which is towards the light source. This is positive phototropism.
Connect differential elongation with positive phototropism
A complete explanation contains the source, movement, unequal distribution, elongation difference and final source-relative direction. Omitting the growth link leaves only a chemical distribution statement; saying that auxin attracts light does not describe the response. The light source is the directional stimulus, auxin is the chemical regulator and the cells’ unequal growth produces the curve.
If the source position is reversed in a new trial, the shaded side is reversed as well, so new growing tissue can bend towards the new source. Do not predict that an already grown curved region instantly straightens or that the whole seedling walks to the lamp. These statements concern subsequent shoot growth under suitable conditions and do not transfer the same dose–elongation rule to roots.
Pause and explain
Why does the shoot bend towards the source?
Worked example
Explain a shoot bend towards light from the left.
Show the worked solution
- Auxin from the tip is greater on the shaded right side.
- Right-side cells elongate more, so the shoot bends towards the less-elongating left side.
Answer Auxin made in the shoot tip diffuses from it and becomes greater on the shaded right side. Greater right-side elongation bends the shoot left towards the source: positive phototropism.
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.
Explain positive phototropism with light from the left using the complete auxin chain.
Give me a hint
Link tip, movement, shaded side, growth and bend.
Compare my reasoning
- Auxin is made in the tip and diffuses from it; more is on the shaded right.
- Right-side elongation is greater, so the shoot bends left towards light.
Auxin is made in the shoot tip and diffuses from it through plant tissue. More is distributed to the shaded right side, where cells elongate more. The shoot bends left towards the light: positive phototropism.
Look for these in your work
- Include source and movement
- Link distribution, elongation and direction
Predict the distribution and new bend if the source is on the right in a separate trial.
Give me a hint
Reverse the lit and shaded sides.
Compare my reasoning
- The left side is shaded and has more auxin in the model.
- Left-side elongation is greater, so the shoot bends right.
More auxin on the shaded left stimulates greater left-side elongation. New shoot growth bends right towards the source.
Look for these in your work
- Identify the new shaded side
- Predict source-relative growth
Correct: more auxin on the shaded side makes the shoot bend away from the lamp towards its fastest-growing side.
Give me a hint
The longer side is outside, not inside, the curve.
Compare my reasoning
- The shaded side elongates more.
- The shoot bends towards the less-elongating lit side.
More auxin on the shaded side causes greater elongation there; the shoot bends towards the less-elongating lit side and therefore towards the lamp.
Look for these in your work
- Retain shaded-side growth
- Correct the bend geometry
Common mistakes
- Putting more auxin on the lit side in this model.
- Predicting bending towards the faster-growing shaded side.
What can you explain now?
Why does the shoot bend towards the source?
Compare with the explanation
The shaded side elongates more, making the lit side the inside of the curve
The direction follows from unequal growth.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
Explain the full shoot-light chain for either source side with the correct distribution and bending geometry.
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.