Interpret controlled evidence for a shoot-tip signal
Use cap and barrier models without overstating what each comparison proves.
Section 14.5 · Tropic responses
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
- 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 control helps separate tip covering from excluding light?
What you will learn
- Compare tip shading with transparent and lower-region controls
- Evaluate movement evidence and the limits of tip removal
Interpret controlled evidence for a shoot-tip signal
Intact cap comparisons separate light exclusion from covering. Supplied permeability models support a mobile chemical signal; tip removal also wounds and removes a source. Original teaching model.
| Feature | Meaning |
|---|---|
| Opaque tip cap | Excludes light from the tip |
| Transparent tip cap | Controls covering while allowing light |
| Barrier model | Permeable versus impermeable signal connection |
| Limits | Tip removal is confounded; model does not prove all transport solely passive |
Compare tip shading with transparent and lower-region controls
A controlled shoot-tip comparison can use intact seedlings in one-sided light with an opaque cap over the tip, a transparent tip cap and an opaque cover over a lower region. Match cap handling and keep growth conditions comparable. In a supplied model, the opaque tip cap prevents a directional curve while the transparent tip cap and lower cover allow it. The comparison supports the importance of light reaching the tip, rather than light simply warming any region.
The transparent cap is an important control because it tests covering and handling without excluding light in the same way. A lower-region opaque cover helps locate the light-sensitive region, but any effects on tissue growth or temperature still need consideration. Record continued growth as well as curvature: a damaged shoot that does not grow supplies a weaker directional-response comparison than an intact growing one.
Evaluate movement evidence and the limits of tip removal
A separate historical-style supplied model places a signal-permeable gelatin connection or an impermeable barrier between the tip and responsive tissue. A response across the permeable connection, but not the impermeable barrier, is consistent with a mobile chemical signal. The practical handling is not assigned here; the model is for interpreting evidence. It does not prove that every aspect of real auxin transport is solely passive diffusion.
Removing the tip is not by itself a clean test of light detection: it also wounds the plant and removes an auxin source. A loss of curvature could therefore involve lost growth or signal supply as well as lost stimulus reception. Interpret several controlled comparisons together with the taught source–movement–elongation account.
Pause and explain
What does the supplied permeable-versus-impermeable connection model support?
Worked example
Why compare opaque and transparent tip caps?
Show the worked solution
- Both involve covering and handling the tip.
- Only the opaque cap excludes light, supporting a light-access comparison.
Answer The transparent cap controls covering and handling while allowing light; comparison with an opaque cap tests the importance of light reaching the tip.
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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 the roles of an opaque tip cap and transparent tip cap in the supplied comparison.
Give me a hint
Separate light exclusion from the covering treatment.
Compare my reasoning
- Opaque tip cap excludes light from the tip.
- Transparent tip cap controls covering while allowing light.
Opaque tip cap excludes light; transparent tip cap controls covering and handling while admitting light. Comparing their responses supports the importance of light reaching the tip.
Look for these in your work
- State both control roles
- Limit the inference to the comparison
Interpret response with a gelatin connection but not an impermeable barrier in the supplied model.
Give me a hint
Focus on passage of a signal, not universal transport mechanisms.
Compare my reasoning
- A mobile signal can cross the permeable connection.
- The contrast is consistent with chemical signalling to growing tissue.
The contrast is consistent with a mobile chemical signal reaching responsive tissue through a permeable connection. It does not prove that all real auxin transport is solely passive diffusion.
Look for these in your work
- Use permeability evidence
- Retain the transport-model limit
Why is tip removal alone insufficient to prove that the tip only detects light?
Give me a hint
Removing the tip changes more than stimulus reception.
Compare my reasoning
- It wounds the plant and removes an auxin source.
- Loss of growth or signal supply can affect the response.
Tip removal also wounds the plant and removes an auxin source, so loss of curvature may reflect altered growth or signal supply. Use intact cap controls and continued-growth records as well.
Look for these in your work
- Identify source and injury confounds
- Add suitable controlled comparisons
Common mistakes
- Claiming tip removal alone proves detection with no confound.
- Treating a gelatin model as proof that all transport is passive diffusion.
What can you explain now?
What does the supplied permeable-versus-impermeable connection model support?
Compare with the explanation
A mobile chemical signal is consistent with the response
It supports signal passage with stated limits.
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Make it stick
Interpret cap and permeability evidence while recognising the source and injury confounds in tip removal.
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.