Explain cell elongation and the direction of a shoot curve
Separate longer cells from more cells and apply the curve geometry.
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
What does cell elongation mean?
What you will learn
- Describe the growth effect of auxin in shoots
- Predict curvature from unequal elongation
Explain cell elongation and the direction of a shoot curve
| Feature | Meaning |
|---|---|
| Elongation | Increase in cell length |
| Division | Increase in cell number |
| Model increments | 20→30 gives 10 µm; 20→50 gives 30 µm |
| Curve | Towards the less-elongating side; no angle calculated from lengths alone |
Describe the growth effect of auxin in shoots
Auxin stimulates cell elongation in the shoot-growth model. Elongation means that a cell increases in length. It is different from cell division, which increases cell number, and from a muscle contraction, which shortens a contracting structure. Shoot growth involves several biological processes, but this syllabus explanation of tropic curvature specifically requires unequal cell elongation.
Compare matching regions on the two sides of a growing shoot. If the same number of cells on one side lengthen more than those on the other, that side becomes longer. It forms the outside of the resulting curve, while the shorter side forms the inside. The shoot therefore bends towards the side that elongates less, not towards the side where cells lengthen more.
Predict curvature from unequal elongation
An invented cell-length comparison begins at 20 µm on each side. One side reaches 30 µm and the other reaches 50 µm. The elongation increments are 10 and 30 µm respectively. Compare the increments when discussing growth, rather than treating the final length ratio of 50 to 30 as the ratio of the growth amounts.
These supplied cell lengths are a simplified matching-region model, not a universal set of plant measurements. They support a prediction of unequal elongation and the qualitative curve direction. They do not supply enough geometry to calculate a shoot curvature angle in degrees. Keep µm for cell-length changes and degrees for an independently measured angle, and do not assume root auxin responses follow the same shoot rule.
Pause and explain
Cells on the right elongate more than matching cells on the left. Which bend is predicted?
Worked example
Cells start at 20 µm; left reaches 30 and right reaches 50 µm. Which side elongates more and which way does the shoot bend?
Show the worked solution
- Left adds 10 µm; right adds 30 µm.
- The shoot bends towards the less-elongating left side.
Answer The right side elongates more; the shoot bends left towards the less-elongating side.
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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.
Distinguish cell elongation from cell division.
Give me a hint
Compare length with number.
Compare my reasoning
- Elongation increases cell length.
- Division increases cell number.
Cell elongation increases cell length; cell division increases cell number.
Look for these in your work
- Define elongation
- Define division
Calculate elongation increments for left 20→30 µm and right 20→50 µm, then predict the bend.
Give me a hint
Subtract the initial length on each side.
Compare my reasoning
- Left adds 10 µm; right adds 30 µm.
- The right is longer, so the shoot bends towards the left.
Left increment 10 µm; right increment 30 µm. The shoot bends left towards the less-elongating side.
Look for these in your work
- Calculate increments with units
- Predict the correct curve direction
Why cannot the two supplied cell-length increments alone determine a shoot curvature angle in degrees?
Give me a hint
Length difference and angular geometry are different measurements.
Compare my reasoning
- The increments show unequal elongation.
- Matching-region dimensions and curve geometry are not supplied.
The increments support unequal growth and a qualitative bend direction, but insufficient region dimensions and geometry are supplied to calculate a curvature angle.
Look for these in your work
- Use the supported qualitative prediction
- Do not invent an angle
Common mistakes
- Calling increased cell length cell division.
- Using final cell lengths as the growth increments.
What can you explain now?
Cells on the right elongate more than matching cells on the left. Which bend is predicted?
Compare with the explanation
Towards the left
The right side becomes the longer outside of the curve.
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Make it stick
Explain elongation, calculate length increments and predict the bend without confusing length with angle.
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