Explain how auxin promotes cell elongation
Link auxin signalling to proton pumping, cell-wall acidification and elongation growth.
A Level; Cambridge International Biology 9700, 2028–2030. Written answers are self-checked; this is not practical-skills certification.
Before you begin
- A control pathway links a stimulus, receptor, coordination system and response.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
How does auxin help acidify the cell-wall space?
What you will learn
- Explain auxin stimulation of proton pumping
- Relate lower cell-wall pH to wall loosening and cell elongation
Auxin and cell-wall extension
| Feature | Meaning |
|---|---|
| Auxin signal | Promotes proton-pump activity in the plasma membrane |
| Acidification | H⁺ ions enter the cell wall space and lower pH |
| Wall loosening | Acid conditions support expansin-mediated loosening |
| Elongation | A yielding wall permits turgor-driven cell expansion |
Build the mechanism
Auxin can stimulate proton pumps in the plasma membrane. Hydrogen ions move into the cell-wall space, lowering its pH. Acidification supports wall-loosening proteins such as expansins, increasing wall extensibility. With turgor pressure acting on the more yielding wall, the cell can expand and elongate.
Keep the sequence precise
Auxin promotes elongation through a sequence; it does not simply push a cell longer. Proton pumping acidifies the wall space, while turgor provides the force for expansion.
Pause and explain
A student says wall loosening alone causes the cell to lengthen. Add the missing force and sequence.
Worked example
Put these events in order: wall loosening, proton pumping, cell elongation, wall acidification.
Show the worked solution
- Auxin stimulates proton pumping.
- H⁺ lowers wall pH; loosening enables turgor-driven expansion.
Answer Auxin stimulates proton pumping → the cell-wall space acidifies → the wall loosens → turgor-driven cell elongation occurs.
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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 how auxin can promote elongation in a growing region.
Give me a hint
Name the first event, then follow each link in order.
Compare my reasoning
- Auxin stimulates proton pumping into the wall space.
- Lower pH supports loosening so turgor can expand the cell.
Auxin stimulates proton pumps that move H⁺ ions into the cell-wall space. The lower pH promotes wall loosening; turgor can then expand the wall and elongate the cell.
Look for these in your work
- Mechanism stated
- Steps linked in order
A treatment blocks proton pumps but leaves auxin concentration high. Predict the effect on auxin-driven elongation.
Give me a hint
Use the terms in the question and state what changes.
Compare my reasoning
- The wall space is less acidified.
- Wall loosening and expansion are reduced.
Auxin-driven elongation is likely reduced because blocked proton pumps prevent normal wall acidification, limiting wall loosening and turgor-driven expansion.
Look for these in your work
- Evidence interpreted
- Biological direction explained
A student says wall loosening alone causes the cell to lengthen. Add the missing force and sequence.
Give me a hint
Apply the mechanism to the new context without adding unsupported claims.
Compare my reasoning
- Acidification enables a more extensible wall.
- Turgor pressure drives expansion of the yielding wall.
Auxin promotes proton pumping and wall acidification, which increases wall extensibility. Turgor pressure then drives expansion and elongation; loosening alone is not the expansion force.
Look for these in your work
- Mechanism applied
- Limits preserved
Common mistakes
- Saying auxin itself is the proton pump
- Omitting turgor-driven expansion from the explanation
What can you explain now?
How does auxin help acidify the cell-wall space?
Compare with the explanation
It stimulates proton pumps
Proton pumps move H⁺ ions into the wall space.
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Make it stick
How do auxin, proton pumps and turgor connect to cell elongation?
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