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A-Level Biology May/June 2025 Q8: The presence of gibberellins in a plant cell leads to the expression of genes involved…
A-Level Biology · Paper 9700/41 · May/June 2025 · Question 8 · [6 marks]
The presence of gibberellins in a plant cell leads to the expression of genes involved in stem elongation. Describe how gibberellin causes stem elongation in plants.
A full-marks model answer with a mark-by-mark examiner breakdown is below.
1 answer
- accepted ✓
Gibberellin is a plant hormone that diffuses into the target cell and binds to a specific gibberellin receptor. This binding event causes a repressor protein, known as a DELLA protein, to be broken down or destroyed.
The destruction of the DELLA protein releases a transcription factor, such as a Phytochrome Interacting Factor (PIF), which was previously bound and inactivated by DELLA. This freed transcription factor can now move to the nucleus and bind to the promoter region of a specific gene on the DNA.
This binding initiates the transcription of genes involved in stem growth. For example, the gene coding for the enzyme xyloglucan endotransglucosylase (XET) is expressed. The XET enzyme acts on the plant cell wall, loosening it by breaking cross-links between cellulose microfibrils. The loosened wall has reduced resistance to stretching, allowing the cell to take up water by osmosis. The resulting increase in turgor pressure causes the cell to expand and elongate, leading to overall stem elongation.
How the marks are awarded
- M1 — States that gibberellin binds to its receptor, which is the initial step in the signalling cascade.
- M1 — Correctly identifies that the DELLA protein is destroyed as a result of gibberellin binding.
- M1 — Explains that a transcription factor (PIF) is released or freed from the DELLA protein.
- M1 — Describes the transcription factor binding to the promoter region of the DNA.
- M1 — States that a specific gene involved in growth, such as the gene for XET, is switched on or transcribed.
- M1 — Links the action of the resulting enzyme to the loosening of the cell wall.
Common mistakes
- Confusing the mechanism of gibberellin with that of auxin, for example by mentioning proton pumps or expansins.
- Providing a vague answer like 'gibberellin activates genes' without mentioning the specific roles of DELLA proteins and transcription factors.
- Forgetting to link the loosening of the cell wall to the subsequent uptake of water by osmosis, which is the direct cause of cell expansion.
- Incorrectly stating that gibberellin binds directly to DNA, omitting the entire signal transduction pathway involving receptors and repressor proteins.
Examiner tip: For questions on signalling pathways, learn the sequence of events as a flowchart, including the names of key molecules like receptors, repressors (DELLA), and transcription factors.
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