3.6.9Inquiry Process
Regulating Plant Growth and Fruit Ripening — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 149 of 160
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5Apical Dominance
Auxin from the tip
- Auxin from the tip The apical bud produces auxin that moves down the stem and inhibits the growth of lateral buds.
Cytokinin balance
- Cytokinin balance Cytokinin from the roots promotes lateral bud growth, so the ratio of the two hormones matters.
Pruning
- Pruning Removing the shoot tip removes the auxin source, so lateral buds grow and the plant becomes bushier.
Advantage
- Advantage Concentrating growth upwards helps a plant compete for light before it branches.
Gibberellin and branching
- Gibberellin and branching Gibberellins can also encourage stem elongation,
- so the combined balance of hormones determines the final shape of the plant.
Part 2 of 5Fruit Ripening and Ethylene
1
Ripening begins when a fruit starts to produce small amounts of ethylene gas.
2
Ethylene stimulates cells to produce more ethylene, which is positive feedback.
3
Enzymes break down chlorophyll, so green fruit changes colour, and others convert starch to sugar so the fruit becomes sweet.
4
Pectinase enzymes break down cell walls, softening the fruit.
5
Ethylene spreads as a gas and ripens neighbouring fruit, so one ripe fruit can speed ripening of those near it.
6
Ripe fruit is soft, sweet and brightly coloured, so animals eat it and disperse the seeds, which supports reproduction of the plant.
Part 3 of 5Commercial Applications
Commercial Applications
Ripening control
- Ripening control Fruit is picked unripe and transported, then exposed to ethylene to ripen it just before it is sold.
Delaying ripening
- Delaying ripening Storing fruit in low oxygen with ethylene removed slows ripening and extends shelf life.
Gibberellins
- Gibberellins Spraying gibberellin makes seedless grapes grow larger and is used in brewing to speed up malting.
Auxin rooting powder
- Auxin rooting powder Cuttings dipped in synthetic auxin form roots faster, helping the propagation of plants.
| Hormone | Use | Effect |
|---|---|---|
| Ethylene | Fruit ripening rooms | Even ripening |
| Auxin analogue | Rooting powder | Roots form |
| Gibberellin | Seedless grapes | Larger fruit |
Part 4 of 5Worked Example
Question
- Banana fruit produces 0.5 units of ethylene per hour.
- If the amount doubles every 6 hours because of positive feedback, calculate the ethylene produced per hour after 24 hours.
- 24 hours contain 24 ÷ 6 = 4 doublings.
- Rate = 0.5 × 2⁴ = 0.5 × 16.
- Rate = 8 units per hour.
Answer 8 units per hour, a 16-fold increase, showing why positive feedback produces rapid ripening.
Part 5 of 5Exam Checklist & Tips
3.6.9 – Key Points for the Exam
- Apical dominance Inhibition of lateral buds by auxin from the apical bud.
- Ethylene Gaseous hormone that stimulates fruit ripening.
- Positive feedback Process in which a response increases the original stimulus.
- Pectinase Enzyme that breaks down pectin in cell walls, softening fruit.
Common Mistakes
- Saying that cutting the tip stops all growth; it releases lateral buds.
- Describing ripening as slowed by ethylene; ethylene speeds it up.
- Confusing positive feedback with a positive outcome.
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