3.6.8Inquiry Process
Plant Hormones — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 148 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.
Section 1 of 5Five Main Plant Hormones
Auxin
- Auxin Promotes cell elongation in shoots, maintains apical dominance and helps control tropisms.
Cytokinins
- Cytokinins Made mainly in roots, they promote cell division and delay leaf senescence.
Gibberellins
- Gibberellins Promote stem elongation and stimulate seed germination by inducing the synthesis of enzymes such as amylase.
Abscisic acid
- Abscisic acid Causes stomata to close under water stress and maintains seed dormancy.
Ethylene
- Ethylene A gas that stimulates fruit ripening and leaf and fruit abscission.
Sites of action
- Sites of action Hormones travel from where they are made to cells that carry the right receptors,
- so one hormone can have different effects in different organs.
| Hormone | Main source | Key effect |
|---|---|---|
| Auxin | Shoot tip | Elongation |
| Cytokinin | Root tips | Cell division |
| Gibberellin | Young leaves, embryos | Germination, stem growth |
| Abscisic acid | Leaves, roots | Stomatal closure |
| Ethylene | Ripening fruit | Ripening |
Section 2 of 5Hormone Interactions
Hormone Interactions
Antagonism
- Antagonism Gibberellins promote germination while abscisic acid maintains dormancy, so the balance between them decides when seeds germinate.
Balance
- Balance The ratio of auxin to cytokinin influences whether tissue grows shoots or roots in tissue culture.
Synergy
- Synergy Auxin and gibberellin together can stimulate more stem growth than either alone.
Concentration
- Concentration The same hormone may stimulate in low concentration and inhibit at high concentration or in different organs.
Commercial relevance
- Commercial relevance Because hormones interact, growers manage crops by altering one hormone and relying on the balance with the others to give the desired result.
Section 3 of 5Plants and Animals Compared
1
Plant hormones are produced by cells in many tissues and not by specialised glands.
2
They move by diffusion, active transport and the phloem or xylem, and so travel more slowly than animal hormones in blood.
3
Plant hormones often act close to where they are made, and effects depend on concentration and tissue type.
4
Responses are mainly changes in growth and development rather than quick movements.
Section 4 of 5Worked Example
Question
- Barley seeds are soaked in gibberellin and others in water.
- After three days, 18 of 20 seeds in gibberellin have germinated and 6 of 20 in water.
- Calculate the percentage germinated in each group.
- Gibberellin group = 18 ÷ 20 × 100 = 90%.
- Water group = 6 ÷ 20 × 100 = 30%.
- Difference = 90 − 30 = 60 percentage points.
Answer 90% versus 30%, so gibberellin increases germination by 60 percentage points.
Section 5 of 5Exam Checklist & Tips
3.6.8 – Key Points for the Exam
- Auxin Hormone promoting cell elongation and apical dominance.
- Gibberellin Hormone promoting germination and stem elongation.
- Abscisic acid Hormone causing stomatal closure and dormancy.
- Ethylene Gaseous hormone promoting fruit ripening.
Common Mistakes
- Saying ethylene is a liquid; it is a gas.
- Claiming abscisic acid promotes growth; it mostly inhibits it.
- Stating that plant hormones are made in glands.
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