OCR A Level Biology A: Plant responses and plant hormones (H420) – Revision Notes

These notes condense learning outcomes 5.1.5(a) to 5.1.5(f) of the OCR A Level Biology A (H420) specification, Version 4.1 (April 2026), for first assessment in 2025. For full explanations and worked data examples, read the study guide. Then try the practice questions, check the course hub and the printable checklist, and take a free diagnostic. For practical-skills revision see development of practical skills.

Key terms

Term	Meaning
Tropism	Directional growth response to a directional stimulus; positive is towards, negative is away
Nastic response	Response whose direction does not depend on the stimulus direction
Abiotic stress	Stress from non-living factors such as drought or cold
Herbivory	Feeding on plants by animals
Alkaloid	Nitrogen-containing chemical defence, toxic or bitter to herbivores
Pheromone	Airborne chemical signal released by a damaged plant, detected by neighbours
Apical dominance	Apical bud suppresses lateral bud growth
Abscission zone	Region at the base of a leaf stalk where the leaf separates
Serial dilution	Repeated dilution by a constant factor to give a concentration series

5.1.5(a) Types of response

• Tropisms: phototropism (light), geotropism or gravitropism (gravity), hydrotropism (water), thigmotropism (touch), chemotropism (chemicals).
• Abiotic stress: water shortage leads to stomatal closure; winter leads to leaf loss in deciduous plants.
• Herbivory: alkaloids (such as caffeine and nicotine), pheromones and folding on touch (Mimosa).
• Folding on touch: turgor is lost from cells at the base of each leaflet because ions leave and water follows by osmosis. It is nastic.

Organ	Light	Gravity
Shoot	Positive	Negative
Root	Often negative	Positive

Auxin in tropisms: steps to quote

1. Auxin (IAA) is made at the tip.
2. The stimulus makes it move sideways: to the shaded side, or to the lower side.
3. In a shoot, more auxin means more cell elongation on that side, so it bends towards light or away from gravity.
4. In a root, the higher auxin level inhibits elongation on the lower side, so the root bends down.
5. Mechanism of elongation: auxin makes cells pump hydrogen ions into the wall, the wall acidifies, cellulose bonds loosen and turgor stretches the cell.

Practical: tropisms (PAG11)

• Seedlings such as cress or mustard; one-sided light or horizontal position.
• Control: darkness, light from all sides, or a clinostat that rotates seedlings.
• Measure the angle of curvature with a protractor or photograph; use at least ten seedlings and calculate a mean.
• Control species, age, temperature, light intensity, humidity.

5.1.5(b) Roles of hormones

Hormone	Role
Auxin	Elongation, tropisms, apical dominance, inhibits leaf fall
Gibberellin	Stem elongation, germination (amylase)
ABA	Stomatal closure, dormancy
Ethene	Leaf fall, ripening

Stomatal closure steps

1. Water stress raises ABA in the leaf.
2. ABA binds to guard-cell receptors; calcium ions rise in the cytoplasm.
3. Ion channels open and potassium ions leave.
4. Guard-cell water potential rises, water leaves by osmosis, turgor falls.
5. Stomata close; water loss falls but carbon dioxide uptake falls too.

Leaf loss steps

1. Shorter days and colder weather lower auxin from the leaf.
2. Ethene stimulates the abscission zone.
3. Separation-layer cells make enzymes that digest cell walls.
4. The leaf falls; a protective layer seals the scar.

Germination steps

1. The seed takes up water; ABA level falls relative to gibberellin.
2. The embryo makes gibberellin.
3. Gibberellin acts on the aleurone layer and switches on transcription of the amylase gene.
4. Amylase hydrolyses starch to maltose, then glucose.
5. Glucose is respired and used for growth until photosynthesis starts.

5.1.5(c) Apical dominance evidence

Treatment	Result	Conclusion
Remove apical bud	Laterals grow	Apex normally suppresses them
Remove apex, add auxin to stump	Laterals stay short	Auxin replaces the apex
Remove apex, add plain carrier	Laterals grow	Carrier has no effect (control)
Higher auxin concentration	Greater inhibition	Effect depends on concentration

5.1.5(d) Gibberellin evidence

• Stem elongation: dwarf plants that grow taller when given gibberellin make too little of it. Dwarf plants that do not respond cannot respond to it (for example, their receptors or response pathway is faulty).
• Concentration: growth typically rises with concentration, then levels off when something else is limiting.
• Germination: embryo-less half-seeds do not digest starch unless gibberellin is added; iodine shows a clear zone where starch has gone.

5.1.5(e) Investigations and serial dilution

• Method for 1 in 10: 1 cm³ of solution plus 9 cm³ of distilled water, mixed, then repeat from the new tube.
• Check: each step divides concentration by 10. From 0.5 g dm⁻³ to 0.0005 g dm⁻³ takes three steps.
• Zero-hormone control uses distilled water.
• Plot concentration on a log scale.
• Analysis: mean and standard deviation for each group; percentage change = (new − control) ÷ control × 100.
• Statistics: Student’s t-test for a difference between two means; degrees of freedom = n₁ + n₂ − 2. When the calculated t is larger than the critical value for p = 0.05, the difference is significant, so reject the null hypothesis. Use a correlation test for a relationship between two variables.
• Variables: species, seed age, volume of solution, temperature, light, time.

5.1.5(f) Commercial uses

Use	Hormone	How it works
Control ripening	Ethene (also removal of it)	Stimulates softening and starch to sugar; positive feedback
Rooting powder	Auxin	Cuttings form roots, giving clones
Weedkiller	Synthetic auxin (such as 2,4-D)	Not broken down, so uncontrolled growth in broad-leaved weeds

Evaluate: higher yield, cheaper labour and uniform fruit supply against run-off, non-target damage and resistance.

Interpreting an unfamiliar hormone experiment

1. Identify the independent variable (treatment or concentration), the dependent variable and the control.
2. Compare every treatment with the control, not with each other, and quote values from the data with units.
3. Calculate percentage change or a rate (change ÷ time) if the data allow it.
4. Describe the trend, then any plateau or reversal, and state at what concentration it happens.
5. Link the result to the hormone’s role. Say what the result shows, and what it cannot show.
6. Suggest a limitation: small sample, no repeats, variable not controlled, only one species.

Must-know distinctions

Compare	Key difference
Tropism and nastic response	Directional against non-directional
Natural and synthetic auxin	Synthetic auxin is not broken down by the plant, so it persists
Dwarf lacking gibberellin and dwarf that cannot respond	Only the first grows when gibberellin is applied
Shoot and root response to auxin	High auxin stimulates shoot elongation but inhibits root elongation
Gibberellin and abscisic acid in seeds	Gibberellin promotes germination; ABA maintains dormancy

Quick self-test

1. Define a tropism.
2. Why does a horizontal root curve downwards?
3. Name the hormone that closes stomata and the ion that leaves guard cells.
4. Which enzyme does gibberellin stimulate in germinating barley, and where?
5. A shoot grows from 18 mm to 27 mm. What is the percentage increase?
6. How many 1 in 10 dilution steps turn 0.5 g dm⁻³ into 0.0005 g dm⁻³?
7. Why is a plain lanolin control used in an apical dominance experiment?
8. Why can a synthetic auxin kill weeds but not the cereal crop?
9. What hormone causes leaf fall, and which hormone’s decline helps trigger it?
10. Why does the leaf of Mimosa fold when touched?

Answers

1. A directional growth response to a directional stimulus.
2. Auxin collects on the lower side, where it inhibits elongation; the upper side grows faster.
3. Abscisic acid; potassium ions.
4. Amylase, in the aleurone layer.
5. (27 − 18) ÷ 18 × 100 = 50%.
6. Three steps (0.5, 0.05, 0.005, 0.0005).
7. It shows the carrier does not affect growth, so any difference is due to the hormone.
8. It causes uncontrolled growth in broad-leaved weeds, while cereals are far less affected.
9. Ethene; the decline of auxin.
10. Cells at the base of each leaflet lose turgor as ions leave and water follows by osmosis.

Where marks are usually lost

• Giving a hormone name without its role in the specific process the question asks about.
• Saying auxin “bends the shoot” without stating that it moves to the shaded side.
• Forgetting that high auxin concentrations inhibit root growth.
• Missing the control in an experiment, or comparing treatments with the wrong control.
• Quoting a percentage change from the wrong baseline.
• Stating “it is a fair test” without naming the variables kept constant.
• Describing a t-test result without stating the degrees of freedom and the null hypothesis.
• Saying stomata “close because guard cells shrink” without osmosis and turgor.
• Describing weedkillers as “poisons” rather than synthetic auxins causing uncontrolled growth.

Official syllabus

OCR, A Level Biology A (H420) specification, Version 4.1 (April 2026), first assessment 2025: learning outcomes 5.1.5(a) to 5.1.5(f).