Plant hormones Control and coordination & Use of plant hormones (HT) (incl. RP8)

SPEC 4.5.4.1, 4.5.4.2 • PAPER 2 • BIOLOGY ONLY • RP8 • ⭐

Plant Hormones & RP8

How auxin makes plants bend, why roots grow down and shoots grow up, and how farmers use plant hormones to control growth. Biology only — combined science students don't take this content. Includes Required Practical 8.

Three Failure Points to Avoid

This is the final lesson of Module 5 — and plant hormones produces some of the highest examiner-flagged errors on the whole paper:

Failure Point 1 — Spelling Trap

The correct term is phototropism (no 'h' after the 't'). Mark schemes explicitly state: "do not accept phototrophic / phototrophism". Spelling alone can cost the mark.

Failure Point 2 — Auxin in Roots (the Killer)

In one recent series, over 50% of students scored zero on a 2-mark question about auxin in roots. Students often confuse which side of the root has the most growth. In ROOTS, more auxin = less growth — opposite to shoots.

Failure Point 3 — Hormone Confusion (HT)

Auxin, gibberellins, ethene — students mix up their roles. "Auxin ripens fruit" or "gibberellins kill weeds" are common wrong answers. Memorise: auxin = weed killer / rooting / growth; ethene = ripening; gibberellins = germination + flowering + fruit size.

By the End of This Lesson Biology only

Five outcomes. The first three are for all biology students (Foundation and Higher); the last two are HT-only additions.

Objective 1

Define phototropism and gravitropism (geotropism) and identify positive/negative responses in shoots and roots.

Objective 2

Explain how unequal distribution of auxin causes unequal growth rates — in both shoots (toward light) and roots (toward gravity).

Objective 3 — RP8

Describe the method, controls, and results for investigating the effect of light or gravity on newly germinated seedlings.

Objective 4 — HT onlyState the roles of gibberellins (seed germination) and ethene (cell division + fruit ripening).

Objective 5 — HT onlyDescribe the uses of plant hormones in agriculture and horticulture; evaluate the impact of weed killers on biodiversity.

Tropisms — How Plants Sense and Respond

Plants don't have nerves or muscles, but they DO respond to their environment by growing differently — and this is coordinated by hormones.

Definition

Tropism — a growth response by a plant to an external stimulus. Two types you need to know:

Phototropism — response to light

Gravitropism (also called geotropism) — response to gravity

Positive vs Negative

A response is positive if growth is TOWARDS the stimulus, negative if growth is AWAY from it.

Plant partPhototropismGravitropism

ShootsPositive — grow toward lightNegative — grow away from gravity (upwards)

Roots(Negative — grow away from light)Positive — grow toward gravity (downwards)

Spelling Trap

It's photo tropism, not "phototrophism." The 'h' after the 't' is wrong. Mark schemes explicitly reject the misspelling.

Auxin — The Master Plant Hormone

One hormone coordinates both tropisms (phototropism and gravitropism). It's called auxin.

Spec Statement

"Unequal distributions of auxin cause unequal growth rates in plant roots and shoots." — this exact phrasing is the spec's key sentence. Memorise it.

Where Auxin Is Made

Auxin is produced at the tips of shoots and roots. From there it diffuses through nearby tissue, accumulating more on one side when a stimulus is present.

The Critical Twist — Auxin Acts Differently in Shoots vs Roots

This is the most-confused point in the topic:

In shoots: more auxin → MORE growth (cells elongate)

In roots: more auxin → LESS growth (auxin inhibits root cell elongation at high concentrations)

Why This Matters

This opposite behaviour is WHY shoots and roots respond in opposite directions to gravity. Same hormone, same redistribution — but opposite growth response, so opposite bending direction.

Auxin in Shoots — Phototropism

How a shoot bends towards light. The classic mechanism.

The 4-step Chain (memorise this for shoot phototropism)

Light hits one side of the shoot tip.

Auxin moves to the shaded side (away from light).

Cells on the shaded side elongate more than cells on the lit side.

Unequal growth bends the shoot TOWARDS the light.

Auxin in Roots — Gravitropism

How a horizontal root bends to grow downwards. This is where over half of students lose marks.

The Critical Flip vs Shoots

In SHOOTS, more auxin = MORE growth.

In ROOTS, more auxin = LESS growth. Auxin INHIBITS root cell elongation at high concentrations.

That's why in roots, the side with MORE auxin grows LESS, the side with LESS auxin grows MORE — making the root bend in the opposite direction to a shoot.

Mark-Scheme Phrasing — Use Comparative Language

Examiners flag that students lack comparative language. Don't write "the lower side has auxin." Write "the LOWER side has MORE auxin than the UPPER side, so the upper side grows MORE, bending the root DOWN."

RP8 — Investigating Tropism

Required Practical 8: investigate the effect of light or gravity on the growth of newly germinated seedlings.

RP8 Method (Either Investigation)

1

Grow seedlings (same species, same age) in identical conditions until just germinated.

2

Set up TWO apparatuses: one experimental (with the stimulus), one control (without).

3

Leave for several days.

4

Record results as length measurements AND careful, labelled biological drawings .

Light Investigation

IV: direction of light (from one side OR from above)

DV: direction and length of shoot growth

Setup: light from one side vs all directions/above (control)

Expected result: shoots bend toward the light source

Gravity Investigation

IV: orientation of seedlings (horizontal or vertical)

DV: direction and length of root and shoot growth

Setup: horizontal petri dish vs vertical (control)

Expected result: roots bend down; shoots bend up

RP8 — Controls & Recording

The most-examined aspect of this practical. Mark schemes test whether students understand WHY each control is needed.

Why a Control Apparatus Is Needed

The control rules out the variable you're investigating. For example, in a gravity experiment, the vertical control proves that any bending you see in the horizontal apparatus is caused by gravity (not by some other factor).

Other Variables to Control

Same species and age of seedling — ensures consistent baseline growth rates.

Same temperature — affects growth rate independently.

Same water supply — both affects growth AND can affect direction (roots grow toward water sources).

Darkness (for gravity investigation) — prevents light affecting direction of growth.

Recording Results — SPEC requirement

The spec specifies students must record BOTH:

1

Length measurements (quantitative — measure shoot/root lengths in mm)

2

Careful, labelled biological drawings (qualitative — show direction of bending)

Skipping either one loses RP8-method marks. Biological drawings should be in pencil, labelled, and show observed structures faithfully.

Mark-Scheme Word Trap

If a question asks how a control seedling grew, the mark scheme accepts "straight" but NOT "straighter". "Straighter" implies some bending — which contradicts a control showing no directional stimulus.

HT — Gibberellins & Ethene HT only

Two more plant hormones HT students need to know — by name and main role. The spec explicitly states mechanisms are NOT required.

Gibberellins

HT onlyGibberellins are important in initiating seed germination — this is the spec phrasing. They trigger seeds to start growing (break dormancy).

Ethene

HT onlyEthene controls cell division and ripening of fruits. Ethene is a gas — unusual for a hormone — released by ripening fruit (which is why one ripe banana ripens nearby ones).

Mechanisms NOT Required

The spec specifically states: "The mechanisms of how gibberellins and ethene work are not required." Just know the hormones, what they do, and their uses (next slide). Don't over-study them.

HT — Uses in Agriculture & Horticulture HT only

Spec section 4.5.4.2. All three hormones have specific named uses — memorise the spec list.

HormoneUses (spec verbatim)

Auxinsas weed killers as rooting powders for promoting growth in tissue culture

EtheneIn the food industry to control ripening of fruit during storage and transport

Gibberellinsto end seed dormancy to promote flowering to increase fruit size

How Auxin Weed Killers Work

Auxin weed killers are selective — they affect broad-leaved plants (weeds) far more than narrow-leaved plants (cereals, grass). The weeds absorb so much auxin that uncontrolled growth kills them.

Highest-Confusion Area

Past papers flag students mixing up the three hormones. A useful memory aid:

Auxin → Agriculture (weed killer, rooting, tissue culture).

Ethene → Eat (ripening fruit for food).

Gibberellins → Germination, growth-promoting (flowering, fruit size).

HT — Weed Killers & Biodiversity HT only

The spec links this to Working Scientifically 1.3 and 1.4 — the everyday use of hormones as weed killers has effects on biodiversity.

The Spec Question

"Understand how the everyday use of hormones as weed killers has an effect on biodiversity." Examiners frame this as evaluation questions in the extended-response section.

✓ Benefits

Removes competition with crops → higher yields

Reduces need for manual weeding (labour, time, fuel)

Selective — targets broad-leaved weeds while sparing cereal crops

✗ Biodiversity Costs

Reduces wildflower populations

Fewer wildflowers → fewer insects (especially pollinators)

Fewer insects → fewer birds and small mammals → ecosystem disruption

Can also affect non-target plant species through soil/water spread

For Evaluation Questions

Include at least one named ecological consequence (e.g. "fewer wildflowers → fewer pollinating insects") to score the biodiversity mark. Vague statements like "harms the environment" don't score.

Examiner Alert — Specific Traps

Three traps that appear in examiner reports every series for this topic.

Trap 1 — Spelling

The correct term is phototropism / phototropic. Mark schemes explicitly state "do not accept phototrophic / phototrophism". Don't add the 'h'.

Trap 2 — Auxin Direction in Shoots

Auxin moves to the side AWAY from the light (the shaded side). Students often reverse this. The mark scheme phrasing: "more auxin in the side furthest from the light."

Trap 3 — Auxin Effect in Roots

In roots, auxin INHIBITS cell elongation. More auxin = LESS growth in roots. So the side with MORE auxin grows LESS, and the root bends in that direction. This is the opposite of shoots.

Worked Example — PEE Framework

The textbook phototropism question. 4 marks, AO2.

Q: Explain why a shoot grown with light coming from one side bends towards the light. 4 marks

P

Point. Light hits one side of the shoot tip more than the other.

E

Evidence. Auxin is produced at the shoot tip and moves to the shaded side (the side furthest from the light), accumulating there.

P

Point. The cells on the shaded side elongate more than the cells on the lit side.

E

Explain. The unequal growth (more on the shaded side, less on the lit side) bends the shoot TOWARDS the light — known as positive phototropism.

Why this scores 4/4

One mark per named step: light asymmetry ✓ · auxin to shaded side ✓ · cells elongate more on shaded side ✓ · bending towards light (positive phototropism) ✓. Spec-aligned phrasing throughout.

Your Turn

Explain how auxin causes the root to bend downwards. 4 marks

This is the highest-failure question type in the topic — over half of students scored zero in a recent series. Focus on comparative language: which side has more auxin, and which side grows more.

1

Gravity causes auxin to accumulate on the LOWER side of the root (the side furthest from "up").

2

In roots, auxin INHIBITS cell elongation at high concentrations.

3

So the upper side (with less auxin) grows MORE than the lower side (with more auxin).

4

Unequal growth — more on top, less on bottom — bends the root downwards (positive gravitropism).

Mark-scheme matches: auxin moves to lower side ✓ · auxin inhibits root growth ✓ · upper side grows more (comparative) ✓ · root bends down ✓. 4 / 4.

⚠ Common loss: writing "the side with more auxin grows more" (which is true in shoots but FALSE in roots) — instant loss of multiple marks.

Common Errors to Avoid

The four most-flagged mistakes from examiner reports.

✗ Wrong

"Auxin makes the plant move."

✓ Right

Auxin causes unequal growth , which makes the plant bend .

✗ Wrong

"In roots, the side with more auxin grows more."

✓ Right

In roots, auxin INHIBITS growth. The side with more auxin grows LESS.

✗ Wrong

"Ethene helps seed germination." (HT)

✓ Right

Gibberellins initiate germination. Ethene controls cell division and ripening of fruit.

✗ Wrong

"Phototrophism" (with an 'h')

✓ Right

Phototropism (no 'h' after 't'). Mark schemes explicitly reject the misspelling.

Recap + Module 5 Wrap-Up

1. Tropisms & Auxin

Phototropism = response to light; gravitropism = response to gravity.

Unequal distributions of auxin cause unequal growth rates (spec verbatim).

SHOOTS: more auxin = more growth → bend TOWARDS light/away from gravity.

ROOTS: more auxin = LESS growth (auxin inhibits) → bend TOWARDS gravity (down).

2. RP8 Essentials

Investigate light OR gravity on newly germinated seedlings.

Controls: same species + age + temperature + water.

Record BOTH length measurements AND labelled biological drawings.

3. HT — Three Hormones, Many Uses

Auxin : weed killers, rooting powders, tissue culture.

Ethene : controls ripening of fruit (food industry).

Gibberellins : end seed dormancy, promote flowering, increase fruit size.

Weed killers reduce biodiversity (fewer wildflowers → fewer pollinators).

End of Module 5 — Homeostasis & Response

You've now completed all ten lessons of Module 5. The module spanned the nervous system (including the brain and eye), body temperature control, the endocrine system and hormonal coordination (blood glucose, water and nitrogen balance, reproduction, infertility, negative feedback), and finally plant hormones. Next is Module 6: Inheritance, Variation and Evolution.