Negative feedback (HT only) — AQA GCSE Biology

SPEC 4.5.3.7 • PAPER 2 • HT ONLY • ⭐ PRIORITY

Negative Feedback — Adrenaline & Thyroxine

Two key hormones, one feedback principle. How the body responds to stress, controls metabolism, and keeps internal conditions stable. Entire lesson is Higher Tier.

Two Mistakes That Cost Marks Every Year

This topic is a Paper 2 staple — and the same two failure points appear in examiner reports every series:

Failure Point 1 — Vague Adrenaline Answers

Phrases like "increased awareness", "more energy", or "makes you alert" are NOT credited. Mark schemes want specific named effects from the spec: increased heart rate and boosts delivery of oxygen and glucose to brain and muscles. In one recent series, only 55% of students scored full marks for this reason.

Failure Point 2 — "Produces" Energy

For thyroxine and metabolism questions: never say respiration "produces", "makes", or "creates" energy. The mark scheme explicitly rejects these. Energy is released by respiration. Same rule applies across the whole spec — a single word change can cost 2 marks.

In the next 10 minutes, you'll know exactly what to write for both hormones and how to interpret any negative feedback diagram.

By the End of This Lesson HT only

Four outcomes, all Higher Tier. This topic appears on every Paper 2.

Objective 1

Explain the role of adrenaline — where it comes from, what triggers it, and its specific effects on the body.

Objective 2

Explain the role of thyroxine — its source, function (basal metabolic rate, growth and development), and how it affects body temperature.

Objective 3

Describe how thyroxine levels are controlled by negative feedback.

Objective 4 — MS 2c

Interpret and explain simple diagrams of negative feedback control.

Adrenaline — Source, Trigger, Effects

The "stress hormone" — released for short-term, immediate responses to threat.

Spec Definition

HT onlyAdrenaline is produced by the adrenal glands in times of fear or stress. It increases the heart rate and boosts the delivery of oxygen and glucose to the brain and muscles, preparing the body for 'flight or fight'.

Source (two adrenal glands)

The adrenal glands sit on top of each kidney — one per side. They release adrenaline directly into the bloodstream.

The Three Named Effects

Increases the heart rate — pumps oxygenated blood around the body faster.

Boosts the delivery of oxygen to brain and muscles.

Boosts the delivery of glucose to brain and muscles.

Spec Wording

Note the spec says "flight or fight" — not "fight or flight." This is the spec's exact ordering, and mark schemes follow spec wording. Either order is usually accepted, but match the spec for safety.

Why "Flight or Fight" Works

The three effects aren't random — together they prepare the body for sudden physical action.

Adrenaline released

↑ heart rate

↑ aerobic respiration in muscles

More energy released → ready to run or fight

The Logic

Muscles need oxygen and glucose for respiration to release the energy they need to contract. More oxygen + more glucose + a faster heartbeat to deliver them = more energy available to run or fight.

Mark-Scheme Phrasing

Examiners do not credit phrases like "gives you more energy" or "makes you stronger." Always go through the respiration step: more oxygen + glucose → more respiration → releases more energy.

Thyroxine — Source & Role

The long-term metabolism hormone. Where adrenaline acts in seconds, thyroxine acts over hours and days.

Spec Definition

HT onlyThyroxine, from the thyroid gland, stimulates the basal metabolic rate. It plays an important role in growth and development.

Source — Thyroid Gland

The thyroid gland sits at the front of the neck, around the windpipe. It releases thyroxine into the bloodstream continuously.

Two Spec Roles

Stimulates basal metabolic rate — the rate of chemical reactions in cells when at rest. Higher thyroxine → faster cellular reactions.

Plays an important role in growth and development — particularly important in childhood and adolescence.

Memory Hook

Thyroid → Thyroxine. The hormone is named after the gland. And the "basal" in basal metabolic rate just means "baseline" — your cellular activity at rest, before exercise.

Thyroxine → Metabolic Rate → Body Temperature

The full causal chain. This is the highest-yield mark-scheme question on the topic.

↑ thyroxine

↑ basal metabolic rate

↑ respiration

Releases energy /

The Two Mark Points

For a typical 2-mark question on thyroxine and body temperature, mark schemes credit:

Thyroxine increases (basal) metabolic rate (allow: "increases respiration" or "increases reactions in cells").

Respiration releases energy (allow: "releases heat", "is exothermic").

Banned Words

Mark schemes explicitly do not accept "produced", "made", or "created" for energy. Energy can only be released, transferred, or stored — never produced.

Examiner Insight

Examiner reports note that on this 2-mark question, only about 1 in 9 students got both marks. The most common miss: skipping the respiration step and jumping straight from "metabolic rate" to "warmer."

Negative Feedback — The General Principle

The core homeostasis idea. Internal conditions are kept stable by responses that oppose any deviation from the ideal.

Definition

Negative feedback — a control mechanism where a deviation from the ideal level triggers a response that reverses the change, bringing the level back to the ideal.

The Core Idea

"Negative" means opposing, not "decreasing." If a level is too high, the response decreases it. If a level is too low, the response increases it. The system always opposes the change.

Negative Feedback Applied to Thyroxine

The spec explicitly names thyroxine as an example of a negative feedback control. The loop involves the thyroid and the pituitary.

The Loop in Two Sentences

Thyroxine too high → pituitary releases less stimulating hormone → thyroid makes less thyroxine → level falls back to ideal.

Thyroxine too low → pituitary releases more stimulating hormone → thyroid makes more thyroxine → level rises back to ideal.

GCSE Note

The "stimulating hormone" the pituitary releases is called TSH (thyroid-stimulating hormone) — but the spec doesn't require you to name it. Just say "the pituitary releases a stimulating hormone." Mark schemes credit both phrasings.

MS 2c — Reading Feedback Diagrams

You may be given a generic negative feedback diagram (with "Hormone A", "Hormone B", "Substance Q" labels) and asked to describe what's happening. Here's how to read any such diagram.

Three-Step Method

Identify the ideal level box — usually in the middle, the "set point."

Trace the "too high" loop — what hormone is released, what effect it has, how it brings the level back down.

Trace the "too low" loop — what (different) hormone is released, how it brings the level back up.

Generic Answer Template (3 marks)

For a typical 3-mark "explain how Substance Q is controlled" question:

If Substance Q becomes high, Hormone A is released.

If Substance Q becomes low, Hormone B is released.

Hormone A (or B) brings Substance Q back to the ideal / normal level.

Three sentences = three marks. Use the diagram's labels exactly (don't rename Hormone A as "insulin" unless the question says so).

Examiner Alert — Banned Words & Vague Phrases

Two specific traps that appear in every examiner report on this topic.

Trap 1 — "Produces" Energy

For any question involving respiration, metabolism, or energy release, mark schemes explicitly reject the words "produced", "made", or "created". Always use "released", "transferred", or "exothermic".

Trap 2 — Vague Adrenaline Effects

For adrenaline questions, generic phrases lose marks. Use named, specific effects only.

✗ Loses Marks

"Adrenaline gives you more energy and makes you more alert."Vague — no specific named effects.

✓ Gets Marks

"Adrenaline increases the heart rate and boosts the delivery of oxygen and glucose to the brain and muscles."Spec wording — full credit.

Worked Example — PEE Framework

A 4-mark adrenaline question — typical of every Paper 2 in this topic.

Q: Explain how adrenaline prepares the body for a 'flight or fight' response. 4 marks

P

Point. Adrenaline is released by the adrenal glands in response to fear or stress.

E

Evidence. Adrenaline increases the heart rate, pumping blood around the body faster.

P

Point. Adrenaline also boosts the delivery of oxygen and glucose to the brain and muscles.

E

Explain. Brain and muscle cells therefore have more oxygen and glucose for respiration, which releases more energy — so the body can react rapidly (fight or flee).

Why this scores 4/4

One mark per named point: source (adrenal glands) ✓ · heart rate ✓ · O₂/glucose delivery to brain and muscles ✓ · respiration releases energy ✓. The "released" wording protects the energy mark.

Your Turn

Using your knowledge of negative feedback, explain how the body returns thyroxine levels to normal. 3 marks

Three marks = three sequential steps in the feedback chain. Think: which gland senses the low level? What does it release? What is the effect?

1

The pituitary releases more stimulating hormone (TSH not required at GCSE — "a stimulating hormone" is accepted).

2

This stimulates the thyroid to release more thyroxine.

3

Thyroxine levels rise back to the ideal / normal level.

Mark-scheme matches: pituitary releases (more) stimulating hormone ✓ · thyroid releases more thyroxine ✓ · levels return to normal ✓. 3 / 3.

⚠ Common loss: skipping the pituitary step and saying "the thyroid senses the low level and releases more thyroxine." The thyroid is the effector — the pituitary is the sensor/controller. Both are needed for full marks.

Common Errors to Avoid

Four mistakes the examiner reports flag every series.

✗ Wrong

"Adrenaline produces energy."

✓ Right

Adrenaline boosts oxygen and glucose delivery so respiration releases more energy.

✗ Wrong

"Thyroxine is made by the adrenal glands."

✓ Right

Thyroxine — thyroid gland. Adrenaline — adrenal glands.

✗ Wrong

"Thyroxine controls weight."

✓ Right

Thyroxine controls basal metabolic rate (broader than weight — affects all cellular reactions).

✗ Wrong

"Negative feedback always reduces the level."

✓ Right

Negative feedback opposes the change — it increases a low level OR decreases a high level.

Summary — Three Takeaways

1. Two Hormones, Two Glands, Two Speeds

Adrenaline — from adrenal glands ; triggered by fear/stress; short-term flight or fight .

Effects: ↑ heart rate; boosts O₂ and glucose delivery to brain and muscles.

Thyroxine — from thyroid gland ; controls basal metabolic rate + growth/development.

2. The Thyroxine Chain (Body Temperature)

↑ thyroxine → ↑ basal metabolic rate → ↑ respiration → releases energy/heat → body warms up.

Critical: respiration releases energy — never "produces", "makes", or "creates."

3. Negative Feedback

System opposes deviation from the ideal level.

Too high → response reduces it. Too low → response raises it. Both directions, both negative feedback.

Thyroxine is the spec-named example — controlled by the pituitary–thyroid loop.

Next Lesson

Lesson 5.10 — Plant hormones: control and coordination, including auxin, gravitropism, phototropism, and Required Practical 8 (RP8). The final lesson of Module 5.