Negative feedback (HT only) — AQA GCSE Biology
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Negative feedback (HT only) explained
Thyroxine and adrenaline are hormones that help the body respond to change.
Read the full explanation
Thyroxine is produced by the thyroid gland and controls the metabolic rate, so it affects how quickly cells release energy from food. Its level is controlled by negative feedback: if thyroxine is too low, the hypothalamus and pituitary gland stimulate the thyroid to release more; if it is too high, release is reduced. Adrenaline is produced by the adrenal glands in times of fear or stress. It increases heart rate and boosts delivery of oxygen and glucose to the brain and muscles, preparing the body for ‘flight or fight’. Together they show how hormones maintain stable internal conditions and enable rapid responses.
Adrenaline is produced by the adrenal glands in times of fear or stress.
Adrenaline is a hormone released by the adrenal glands, which sit above the kidneys. It is produced in times of fear or stress, for example when a person is startled or faces danger. Once in the bloodstream, adrenaline travels to target organs and triggers rapid responses. It increases heart rate and boosts the delivery of oxygen and glucose to the brain and muscles, preparing the body for ‘flight or fight’. This is an example of hormonal coordination: a gland detects a change and secretes a chemical messenger that affects distant target organs. Students should be able to name the gland, state the trigger and link the hormone to its effects.
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’.
When adrenaline is released in times of fear or stress, it increases the heart rate. A faster heartbeat circulates blood more quickly, so oxygen and glucose are delivered to the brain and muscles at a higher rate. Oxygen is needed for aerobic respiration, and glucose is the fuel respired to release energy for muscle contraction. This prepares the body for ‘flight or fight’: either running away from danger or staying to confront it. Students should trace the sequence from adrenaline release to increased heart rate, then to faster delivery of oxygen and glucose, and finally to more energy available in the brain and muscles.
Thyroxine from the thyroid gland stimulates the basal metabolic rate. It plays an important role in growth and development.
Thyroxine is a hormone secreted by the thyroid gland in the neck. It travels in the blood and raises the basal metabolic rate, the rate at which the body uses oxygen and energy while at rest. A useful method is to compare oxygen consumption or heat release in resting animals: more thyroxine means more respiration, so more oxygen is used and more heat is released. Thyroxine also controls growth and development, for example it is needed for normal bone growth and for the development of the brain and nervous system in young mammals. In amphibians such as frogs, thyroxine triggers metamorphosis from tadpole to adult. Because it affects almost every cell, thyroxine must be kept within a narrow range; too little slows metabolism and growth, while too much speeds metabolism and can cause weight loss and a rapid heartbeat.
Thyroxine levels are controlled by negative feedback.
Negative feedback keeps thyroxine concentration close to a set point. The pituitary gland detects the level of thyroxine in the blood and releases thyroid stimulating hormone (TSH). TSH travels to the thyroid gland and stimulates it to release thyroxine. As thyroxine rises, it inhibits the pituitary gland, so less TSH is released and thyroxine production falls. When thyroxine falls below the set point, inhibition is reduced, so more TSH is released and thyroxine rises again. This is a loop: a change is detected, a response opposes the change, and the original level is restored. The same principle explains why thyroxine stays stable despite changes in activity or temperature.
Your focus
- Describe the role of thyroxine in controlling metabolic rate.
- Explain how negative feedback controls thyroxine levels.
- Explain how adrenaline prepares the body for ‘flight or fight’.
Show all 15 objectives
- Identify the adrenal glands as the source of adrenaline.
- State that adrenaline is produced in times of fear or stress.
- Describe adrenaline as a hormone that prepares the body for ‘flight or fight’.
- Describe how adrenaline increases heart rate.
- Explain how increased heart rate boosts delivery of oxygen and glucose to the brain and muscles.
- Explain how these changes prepare the body for ‘flight or fight’.
- State that the thyroid gland releases thyroxine into the blood.
- Explain how thyroxine stimulates the basal metabolic rate and why this affects oxygen use and heat release.
- Describe the role of thyroxine in growth and development, using at least one named example.
- Identify the pituitary gland and thyroid gland in the thyroxine control pathway.
- Explain how TSH changes when thyroxine concentration rises or falls.
- Describe negative feedback as a loop that opposes a change and restores a set point.
Negative feedback (HT only) exam tips
Marking Points
- State that thyroxine is produced by the thyroid gland and controls the metabolic rate.
- Explain that thyroxine levels are controlled by negative feedback involving the hypothalamus and pituitary gland.
- State that adrenaline is produced by the adrenal glands in times of fear or stress.
- Explain that adrenaline increases heart rate and boosts delivery of oxygen and glucose to the brain and muscles.
- Link adrenaline to preparation for ‘flight or fight’ and thyroxine to long-term control of metabolism.
- Identify the adrenal glands as the source of adrenaline.
- State that adrenaline is produced in times of fear or stress.
- Describe adrenaline as a hormone transported in the blood to target organs.
- Link adrenaline release to preparation for ‘flight or fight’.
- Give a concrete example, such as a sudden scare causing a faster heartbeat.
- State that adrenaline increases the heart rate.
- Explain that a faster heart rate increases blood flow to the brain and muscles.
- State that this boosts the delivery of oxygen and glucose to the brain and muscles.
- Link oxygen and glucose to aerobic respiration and energy release for muscle contraction.
- Conclude that these changes prepare the body for ‘flight or fight’.
- Thyroxine is produced by the thyroid gland and transported in the blood plasma to target cells around the body.
- It stimulates the basal metabolic rate, increasing the rate of aerobic respiration in cells at rest.
- A higher basal metabolic rate means more oxygen is used and more energy is released as heat.
- Thyroxine is essential for normal growth and development, including bone growth and development of the brain and nervous system.
- In amphibians, thyroxine controls metamorphosis, showing its role in development.
- Thyroxine concentration must be controlled within narrow limits because it affects almost all cells.
- The pituitary gland detects thyroxine concentration and releases TSH.
- TSH stimulates the thyroid gland to release thyroxine into the blood.
- Rising thyroxine levels inhibit the pituitary gland, reducing TSH release.
- Falling thyroxine reduces inhibition, so TSH release increases and thyroxine rises.
- The loop restores thyroxine concentration towards its normal level, which is an example of negative feedback.
Examiner Tips
- 💡Name the gland and the hormone together in every answer to avoid confusion.
- 💡For thyroxine, mention negative feedback and the role of the hypothalamus and pituitary gland.
- 💡For adrenaline, link the increased heart rate to faster delivery of oxygen and glucose to the brain and muscles.
- 💡Use the phrase ‘in times of fear or stress’ to show the trigger clearly.
- 💡Name the adrenal glands precisely rather than writing ‘glands’ alone.
- 💡Link the hormone to a rapid body response to show understanding of its function.
- 💡Write the sequence in order: adrenaline, increased heart rate, faster blood flow, more oxygen and glucose to brain and muscles.
- 💡Link oxygen and glucose to respiration so the answer explains why delivery matters.
- 💡Use the phrase ‘flight or fight’ to show the purpose of the response.
- 💡Link thyroxine to respiration by writing that a higher basal metabolic rate means more aerobic respiration, so more oxygen is used and more heat is released.
- 💡Use a named example, such as metamorphosis in a frog, to show that thyroxine controls development as well as metabolism.
- 💡When a question asks why thyroxine concentration must be controlled, refer to its wide-ranging effects on cells and the need to keep metabolism and growth within normal limits.
- 💡Name the two organs in order: pituitary gland and thyroid gland, and state the hormone each releases.
- 💡Use the phrase 'opposes the change' when explaining negative feedback, then apply it to thyroxine by saying that high thyroxine reduces TSH release.
- 💡If asked to predict the effect of an underactive thyroid, trace the loop: less thyroxine means less inhibition, so TSH levels rise.
Common Mistakes
- Confusing the glands: thyroxine comes from the thyroid gland, while adrenaline comes from the adrenal glands; correct this by naming each gland with its hormone.
- Thinking adrenaline lowers heart rate; correct this by stating that adrenaline increases heart rate.
- Describing thyroxine as a fast stress hormone; correct this by explaining that thyroxine controls metabolic rate and is regulated by negative feedback.
- Naming the thyroid gland instead of the adrenal glands; correct this by stating that adrenaline is produced by the adrenal glands.
- Saying adrenaline is a nerve impulse; correct this by describing it as a hormone carried in the blood.
- Stating that adrenaline is released only during exercise; correct this by explaining that fear or stress is the trigger.
- Saying adrenaline decreases heart rate; correct this by stating that it increases heart rate.
- Writing that adrenaline delivers only oxygen; correct this by including glucose as well as oxygen.
- Forgetting the target organs; correct this by naming the brain and muscles as the destinations for the increased oxygen and glucose.
- Saying thyroxine is made by the pituitary gland; the correction is that the thyroid gland produces thyroxine, while the pituitary gland releases TSH, which stimulates the thyroid.
- Confusing thyroxine with adrenaline; the correction is that adrenaline prepares the body for rapid action, whereas thyroxine raises the resting metabolic rate over a longer period.
- Stating that thyroxine only affects growth; the correction is that it also stimulates the basal metabolic rate in adults as well as supporting growth and development.
- Writing that thyroxine stimulates its own release; the correction is that high thyroxine inhibits the pituitary gland, reducing further release.
- Confusing the roles of the pituitary and thyroid glands; the correction is that the pituitary releases TSH, which stimulates the thyroid to release thyroxine.
- Describing negative feedback as a process that increases the original change; the correction is that negative feedback opposes the change and returns the level towards the set point.