AQA GCSE Biology 8461: Homeostasis and response – Revision Notes

Condensed recall for the final weeks. For full explanations and worked examples, use the Homeostasis and response study guide.

These notes cover topic 5, Homeostasis and response (sections 4.5.1–4.5.4), of the AQA GCSE Biology (8461) specification, for teaching from September 2016 and exams from 2018 onwards. It is examined on Paper 2 at Foundation and Higher tier; lines marked Higher tier only match the specification’s “(HT only)” content. Course hub: AQA GCSE Biology. Track progress with the printable checklist, and test yourself with the practice questions.

4.5.1 Homeostasis in one box

Homeostasis = regulation of the internal conditions of a cell or organism to maintain optimum conditions for function, in response to internal and external changes.

Controlled in humans: blood glucose, body temperature, water levels.

Parts: receptor (detects stimulus) → coordination centre (brain, spinal cord, pancreas) → effector (muscle or gland).

4.5.2 Nervous system

Nerves vs hormones

	Nervous	Hormonal
Signal	Electrical impulse along neurones	Chemical in the blood
Speed	Very fast	Slower
Duration	Short	Longer lasting
Target	Precise area	Target organs anywhere the blood reaches

Reflex arc – method in steps

1. Stimulus detected by a receptor.
2. Impulse along a sensory neurone to the spinal cord.
3. Chemical crosses a synapse.
4. Relay neurone in the CNS.
5. Synapse again, then motor neurone.
6. Effector responds (muscle contracts, gland secretes).

Reflexes are automatic and rapid and bypass the conscious brain.

Required practical 7: effect of a factor on reaction time. Independent variable: the factor (e.g. caffeine). Dependent: reaction time. Control: same person, same hand, same test. Take repeats and a mean.

Brain

Region	Where	Function
Cerebral cortex	Outer folded layer	Consciousness, memory, language, intelligence
Cerebellum	Back, underneath	Muscle coordination and balance
Medulla	Base, joins spinal cord	Unconscious actions: heart rate, breathing

Higher tier only: mapped using brain-damaged patients, electrical stimulation and MRI. Treatment is hard because the brain is complex and delicate.

Eye

Near object	Distant object
Ciliary muscles contract	Ciliary muscles relax
Suspensory ligaments slacken	Suspensory ligaments tighten
Lens thicker, refracts more	Lens thinner, refracts less

• Myopia (short sight): image forms in front of the retina; concave spectacle lens.
• Hyperopia (long sight): image forms behind the retina; convex spectacle lens.
• Also: contact lenses, laser surgery on the cornea, replacement lens.
• Dim light: pupil widens so more light reaches the retina.

Temperature control

Too hot	Too cold
Vasodilation	Vasoconstriction
Sweating	Sweating stops
–	Shivering (skeletal muscles contract)

Monitored by the thermoregulatory centre in the brain (blood temperature) plus skin receptors. Higher tier only: explain each in context – more or less energy transferred from the skin; shivering respires and releases energy.

4.5.3 Hormones

Glands to locate

Pituitary (brain, “master gland”) · thyroid (neck) · pancreas · adrenal glands (on kidneys) · ovaries · testes.

Hormone table

Hormone	Gland	Effect
Insulin	Pancreas	Glucose moves from blood into cells; liver and muscle store it as glycogen
Glucagon (Higher tier only)	Pancreas	Glycogen → glucose, released into blood
ADH (Higher tier only)	Pituitary	Kidney tubules more permeable → more water reabsorbed
FSH	Pituitary	Egg matures in ovary
LH	Pituitary	Egg released (ovulation)
Oestrogen	Ovary	Maintains uterus lining; main female hormone
Progesterone	Ovary	Maintains uterus lining
Testosterone	Testes	Stimulates sperm production
Adrenaline (Higher tier only)	Adrenal glands	Raises heart rate; more oxygen and glucose to brain and muscles
Thyroxine (Higher tier only)	Thyroid	Stimulates basal metabolic rate; growth and development

Diabetes

	Type 1	Type 2
Problem	Pancreas makes too little insulin	Cells stop responding to insulin
Treatment	Insulin injections	Carbohydrate-controlled diet, exercise
Link	–	Obesity is a risk factor

Higher tier only – negative feedback, glucose: glucose high → insulin → glucose falls. Glucose low → glucagon → glucose rises. Each response reverses the change.

Kidneys

• Urine is made by filtration then selective reabsorption (glucose, some ions, water).
• Lungs and skin lose water (and ions and urea in sweat) with no control.
• Higher tier only: excess amino acids → deamination in the liver → ammonia (toxic) → urea.
• Higher tier only: blood too concentrated → pituitary releases more ADH → more water reabsorbed → blood less concentrated (negative feedback).
• Dialysis: partially permeable membrane; fluid has normal glucose and ion levels but no urea, so only unwanted substances diffuse out.
• Transplant vs dialysis: transplant frees the patient from sessions but needs a donor match and anti-rejection drugs.

Menstrual cycle (Higher tier only interactions)

FSH → egg matures and ovary makes oestrogen. Oestrogen → inhibits FSH, stimulates LH. LH peak → ovulation (about day 14). Progesterone → keeps lining thick, inhibits FSH and LH. Progesterone falls → lining breaks down.

Contraception – sort into two groups

• Hormonal: pill (inhibits FSH), progesterone injection, implant or patch, some IUDs.
• Non-hormonal: condom, diaphragm, spermicide, an IUD that only prevents implantation, abstinence when an egg may be in the oviduct, sterilisation.

IVF (Higher tier only)

FSH and LH → several eggs mature → eggs collected → fertilised in the lab → embryos grow → one or two inserted into uterus. Drawbacks: stressful, low success rate, multiple-birth risk.

4.5.4 Plant hormones

• Auxin: unequal distribution → unequal growth. Shoots bend towards light (phototropism); roots grow down (gravitropism).
• Required practical 8: light or gravity on seedlings; record lengths and labelled drawings.
• Higher tier only: gibberellins start germination; ethene controls cell division and ripening.
• Higher tier only – uses: auxin (weed killer, rooting powder, tissue culture); ethene (control ripening in storage and transport); gibberellins (end dormancy, promote flowering, bigger fruit).

Method boxes

Reading a blood glucose graph (MS 2c)

1. Read the starting level and the peak from the axes, with units.
2. Compare the peak and how fast the level returns to the start.
3. Rate of change = change in concentration ÷ time taken.
4. Without diabetes, insulin brings the level back towards the start; in untreated Type 1 diabetes it stays high because too little insulin is made.
5. With Type 2, the level falls more slowly because cells respond poorly to insulin.

Explaining negative feedback (Higher tier only)

1. Name the change (e.g. blood too concentrated).
2. Name the detector and gland (pituitary) and the hormone (ADH).
3. Say what the hormone does at the target (tubules reabsorb more water).
4. Finish with the result: the change is reversed, so the hormone release falls again.

Tables of glucose, ions and urea (MS 4a)

• Glucose: present in filtrate, absent from urine – all reabsorbed.
• Urea: concentration higher in urine than filtrate – much more water than urea has been reabsorbed.
• Ions: some reabsorbed, the rest excreted to keep the balance.

Must-know distinctions

• Glucagon (hormone) vs glycogen (storage carbohydrate).
• Vasodilation (hot) vs vasoconstriction (cold).
• FSH (egg matures) vs LH (egg released).
• Filtration (everything small leaves the blood) vs selective reabsorption (useful things return).
• Type 1 (no insulin made) vs Type 2 (cells do not respond).
• Phototropism (light) vs gravitropism (gravity).

Quick self-test

1. Name the coordination centre for blood glucose.
2. Which neurone carries impulses from the CNS to an effector?
3. Name the part of the brain that coordinates balance.
4. What happens to the suspensory ligaments when you look at a near object?
5. Give two responses when body temperature is too low.
6. A student’s reaction times were 0.24 s, 0.27 s and 0.21 s. Calculate the mean.
7. The kidneys filter 170 dm³ of fluid in a day and 1.7 dm³ of urine is produced. What percentage of the water is reabsorbed?
8. Blood glucose falls from 6.0 to 4.5 mmol/dm³ in 30 minutes. Calculate the rate of fall.
9. Why does dialysis fluid contain glucose?
10. (Higher tier only) What is the effect of ADH on the kidney tubules?
11. Name the hormone that triggers ovulation.
12. Which way does a shoot bend when light comes from the left?

Answers

1. The pancreas.
2. Motor neurone.
3. Cerebellum.
4. They loosen (slacken).
5. Vasoconstriction; sweating stops; shivering (any two).
6. (0.24 + 0.27 + 0.21) / 3 = 0.72 / 3 = 0.24 s.
7. (170 − 1.7) / 170 × 100 = 99%.
8. 1.5 / 30 = 0.05 mmol/dm³ per minute.
9. It has the same glucose concentration as normal blood, so glucose does not diffuse out of the blood.
10. It makes them more permeable, so more water is reabsorbed into the blood.
11. LH.
12. To the left, towards the light.

Where marks are usually lost

• Writing “the brain” for the coordination centre in blood glucose control instead of the pancreas.
• Describing a reflex without naming all three neurones and the synapse.
• Saying ciliary muscles “tighten” the lens; they contract or relax, and the ligaments tighten or slacken.
• Saying sweat “cools you” without mentioning evaporation or energy transfer from the skin.
• Writing that Type 2 diabetes is caused by eating sugar; the spec names cells no longer responding to insulin, with obesity as a risk factor.
• Saying glucagon is stored in the liver.
• Claiming urine from a healthy person contains glucose.
• Giving ADH as “making more urine”; it reduces urine volume by increasing reabsorption.
• Treating the pill as a barrier method; it works by inhibiting FSH.
• Writing that auxin is destroyed by light instead of saying it collects on the shaded side.

Official syllabus

AQA GCSE Biology (8461) specification, Version 1.0 (21 April 2016), for teaching from September 2016 and exams from 2018 onwards, AQA – section 4.5, Homeostasis and response.