AQA GCSE Biology 8461: Homeostasis and response – Study Guide

This guide teaches 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. The topic is examined on Paper 2 (1 hour 45 minutes, 100 marks, 50% of the GCSE), which is set at Foundation and Higher tier. Content the specification marks “(HT only)” is labelled Higher tier only here; everything else is for both tiers.

The AQA GCSE Biology hub lists every topic, and the printable checklist lets you tick off each statement.

What this topic covers

Spec section	Content	Higher tier only parts
4.5.1	Homeostasis; receptors, coordination centres, effectors	–
4.5.2.1	Reflex arc; Required practical 7	–
4.5.2.2	Brain regions	Studying and treating the brain
4.5.2.3	The eye, accommodation, sight defects	–
4.5.2.4	Body temperature	Explaining in context
4.5.3.1–2	Endocrine glands; insulin, diabetes	Glucagon
4.5.3.3	Water balance, kidneys, dialysis	Deamination, ADH
4.5.3.4–5	Reproductive hormones; contraception	Hormone interactions
4.5.3.6–7	Infertility; adrenaline, thyroxine	Whole sections
4.5.4	Auxin; Required practical 8	Gibberellins, ethene, uses

Homeostasis (4.5.1)

Homeostasis is the regulation of the internal conditions of a cell or organism to maintain optimum conditions for function, in response to internal and external changes – including optimum conditions for enzyme action. In humans it includes control of blood glucose concentration, body temperature and water levels.

All control systems include:

• Receptors – cells that detect stimuli (changes in the environment).
• Coordination centres – such as the brain, spinal cord and pancreas – which receive and process the information.
• Effectors – muscles or glands – which bring about responses that restore optimum levels.

These automatic systems may use nervous or chemical responses.

The nervous system (4.5.2)

Structure and the reflex arc

Impulses travel from receptors along neurones to the central nervous system (CNS) – the brain and spinal cord – which coordinates effectors: muscles contract or glands secrete hormones.

stimulus → receptor → coordinator → effector → response

A reflex action is automatic and rapid and does not involve the conscious part of the brain. In a reflex arc:

1. A receptor detects the stimulus (for example, a pin in the skin).
2. A sensory neurone carries impulses to the CNS.
3. At a synapse a chemical diffuses across the gap and starts an impulse in the next neurone.
4. A relay neurone in the spinal cord passes it on to a motor neurone, which carries impulses to the effector.

Reflexes protect you from harm before you have time to think.

Required practical 7: reaction time

You investigate the effect of a factor (such as caffeine or practice) on human reaction time, often with a ruler drop or computer test. Control the hand used and the starting position; repeat and calculate a mean.

Worked example. A student’s reaction times (s) were measured before and after drinking cola.

Test	1	2	3	4	5
Before	0.32	0.29	0.35	0.30	0.34
After	0.27	0.25	0.28	0.26	0.29

Mean before = (0.32 + 0.29 + 0.35 + 0.30 + 0.34) / 5 = 1.60 / 5 = 0.32 s
Mean after  = (0.27 + 0.25 + 0.28 + 0.26 + 0.29) / 5 = 1.35 / 5 = 0.27 s
Percentage change = (0.27 − 0.32) / 0.32 × 100 = −15.6%

Reaction time fell by 15.6%.

The brain (4.5.2.2)

Identify three regions on a diagram:

• Cerebral cortex – the large outer layer: consciousness, intelligence, memory and language.
• Cerebellum – at the back, underneath: coordinates muscular activity and balance.
• Medulla – at the base, joined to the spinal cord: controls unconscious activities such as heart rate and breathing.

Higher tier only. Scientists mapped brain regions by studying patients with brain damage, electrically stimulating parts of the brain, and using MRI scanning. The brain is complex and delicate, so investigating and treating brain disorders is very difficult.

The eye (4.5.2.3)

Structure	Function
Retina	Contains receptors sensitive to light intensity and colour
Optic nerve	Carries impulses from the retina to the brain
Sclera	Tough white outer layer
Cornea	Transparent front; refracts light into the eye
Iris	Controls pupil size
Ciliary muscles	Contract or relax to change lens shape
Suspensory ligaments	Connect ciliary muscles to the lens

Accommodation is changing the shape of the lens to focus.

• Near object: ciliary muscles contract, suspensory ligaments loosen, the lens becomes thicker and refracts light strongly.
• Distant object: ciliary muscles relax, suspensory ligaments are pulled tight, the lens is pulled thin and refracts light only slightly.

In dim light the iris widens the pupil so more light reaches the retina.

Myopia (short sight): light from distant objects focuses in front of the retina. It is corrected with a concave (diverging) spectacle lens. Hyperopia (long sight): light from near objects focuses behind the retina, corrected with a convex (converging) lens. Newer options: contact lenses, laser surgery to reshape the cornea, and a replacement lens.

Control of body temperature (4.5.2.4)

The thermoregulatory centre in the brain contains receptors sensitive to blood temperature. Skin temperature receptors also send it impulses.

• Too hot: blood vessels in the skin dilate (vasodilation) and sweat glands produce sweat. Both transfer energy from the skin to the environment.
• Too cold: blood vessels constrict (vasoconstriction), sweating stops, and skeletal muscles contract (shiver).

Higher tier only. Explain these in context: vasodilation sends more blood near the surface, so more energy is transferred to the surroundings; evaporating sweat transfers energy from the skin. Vasoconstriction does the reverse. Shivering needs respiration, which releases energy that warms the body.

Hormonal coordination (4.5.3)

Endocrine glands secrete hormones directly into the blood, which carries them to a target organ. Compared with nerves, effects are slower but last longer. The pituitary gland in the brain is a “master gland”: its hormones act on other glands, which release other hormones. Learn the positions of the pituitary, thyroid, pancreas, adrenal glands, ovaries and testes.

Blood glucose (4.5.3.2)

The pancreas monitors and controls blood glucose concentration. If it is too high, the pancreas releases insulin, which causes glucose to move from the blood into cells. In liver and muscle cells excess glucose is converted to glycogen for storage.

Higher tier only. If blood glucose is too low, the pancreas releases glucagon, which causes glycogen to be converted into glucose and released into the blood. Insulin and glucagon act in a negative feedback cycle: a change in one direction triggers a response that reverses it.

	Type 1 diabetes	Type 2 diabetes
Cause	Pancreas makes too little insulin	Cells no longer respond to insulin
Treatment	Insulin injections	Carbohydrate-controlled diet and exercise
Risk factor	–	Obesity

Worked example. After a glucose drink, a person without diabetes has a blood glucose peak of 8.0 mmol/dm³ at 30 minutes, falling to 5.3 mmol/dm³ at 120 minutes. Calculate the mean rate of fall.

Fall = 8.0 − 5.3 = 2.7 mmol/dm³
Time = 120 − 30 = 90 min
Rate = 2.7 / 90 = 0.03 mmol/dm³ per minute

Water and nitrogen balance (4.5.3.3)

Cells that lose or gain too much water by osmosis do not function efficiently. Water is lost uncontrolled from the lungs, and water, ions and urea in sweat. Excess water, ions and urea are removed by the kidneys in urine.

The kidneys make urine by filtration of the blood and selective reabsorption of useful substances such as glucose, some ions and water.

Worked example. In one day the kidneys filter 180 dm³ of fluid and produce 1.5 dm³ of urine. The filtrate holds 54 g of urea; the urine holds 30 g.

Water reabsorbed = (180 − 1.5) / 180 × 100 = 99.2%
Urea excreted    = 30 / 54 × 100 = 55.6%

Higher tier only. Digesting protein gives excess amino acids. In the liver they are deaminated to form ammonia, which is toxic, so it is immediately converted to urea for safe excretion. The water level is controlled by ADH, released by the pituitary gland when the blood is too concentrated. ADH makes the kidney tubules more permeable, so more water is reabsorbed into the blood. This is negative feedback.

Kidney failure is treated by dialysis or a transplant. In dialysis, blood flows past a partially permeable membrane. The dialysis fluid has the same glucose and ion concentrations as normal blood, so these are not lost, but no urea, so urea diffuses out.

Hormones in reproduction (4.5.3.4)

At puberty, reproductive hormones cause secondary sex characteristics. Oestrogen is the main female hormone, made in the ovary. An egg is released about every 28 days: ovulation. Testosterone, made by the testes, stimulates sperm production.

• FSH (pituitary) causes an egg to mature in the ovary.
• LH (pituitary) stimulates release of the egg.
• Oestrogen and progesterone (ovary) maintain the uterus lining.

Higher tier only. FSH stimulates the ovary to make oestrogen. Rising oestrogen inhibits FSH and stimulates LH. The LH surge triggers ovulation around day 14. Progesterone keeps the lining thick and inhibits FSH and LH. If no pregnancy occurs, progesterone falls, the lining breaks down, and FSH can rise again.

Contraception (4.5.3.5)

Method	How it works
Oral contraceptive	Hormones inhibit FSH so no eggs mature
Injection, implant or skin patch	Slow-release progesterone inhibits maturation and release of eggs for months or years
Condom, diaphragm	Barrier: sperm cannot reach the egg
Intrauterine device	Prevents implantation of an embryo, or releases a hormone
Spermicide	Kills or disables sperm
Abstinence	No intercourse when an egg may be in the oviduct
Sterilisation	Surgical, male or female

Infertility treatment (4.5.3.6) – Higher tier only

A fertility drug of FSH and LH can help a woman become pregnant normally. In IVF, the mother is given FSH and LH so several eggs mature; eggs are collected and fertilised by the father’s sperm in the laboratory; the fertilised eggs develop into embryos; one or two tiny balls of cells are inserted into the uterus. IVF is emotionally and physically stressful, success rates are not high, and multiple births risk the health of babies and mother. If a clinic records 104 births from 400 treatment cycles, the success rate is 104 / 400 × 100 = 26%.

Adrenaline and thyroxine (4.5.3.7) – Higher tier only

Adrenaline from the adrenal glands is released in fear or stress. It increases heart rate and boosts delivery of oxygen and glucose to the brain and muscles, preparing the body for “fight or flight”. Thyroxine from the thyroid gland stimulates the basal metabolic rate and is important in growth and development. Thyroxine levels are controlled by negative feedback: a fall in thyroxine leads to more stimulation of the thyroid, and a rise leads to less.

Plant hormones (4.5.4)

Plant hormones control growth and responses to light (phototropism) and gravity (gravitropism or geotropism). Unequal distribution of auxin causes unequal growth rates. In a shoot lit from one side, auxin collects on the shaded side and the cells there elongate more, so the shoot bends towards the light. In a horizontal root, auxin collects on the lower side and slows growth, so the root bends down.

Required practical 8

Investigate the effect of light or gravity on newly germinated seedlings; record lengths and careful, labelled biological drawings. Keep temperature, water and seed type the same, and calculate mean lengths.

Higher tier only. Gibberellins start seed germination; ethene controls cell division and fruit ripening (their mechanisms are not required). Uses:

• Auxins: weed killers, rooting powders, promoting growth in tissue culture.
• Ethene: controls ripening of fruit during storage and transport.
• Gibberellins: end seed dormancy, promote flowering, increase fruit size.

Common errors

• Describing accommodation without the ciliary muscles and ligaments in the right order.
• Saying insulin converts glucose to glycogen in the blood – it happens in liver and muscle cells.
• Confusing glucagon with glycogen.
• Saying blood vessels “move” towards the skin; they dilate or constrict.
• Saying auxin “moves towards the light”; it collects on the shaded side.

Next, condense this with the revision notes, work through the practice questions, or try the free diagnostics.

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.