AQA GCSE Biology (8461) · Paper 2
🧠 Homeostasis & Response
Revision notes written to the specification, with examiner tips and the required practicals. Every point here has flashcards in the Stickwise app.
Homeostasis
Homeostasis is the regulation of the body's internal conditions to keep them constant despite changes inside the body and in its surroundings. This matters because enzymes and cells only work properly within a narrow range of temperature, water content and glucose concentration.
Human homeostasis controls three things: blood glucose concentration, body temperature and water levels.
Every automatic control system, whether nervous or hormonal, has the same three parts:
- Receptors detect the change, which is called the stimulus.
- A coordination centre, such as the brain, spinal cord or pancreas, processes the information and decides on a response.
- Effectors, which are muscles or glands, carry out the response.
The nervous system
Neurones are long cells that carry electrical impulses at high speed. Information passes through the nervous system in a fixed sequence:
The central nervous system (CNS) is the brain and spinal cord. It receives impulses from receptors along sensory neurones and sends impulses out along motor neurones to effectors.
Reflexes
A reflex is a response that happens automatically, without conscious thought, so it protects the body before there is time to notice the danger — for example, pulling a hand off a hot pan. This pathway is called the reflex arc:
Between neurones there is a tiny gap called the synapse. The impulse cannot cross this gap electrically, so a chemical diffuses across it and triggers a new impulse on the far side. This chemical crossing causes the slight delay in the reflex pathway.
The brain (Triple)
The brain is hard to investigate and treat (Higher content): it is protected by the skull, its tissue is delicate and complex, and surgery risks damaging something vital. Current understanding comes from studying patients with brain damage, electrically stimulating parts of the brain, and MRI scans.
The eye (Triple)
Accommodation is the process of changing the shape of the lens to focus on objects at different distances. For a near object, the ciliary muscles contract and the suspensory ligaments loosen, so the lens becomes fatter and bends light more strongly. For a distant object, the ciliary muscles relax and the ligaments pull tight, so the lens becomes thinner.
In myopia, or short sight, the image focuses in front of the retina; this is corrected with a concave lens. In hyperopia, or long sight, the image focuses behind the retina; this is corrected with a convex lens. Contact lenses and laser surgery can correct both in the same way.
Body temperature (Triple)
The thermoregulatory centre in the brain monitors the temperature of the blood, and skin receptors also send it information. When the body is too hot, it produces sweat, which cools the skin as it evaporates, and blood vessels near the skin widen in a process called vasodilation, which loses more heat. When the body is too cold, shivering makes muscles respire faster and release heat, blood vessels narrow in a process called vasoconstriction, and body hairs stand up to trap an insulating layer of air.
Hormonal coordination
A hormone is a chemical messenger made in a gland and carried in the blood to target organs. Compared with nervous signals, hormones act more slowly but have a longer-lasting and more general effect, since they work by chemistry rather than electrical impulses.
The pituitary gland is known as the 'master gland': its hormones instruct other glands to release their own hormones. The main glands and hormones are thyroxine from the thyroid, insulin from the pancreas, adrenaline from the adrenal glands, oestrogen from the ovaries and testosterone from the testes.
Blood glucose
When blood glucose is too high, for example after a meal, the pancreas releases insulin, which causes cells to take up glucose and the liver to store the excess as glycogen.
Higher content: when blood glucose is too low, the pancreas releases glucagon, which causes the liver to break glycogen back down into glucose. Insulin and glucagon push blood glucose in opposite directions, forming a negative feedback cycle.
Water balance and the kidneys (Triple)
Water is lost in ways the body cannot control, through the lungs when breathing out and through sweat, so the kidneys must balance the body's water content. The kidneys filter the blood, then selectively reabsorb all the glucose, the ions the body needs, and the right amount of water. What is left, which is excess water, excess ions and urea (made in the liver from excess amino acids), leaves the body as urine.
Higher content: the hormone ADH, from the pituitary gland, controls how much water the kidneys reabsorb. If the blood becomes too concentrated, more ADH is released, so the kidney tubules reabsorb more water and produce a small volume of concentrated urine. This is a classic example of negative feedback.
Kidney failure can be treated with dialysis, which removes waste and excess substances through a partially permeable membrane, or a kidney transplant. A transplant may remove the need for regular dialysis but is a long-term treatment, not a cure. Donor availability, surgery and rejection are limitations, and immunosuppressant medicines are usually needed.
Puberty and the menstrual cycle
At puberty, testosterone, produced in the testes, drives sperm production, while oestrogen, produced in the ovaries, drives the development of female secondary characteristics and controls the menstrual cycle. This is the roughly 28-day cycle in which an egg and a womb lining are prepared.
Higher content: these hormones control each other. Oestrogen inhibits FSH, so that normally only one egg matures, and stimulates LH; progesterone inhibits both FSH and LH. This interaction between the hormones is often tested in a 6-mark exam question.
Contraception and fertility
Hormonal contraception, such as the pill, implant, injection or patch, uses oestrogen or progesterone to inhibit FSH, so that no egg matures. Non-hormonal methods include condoms, which are the only method that also protects against STIs, diaphragms, IUDs (some of which are hormonal), spermicides, surgical sterilisation, and abstinence.
Higher content: fertility treatment reverses this logic by giving FSH and LH to trigger egg maturation. In IVF, several eggs are stimulated and collected, fertilised with sperm in the laboratory, grown into embryos, and one or two are implanted in the womb. IVF can give infertile couples a child, but success rates are modest at around 30%, the process is physically and emotionally demanding, and multiple births carry additional risks.
Adrenaline and thyroxine (Higher)
Adrenaline produces the fight-or-flight response: heart rate increases and oxygen and glucose are directed to the brain and muscles. It is not controlled by negative feedback; instead its level spikes and then fades. Thyroxine, from the thyroid, sets the basal metabolic rate and supports growth; unlike adrenaline, its level is held steady by classic negative feedback, in which the response reverses the change in either direction.
Plant hormones (Triple) Separate science only
Plants respond to their surroundings by growing. In phototropism, shoots grow towards light; in gravitropism, roots grow down in the direction of gravity, while shoots grow away from it. Both responses are controlled by the hormone auxin, which builds up on the shaded or lower side of the plant. In shoots, this extra auxin makes cells elongate faster, bending the shoot towards the light; in roots, auxin instead inhibits growth, bending the root downwards.
Higher content: plant hormones have several uses in horticulture. Auxins are used as selective weedkillers, as rooting powders for cuttings and to promote growth in tissue culture. Gibberellins are used to end seed dormancy, to promote flowering and to increase fruit size. Ethene, a gas, is used to control fruit ripening during storage and transport.