GCSE Combined Science Homeostasis & response revision

Homeostasis is the regulation of the conditions inside your cells and body to keep them close to an optimum — blood glucose concentration, body temperature, water and ion levels. It matters because enzymes only work inside narrow ranges: drift too far and reactions slow or stop. Conditions are not frozen rigid; they oscillate gently around a set point, nudged back whenever they wander.

Every automatic control system has the same three-part wiring. Receptors detect a change (the stimulus). A coordination centre — the brain, spinal cord or pancreas — receives the signal, compares it with the set point and decides the response. Effectors — muscles or glands — carry the response out. Most systems use negative feedback: when a level moves away from the set point, the response pushes it back the other way.

The nervous system is built from neurones carrying electrical impulses. The central nervous system (CNS) — brain plus spinal cord — coordinates responses; sensory and motor neurones connect it to receptors and effectors. A reflex is the fastest route of all: an automatic, protective response that does not involve the conscious parts of the brain.

Between two neurones sits a tiny gap called a synapse. The impulse triggers chemicals called neurotransmitters, which diffuse across the gap and bind to the next neurone, restarting the electrical signal. Reflexes stay fast because the whole loop runs through a few neurones — the conscious brain is informed afterwards, not consulted first.

1. 11. Set up

   Sit with your forearm resting on a table; a partner holds a 30 cm ruler so the zero hangs between your thumb and finger.

2. 22. Drop it

   Without warning, your partner releases the ruler; you catch it as fast as possible.

3. 33. Read

   Record the distance the ruler fell (the marking at your thumb). A longer drop means a slower reaction.

4. 44. Repeat

   Do at least three trials, then average — and swap roles to test your partner fairly.

5. 55. Change the factor

   Test a factor such as caffeine, practice, background music or loudness of a sound stimulus, keeping everything else constant.

The endocrine system is a collection of glands that secrete hormones — chemical messengers — directly into the blood. The blood carries them everywhere, but only target organs with the right receptors respond. The pituitary gland in the brain is the master gland: it secretes hormones that instruct other glands, such as telling the ovaries or testes to act.

The pancreas monitors blood glucose. After a sugary meal, glucose climbs; the pancreas responds by releasing insulin, which makes the liver and muscles take glucose out of the blood and store it as glycogen. When glucose dips too low — between meals or during exercise — the pancreas releases glucagon, which makes the liver break glycogen back down into glucose and release it. The two hormones work in opposite directions: the definition of negative feedback.

Diabetes is what happens when this loop breaks. Type 1 is an autoimmune condition, usually starting in childhood, in which the pancreas produces little or no insulin; it is treated with insulin injections at mealtimes and a carbohydrate-aware diet. Type 2 usually develops later in life: the cells stop responding to insulin (the pancreas may still make it). It is managed first with a carbohydrate-controlled diet, weight loss and exercise, with drugs if needed. Obesity is a major risk factor for Type 2 — the correlation is usually quantified with body mass index.

Examination questions often give a scatter-style description of population data: as BMI category rises, the percentage of people with Type 2 diabetes rises too. That is a correlation, not proof on its own — but the mechanism (excess fat reducing insulin response) supports a causal link, so advice follows: keep a healthy weight, eat a balanced low-sugar diet, and exercise regularly.

At puberty the reproductive hormone systems switch on: the testes begin producing testosterone, driving male secondary sexual characteristics; the ovaries begin producing oestrogen, driving female secondary sexual characteristics and starting the menstrual cycle. The cycle runs on roughly a 28-day rhythm: the uterus lining builds up, an egg is released (ovulation) at about day 14, and if no fertilisation happens the lining breaks down (menstruation).

The interactions are a Higher-tier favourite: rising oestrogen suppresses FSH (so only one egg matures) but, above a threshold, switches on a surge of LH that pops the egg; progesterone then holds both pituitary hormones down, keeping the lining ready in case of pregnancy. When levels fall, the lining sheds and FSH is released again — the loop restarts.

Contraception exploits these controls. The hormonal pill supplies oestrogen and progesterone, which inhibit FSH release — with no FSH, no egg matures and nothing can be fertilised. Non-hormonal options physically block sperm (condoms, diaphragms) or the womb (intrauterine devices). Fertility treatment runs the loop the other way: FSH and LH “fertility drugs” stimulate several eggs to mature, which are collected for IVF — fertilisation happens in a laboratory dish, and one or two resulting embryos are placed in the uterus. IVF has created millions of families, but success rates per cycle are modest, treatment is physically and emotionally demanding, and it carries a chance of multiple births and side effects from the hormone doses.