4.5 Homeostasis and response

Unit Summary

This unit introduces homeostasis - how the body maintains stable internal conditions so cells can function within narrow limits  and is fully aligned with the AQA GCSE Combined Science Trilogy specification. It builds on prior knowledge of cells and enzymes, and prepares students for later topics such as inheritance and ecology.

Students first establish the general principle of homeostasis: that control systems made up of receptors, coordination centres and effectors work together to keep conditions such as blood glucose concentration, body temperature and water levels within optimal limits. They then study the human nervous system, learning how information passes from receptors along neurones to the central nervous system, which coordinates a response through effectors. The reflex arc (sensory neurone, synapse, relay neurone, motor neurone) is taught in detail, with reflex actions explained as rapid, automatic responses that do not involve conscious thought.

Hormonal coordination covers the structure and role of the human endocrine system, including the position of the pituitary gland, pancreas, thyroid, adrenal glands, ovaries and testes. Blood glucose control is a key focus: insulin lowers blood glucose by causing glucose uptake into cells, and Type 1 and Type 2 diabetes are studied as contrasting disorders, including their treatment. (HT only) students go further to explain how glucagon raises blood glucose and how insulin and glucagon interact in a negative feedback cycle.

Reproduction receives particular emphasis, since hormonal coordination controls the menstrual cycle. Students learn the roles of oestrogen, testosterone, FSH and LH, with (HT only) the detailed interactions between FSH, oestrogen, LH and progesterone across the cycle. This understanding underpins the evaluation of hormonal and non-hormonal methods of contraception, which all students cover, while (HT only) the use of hormones to treat infertility, including IVF and its associated social and ethical issues, extends this further.

Finally, (HT only) students study feedback systems in more depth, explaining the roles of adrenaline, produced by the adrenal glands in response to fear or stress to prepare the body for "fight or flight," and thyroxine, produced by the thyroid gland to stimulate basal metabolic rate and support growth and development. Thyroxine levels are controlled by negative feedback, reinforcing this as a recurring theme across the unit.

The required practical investigates human reaction time, developing skills in planning, identifying variables and evaluating sources of error. Real-world links include diabetes management and contraception, with infertility treatment and the stress/fight-or-flight response relevant for Higher Tier classes, while ongoing retrieval practice and exam-style questioning build progressively towards exam readiness.