Disease and response · GCSE Biology
Homeostasis
Nervous and chemical coordination, reflex arcs, synapses, blood glucose, insulin and the control of body temperature.
Homeostasis keeps conditions in a narrow range. The nervous system is fast and electrical; hormones are slower and chemical. Reflexes skip conscious control so damage is limited.
The important bits
What you need to know
- 1
Homeostasis is the regulation of the internal environment — blood glucose, temperature and water — so that cells can function. Enzymes, in particular, need a stable optimum.
- 2
A receptor detects a stimulus. A coordinator (spinal cord or brain) processes the information. An effector (muscle or gland) produces a response.
- 3
A reflex arc: stimulus → receptor → sensory neurone → relay neurone in the CNS → motor neurone → effector. The synapse uses a chemical neurotransmitter. Reflexes are rapid and automatic.
- 4
Hormones are chemical messengers secreted by glands into the blood. They act on target organs. The endocrine system is slower than the nervous system but often has a longer-lasting effect.
- 5
When blood glucose is too high, the pancreas releases insulin, which makes liver and muscle cells take up glucose and store it as glycogen. Type 1 diabetes is a lack of insulin; Type 2 is cells that do not respond properly.
- 6
On Higher tier, glucagon is released when blood glucose is too low and causes glycogen to be broken down to glucose.
- 7
Body temperature is monitored by the thermoregulatory centre. If too hot: sweating and vasodilation. If too cold: shivering, vasoconstriction and less sweating. These restore the set point.
Go deeper
The reflex is a one-way street
Learn the sequence as a chain you can write in order. The receptor, not the brain, starts it. The sensory neurone carries the impulse to the central nervous system. A relay neurone connects to a motor neurone, which stimulates a muscle or gland. Synapses can only fire in one direction because the transmitter is released from one side and received on the other. That is why the pathway does not reverse. “The brain thinks about it” is the wrong model for a reflex: the point is that conscious processing is too slow to withdraw a hand from a hot plate. If a diagram is given, label neurones by function, not by a generic “nerve”.
Go deeper
Blood glucose is a two-way control on Higher tier
Insulin is the Foundation story: high glucose, insulin released, glucose into cells, stored as glycogen, level falls. Higher tier adds glucagon: low glucose, glucagon released, glycogen to glucose, level rises. Together they are negative feedback. Type 1 diabetes usually begins when the pancreas cannot make insulin, so glucose is injected as a drug and carbohydrate intake is managed. Type 2 is linked to obesity and a poor response of cells to insulin; first-line control is diet, exercise and then medication. Do not write that diabetics “have sugar in the blood” as if healthy people do not. Everyone has blood glucose; the issue is control.
See the idea in action
A student steps on a pin. Pain receptors in the skin are stimulated. A sensory neurone carries an electrical impulse to the spinal cord. A relay neurone passes the impulse across synapses using neurotransmitter to a motor neurone. The motor neurone stimulates leg muscles, which contract and lift the foot. The brain becomes aware afterwards. The sequence is still receptor, coordinator, effector — just faster than a considered decision.
Exam technique
Turn knowledge into marks
Write reflex pathways in order and name each neurone. For diabetes, say Type 1 or Type 2 and match the treatment. For temperature, name the mechanism (vasodilation, sweating) and the effect (more heat transferred to the surroundings).
Common mistakes
Do not give these marks away
- 01
Putting the brain as a required stop in every reflex, or listing neurones in the wrong order.
- 02
Saying insulin “breaks down sugar” instead of causing cells to take up glucose and store it as glycogen.
- 03
Confusing Type 1 (no insulin, injections) with Type 2 (cells do not respond, lifestyle then drugs).
What does insulin do when blood glucose is too high?
AIt causes glycogen to be broken down to glucose
BIt causes liver and muscle cells to take up glucose and store it as glycogen
CIt raises body temperature
DIt stimulates pain receptors
Show the answer
It causes liver and muscle cells to take up glucose and store it as glycogen. Insulin is released by the pancreas when glucose is high. Cells take up glucose and store it as glycogen, so blood glucose falls. Glucagon does the reverse on Higher tier.
Quick questions
If this is the bit you searched
What is a reflex arc?
An automatic pathway: stimulus, receptor, sensory neurone, relay neurone in the CNS, motor neurone, effector. It is fast and does not wait for conscious thought.
What is the difference between Type 1 and Type 2 diabetes?
Type 1: the pancreas makes little or no insulin, treated with insulin. Type 2: cells do not respond properly to insulin, often linked to obesity and treated first with diet and exercise.