GCSE Biology Homeostasis & nerves revision

Enzymes are fussy: they work in a narrow band of temperature and pH, and cells need a steady supply of glucose and water. Homeostasis is the regulation of the internal environment — of cells and of the whole organism — to keep those optimal conditions despite changes outside and inside. The GCSE examples: blood glucose concentration, body temperature, and water and ion levels.

The word “regulation” hides a three-part machine. A receptor detects a change (a stimulus). A coordination centre — the brain, spinal cord or pancreas — receives and processes the information. An effector — a muscle or gland — carries out the response. The response feeds back on the original change: that is negative feedback, and it keeps conditions oscillating tightly around a set point.

The nervous system lets animals react fast to their surroundings and coordinate behaviour. Its hub is the central nervous system (CNS) — the brain and spinal cord. Information travels to and from the CNS along neurones as fast electrical impulses.

The journey starts at receptors: specialised cells or proteins that detect stimuli — light in the eye, sound in the ear, chemicals on the tongue, temperature and pressure and touch in the skin. Each type of receptor matches one kind of stimulus, so the brain can tell what is happening from which line rings.

A reflex is a fast, automatic response to a stimulus — blinking, knee-jerk, hand-withdrawal. It does not involve the conscious part of the brain, which is why it is so quick: the impulse takes a shortcut through the spinal cord instead of waiting for a decision.

1. 11. Stimulus

   A pinprick touches the finger — the dangerous change.

2. 22. Receptor

   Pain receptors in the skin detect it and fire an impulse.

3. 33. Sensory neurone

   The impulse travels to the spinal cord.

4. 44. Coordination centre

   A relay connection in the spinal cord passes the impulse straight to a motor neurone.

5. 55. Motor neurone

   The impulse speeds out to the arm.

6. 66. Effector

   The biceps muscle contracts and the hand pulls away.

The conscious brain is told afterwards — you feel the pain a split second after your hand has already moved. That order of events (“responded before aware”) is a favourite exam point.

Neurones do not quite touch. Between them lies a microscopic gap called a synapse. An electrical impulse cannot jump the gap, so the signal changes form: the arriving impulse triggers release of chemicals called neurotransmitters, which diffuse across the synapse and bind to receptor molecules on the next neurone. That binding starts a new electrical impulse, and the message continues.

The chemical handover is why synapses matter in exam answers: they are where the impulse is passed between neurones, and where many drugs act — some block the receptors, some mimic the neurotransmitter. One-direction flow is guaranteed because only the sending side releases the transmitter and only the receiving side has the receptors.

This practical measures how fast the nervous system responds. One person holds a 30 cm ruler vertically; the sitter’s thumb and finger hover at the zero mark. Without warning, the ruler is released — and caught as fast as possible. The distance the ruler falls is read off and converted into a reaction time.

1. 1Set up

   Sitter rests their forearm on a table edge; thumb and index finger frame the ruler’s zero mark.

2. 2Drop

   The holder releases the ruler with no warning — no countdown, no rhythm.

3. 3Catch

   The sitter catches it as fast as possible; the reading at the thumb gives the drop distance in cm.

4. 4Repeat

   Repeat at least five times and calculate the mean, discarding obvious “false start” catches.

5. 5Compare

   Repeat the whole procedure under a second condition — for example after a measured dose of caffeine, or with the non-dominant hand — and compare the means.

Variables to control: the same ruler and same starting height, the same catcher and holder, no talking or warning cues, and the same hand. A computer-measured reaction time (clicking on a screen stimulus) removes the catching skill entirely and can be more valid.