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This lesson introduces the concept of homeostasis and explores the structure and function of the human nervous system. Understanding how the body maintains a stable internal environment is central to AQA GCSE Biology and provides the foundation for the rest of the Homeostasis and Response topic.
Homeostasis is the regulation of the internal conditions of a cell or organism to maintain optimum conditions for function, in response to internal and external changes.
The conditions that need to be controlled in the human body include:
| Condition | Why It Must Be Controlled | What Happens If It Is Not |
|---|---|---|
| Body temperature | Enzymes work best at 37 degrees C | Enzymes denature at high temperatures; reactions slow at low temperatures |
| Blood glucose concentration | Cells need a constant supply of glucose for respiration | Too much or too little glucose can be dangerous or fatal |
| Water balance | Cells need the correct concentration of water for reactions | Cells may swell and burst (lysis) or shrink and die (crenation) |
| pH levels | Enzymes are sensitive to pH changes | Incorrect pH causes enzymes to denature |
| Carbon dioxide levels | CO2 is a waste product that can lower blood pH | Excess CO2 makes blood too acidic, harming enzymes |
Exam Tip: Homeostasis literally means "staying the same". In the exam, always emphasise that it is about maintaining optimum conditions for enzyme action and cell function, not just keeping things constant.
All homeostatic control systems involve three key components working together:
flowchart LR
A[Stimulus] --> B[Receptor]
B --> C[Coordination Centre]
C --> D[Effector]
D --> E[Response]
E -->|Negative feedback| A
Most homeostatic mechanisms use negative feedback. This is a process where the response produced by the effector works to reverse (counteract) the original change, returning the condition to its normal (set) level.
For example:
flowchart TD
A[Normal level] --> B{Change detected}
B -->|Level rises| C[Receptor detects increase]
C --> D[Coordination centre processes]
D --> E[Effector acts to decrease level]
E --> A
B -->|Level falls| F[Receptor detects decrease]
F --> G[Coordination centre processes]
G --> H[Effector acts to increase level]
H --> A
Exam Tip: Negative feedback is called "negative" because the response is in the opposite direction to the change. Do not confuse this with something bad happening.
The nervous system enables humans to react to their surroundings and coordinate their behaviour. It uses electrical impulses to communicate rapidly.
| Component | Role |
|---|---|
| Central nervous system (CNS) | The brain and spinal cord — processes information and coordinates responses |
| Peripheral nervous system (PNS) | All the nerves outside the CNS that connect receptors and effectors to the CNS |
| Sensory neurones | Carry impulses from receptors to the CNS |
| Relay neurones | Found within the CNS; connect sensory and motor neurones |
| Motor neurones | Carry impulses from the CNS to effectors (muscles or glands) |
Receptors are specialised cells that detect changes in the environment called stimuli. Different receptors detect different stimuli:
| Receptor Location | Stimulus Detected |
|---|---|
| Eyes | Light |
| Ears | Sound and changes in position (balance) |
| Nose | Chemicals (smell) |
| Tongue | Chemicals (taste) |
| Skin | Touch, pressure, temperature, pain |
Neurones (nerve cells) are specialised cells that carry electrical impulses. They are elongated to cover long distances and have adaptations for rapid signal transmission.
| Feature | Sensory Neurone | Relay Neurone | Motor Neurone |
|---|---|---|---|
| Direction of impulse | Receptor to CNS | Within the CNS | CNS to effector |
| Cell body position | In the middle of the axon | In the CNS | At one end |
| Axon length | Long | Short | Long |
| Dendrites | At one end | Multiple short branches | Short dendrites at cell body end |
A synapse is the junction between two neurones. There is a tiny gap (the synaptic cleft) between them, so the electrical impulse cannot simply jump across. Instead, chemical transmission occurs:
flowchart LR
A[Electrical impulse arrives] --> B[Neurotransmitter released into cleft]
B --> C[Neurotransmitter diffuses across gap]
C --> D[Binds to receptors on post-synaptic neurone]
D --> E[New electrical impulse generated]
E --> F[Neurotransmitter removed from cleft]
Exam Tip: Many students confuse receptor cells (which detect stimuli) with receptor molecules on neurones (which bind neurotransmitters at synapses). Make sure you specify which type of receptor you mean in your answers.
Effectors are the parts of the body that carry out a response. There are two types:
The full pathway from stimulus to response is:
Stimulus → Receptor → Sensory neurone → CNS (relay neurone) → Motor neurone → Effector → Response
The body uses two communication systems: the nervous system and the endocrine (hormonal) system. Although both coordinate body functions, they work in very different ways.
| Feature | Nervous System | Endocrine System |
|---|---|---|
| Signal type | Electrical impulses | Chemical hormones |
| Transmission | Along neurones | In the blood |
| Speed | Very fast (milliseconds) | Slower (seconds to minutes) |
| Duration of effect | Short-lived | Longer-lasting |
| Area affected | Precise, localised target | Widespread, general target |
Exam Tip: When describing a nervous pathway in an exam, always use the correct sequence: stimulus, receptor, sensory neurone, relay neurone (in CNS), motor neurone, effector, response. Missing a step will lose you marks.
A student walks from a warm room into a cold garden. Within seconds their skin feels cold, and soon afterwards they notice they are shivering. Identify the receptor, coordination centre, effector, and response.
Model working:
| Component | Identification | Reason |
|---|---|---|
| Stimulus | Fall in external temperature | The change that the body must detect |
| Receptor | Thermoreceptors in the skin (and hypothalamus) | Specialised cells that detect temperature change |
| Coordination centre | The hypothalamus in the brain | Processes the information and organises a response |
| Effector | Skeletal muscles | Contract rapidly and repeatedly to produce heat |
| Response | Shivering (plus vasoconstriction, erector-pili muscle contraction) | The corrective action that warms the body |
| Feedback | Negative feedback | Rising body temperature switches the shivering off |
This structure can be applied to any homeostatic question. In the exam, labelling each component in a short sentence is often worth a separate mark.
Common mistake: Students sometimes name the brain as the "effector". The brain is a coordination centre. An effector must be a muscle or a gland that physically carries out the response.
Common mistake: Confusing the nervous and endocrine systems. Remember that nervous responses use electrical impulses along neurones and act on a specific target very quickly, whereas hormonal responses use chemical messengers carried in the blood and tend to be slower but longer-lasting. If an answer describes a signal travelling in the blood, it is not a nervous response.
| Feature | Nervous System | Endocrine System |
|---|---|---|
| Signal type | Electrical impulse | Chemical hormone |
| Transmission medium | Along neurones | In the bloodstream |
| Speed | Very fast (milliseconds) | Slower (seconds to hours) |
| Duration | Short-lived | Longer-lasting |
| Specificity | Precise target (single effector) | Widespread — only cells with matching receptors respond |
| Typical examples | Reflex arc, blinking, muscle contraction | Insulin and blood glucose, thyroxine and metabolic rate |
| Off-switch | Stops when impulse stops | Relies on the hormone being broken down or negative feedback shutting off secretion |
The two systems often work together. For example, during the fight-or-flight response the nervous system triggers the adrenal gland (nervous signal) to release adrenaline (hormonal signal) so the body can react quickly and sustain the response.
Exam-style question: Explain how the body uses the principle of negative feedback to keep core body temperature close to 37 degrees C when a person stands in a cold room. (6 marks)
Grade 4–5 response (illustrative): When the person is cold, receptors in the skin detect the temperature drop. The brain receives the information and sends a signal to the muscles to shiver. Shivering warms the person up. This is negative feedback because the body is brought back to normal.
Why this is mid-band: It identifies the receptor, a coordination centre, and a response, and attempts to link to negative feedback. It is not precise about the thermoreceptors, the hypothalamus, or the idea of antagonistic processes switching off once the set point is restored.
Grade 8–9 response (illustrative): Thermoreceptors in the skin detect the fall in temperature below the set point of about 37 degrees C. They send nervous impulses along sensory neurones to the hypothalamus, which acts as the coordination centre. The hypothalamus activates effectors: skeletal muscles contract rapidly to produce heat through shivering, and arterioles in the skin undergo vasoconstriction to reduce heat loss. Once the core temperature rises back towards 37 degrees C, the thermoreceptors detect the reversal and the hypothalamus reduces its output, switching the effectors off. Because the response opposes the original change, this is an example of negative feedback, which maintains the stable internal environment required for enzyme-controlled reactions.
Why this is top-band: Uses precise terms (thermoreceptor, hypothalamus, effector, vasoconstriction, set point), names two effectors, explicitly identifies the response as opposing the change, and links the mechanism to enzyme function.
AQA alignment: This content is aligned with AQA GCSE Biology (8461) specification section 4.5 Homeostasis and response — specifically 4.5.1 Homeostasis and 4.5.2.1 Structure and function of the nervous system. Assessed on Paper 2.