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This lesson introduces the concept of homeostasis and explains why the human body must maintain a stable internal environment. Understanding homeostasis is essential for the AQA GCSE Combined Science Trilogy specification (8464) and underpins every other topic in the Homeostasis and Response unit.
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.
All cells depend on enzyme-controlled reactions. Enzymes function most efficiently at a particular temperature and pH, so the body must continuously adjust its internal environment to keep conditions as close to the optimum as possible.
Exam Tip: When defining homeostasis in the exam, always include the phrase "to maintain optimum conditions for function". Simply saying "keeping things the same" will not earn full marks.
Cells rely on enzymes to catalyse the biochemical reactions that sustain life — respiration, protein synthesis, DNA replication, and so on. If internal conditions deviate too far from the optimum, enzymes may denature and metabolic reactions slow down or stop entirely. Without homeostasis, cells cannot function, and the organism dies.
The human body must regulate several internal conditions:
| Condition | Why It Must Be Controlled | Consequence of Failure |
|---|---|---|
| Body temperature | Enzymes have an optimum around $$37,°\text{C}$$ | Above optimum — enzymes denature; below optimum — reaction rates fall |
| Blood glucose concentration | Cells require a constant supply of glucose for respiration | Hyperglycaemia or hypoglycaemia — both can be life-threatening |
| Water balance (osmoregulation) | Cells need the correct water potential for chemical reactions | Cells may lyse (burst) if too much water enters, or crenate (shrink) if too much leaves |
| Blood pH | Enzymes are highly sensitive to pH changes | Denaturation of enzymes and failure of metabolic pathways |
| Carbon dioxide levels | Waste product of respiration that affects blood pH | Excess $$\text{CO}_2$$ makes blood acidic (carbonic acid formation) |
Explain why it is important for the body to maintain a constant internal temperature of approximately 37 °C.
Model answer: Enzymes in the human body have an optimum temperature of approximately 37 °C. If body temperature rises significantly above this, the active sites of enzymes change shape (denature) and substrates can no longer bind, so metabolic reactions stop. If temperature falls significantly below 37 °C, the kinetic energy of enzyme and substrate molecules decreases, so the rate of successful collisions falls and metabolic reactions slow down. Both situations can be fatal if not corrected.
All homeostatic control systems involve the same three key components:
flowchart LR
A["Stimulus\n(change in condition)"] --> B["Receptor\n(detects the change)"]
B --> C["Coordination Centre\n(brain / spinal cord / pancreas)"]
C --> D["Effector\n(muscle or gland)"]
D --> E["Response\n(counteracts the change)"]
The body uses two main communication pathways to send information between receptors, coordination centres, and effectors:
| Feature | Nervous System | Endocrine System |
|---|---|---|
| Signal type | Electrical impulses along neurones | Chemical hormones in the blood |
| Speed | Very fast | Slower |
| Duration of effect | Short-lived | Longer-lasting |
| Target | Specific part of the body | Specific target organs (or widespread) |
Both systems work together to maintain homeostasis. You will study each in detail in the lessons that follow.