Describe homeostasis as a stable internal environment
Distinguish regulated internal conditions from changing surroundings.
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Recall hormone production and blood transport, respiration, liver storage and electrical impulses along neurones.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which environment is maintained by homeostasis?
What you will learn
- Define the internal environment
- Explain stability without requiring a perfectly flat value
Describe homeostasis as a stable internal environment
Homeostasis maintains internal conditions within suitable limits. External conditions may change; internal values need not be perfectly identical at every instant. Original teaching model.
| Feature | Meaning |
|---|---|
| Definition | Maintenance of a constant internal environment |
| Internal examples | Body-fluid composition and temperature |
| Constant | Controlled within suitable limits |
| External conditions | May change while internal stability is maintained |
Define the internal environment
Homeostasis is the maintenance of a constant internal environment. Internal conditions include the surroundings of body cells and the composition and temperature of body fluids. Maintaining suitable internal conditions helps cells and enzymes function even when the environment outside the organism changes. The word internal is essential: homeostasis does not keep the outside air at a fixed temperature.
In biology, constant means controlled within suitable limits rather than every measured value remaining exactly identical at every instant. Blood glucose can rise after absorbed food and then move back towards its regulated level. Body temperature can vary slightly while remaining controlled. A small temporary change does not by itself show that all regulation has failed.
Explain stability without requiring a perfectly flat value
A complete definition identifies both maintenance and the internal environment. Saying that an organism responds to stimuli is too broad: responses may occur without keeping an internal condition stable. Saying that the body never changes is also too strong. Focus on the controlled condition and the fact that stability is maintained despite disturbances.
In a supplied record, compare external change with the internal measurement. If outside temperature falls while internal temperature remains close to its stated level, the pattern is consistent with maintaining internal stability. The record alone may not identify every controlling mechanism, so use it to support the definition rather than inventing an unmeasured hormone or nerve response.
Sources for this lesson
Pause and explain
Which definition is appropriate?
Worked example
Outside temperature falls while a model internal value stays close to its stated level. Explain the pattern.
Show the worked solution
- The surroundings change.
- The internal condition remains regulated within a narrow range.
Answer The pattern is consistent with homeostasis: maintenance of a constant internal environment despite changing surroundings.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Define homeostasis.
Give me a hint
Include both maintenance and internal conditions.
Compare my reasoning
- It involves maintaining stable conditions.
- Those conditions form the internal environment.
Homeostasis is the maintenance of a constant internal environment.
Look for these in your work
- Include maintenance
- Specify internal environment
Why does a small temporary change in a regulated value not automatically disprove homeostasis?
Give me a hint
Constant does not require identical readings at every instant.
Compare my reasoning
- Regulation keeps conditions within suitable limits.
- Temporary deviations can be corrected.
Homeostasis maintains conditions within suitable limits; small temporary deviations do not automatically mean regulation has failed.
Look for these in your work
- Distinguish stability and exact equality
- Allow controlled temporary variation
A model animal has outside temperatures 15, 10 and 5°C but internal temperatures 37.0, 37.1 and 37.0°C. Explain what the comparison supports.
Give me a hint
Compare the large external change with the small internal variation.
Compare my reasoning
- External temperature changes by 10°C.
- Internal readings remain close to the supplied level.
The record is consistent with homeostasis: internal temperature stays close to its stated level despite a large external change. It does not identify every mechanism.
Look for these in your work
- Compare internal and external records
- Keep the conclusion supported
Common mistakes
- Defining homeostasis as constant outside temperature.
- Demanding that every reading be exactly identical.
What can you explain now?
Which definition is appropriate?
Compare with the explanation
Maintenance of a constant internal environment
It gives both regulation and its location.
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Make it stick
Define homeostasis and distinguish regulated stability from an impossible exact constancy.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.