lesson
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Whether you are trekking across a freezing snowfield or baking in a desert sun, the temperature deep inside your brain and organs barely budges from 37°C (98.6°F).
This automatic balancing act is called homeostasis, which is the regulation of internal conditions inside a cell or organism to maintain optimum conditions for function in response to internal and external changes.
In 1854, French physiologist Claude Bernard first realized that complex life survives because it shields its cells within a stable "internal sea" (milieu intérieur), a concept Walter Cannon later named homeostasis in 1926.
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Why must our bodies spend so much continuous energy defending these specific set points?
The Enzyme Connection
Every metabolic reaction in your cells—from releasing energy in respiration to synthesizing new DNA—relies on enzymes, which are biological catalysts made of protein.
Each enzyme has a precisely shaped active site where its specific substrate molecule binds, and this shape is held together by delicate intermolecular bonds.
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If body temperature drops too low, molecules move too slowly, causing too few collisions between enzymes and substrates to sustain life.
If temperature rises above roughly 40°C, the high thermal energy breaks the bonds holding the protein together; the active site deforms permanently, and the enzyme is denatured.
A common mistake on exams is saying that high temperatures "kill" enzymes—enzymes are non-living chemical molecules, so we say they denature, not die.
Temperature isn't the only threat to your cells—what happens if the fluid surrounding them becomes too concentrated with sugar or salt?