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Metabolism converts food into energy; the heat it produces both fuels and reflects metabolic activity. In ectotherms, external temperature directly governs metabolic rate, whereas endotherms maintain a core temperature around 37 °C to optimize enzyme function.
Metabolism encompasses every chemical reaction that sustains life. It can be divided into two complementary pathways:
Because spontaneous reactions can be unreliable, cells rely on enzymes as catalysts to bring reactants together, increase reaction speed, and maintain metabolic balance.
Only a small fraction of the energy from food becomes usable work; the rest dissipates as heat. This thermal output keeps endothermic animals warm and influences metabolic rate. Humans, for example, maintain a core temperature of about 37 °C (98.6 °F) to keep enzymes operating near their optimal activity.
Enzymes function best within a narrow temperature window. Their activity follows a bell-shaped curve: sluggish at low temperatures, peaking at the optimum, and declining sharply when temperatures rise beyond the optimum. Above 98.6 °F, enzymes can denature—losing structure and function—thereby halting metabolic reactions.
Animals that cannot regulate their own body temperature—lizards, fish, amphibians—experience metabolic rates that rise with ambient heat. Cold conditions suppress their metabolic activity, limiting movement and energy availability. Conversely, high temperatures accelerate chemical kinetics, enabling rapid activity but requiring increased food intake to sustain the elevated metabolism.
Endotherms (birds and mammals) expend metabolic energy to maintain body temperature. Shivering, sweating, or panting are all energetically costly, thereby elevating metabolic rate. These adaptations illustrate the tight link between thermoregulation and overall energy expenditure.
