Energy and matter: the accounting behind every biology number
Biology runs on two accounting systems at once. Matter cycles — carbon, nitrogen, and phosphorus move through organisms and back to the environment in closed loops, because atoms are neither created nor destroyed. Energy flows — it enters (usually as sunlight or food), is transformed at each trophic step, and leaves as heat. Every ecology and physiology calculation is, at bottom, an entry in one of these ledgers.
The constraints are quantitative. First-law thermodynamics caps energy transformations at 100%; the second law ensures each transfer loses usable energy — typically 90% at a trophic step, which is why food chains rarely exceed four or five links and why an ecosystem's total biomass shrinks as you climb it. In organisms, metabolic rate scales with body mass, usually as a power law near mass^0.75, so a gram of shrew burns roughly thirty times the energy of a gram of elephant per unit time.
Temperature governs the pace. Biological reaction rates roughly double per 10 °C within a viable range (a Q₁₀ near 2), until enzymes denature — which is why fever, cold-blooded activity levels, and compost-heap temperatures all behave the way they do.