metabolic rate

Meaning: Metabolic rate is the total energy an organism uses per unit time, summing all its catabolic and anabolic reactions. Because cells release much of that energy as heat, it can be measured by heat output or indirectly by O₂ consumed or CO₂ produced. Basal metabolic rate (BMR) is the rate at rest, fasting, and at a comfortable temperature. Activity, digestion, cold exposure in endotherms, and thyroid hormone raise the rate. Endotherms spend energy making heat to hold body temperature steady, so they have much higher rates than ectotherms of similar size. Small animals have a high surface-area-to-volume ratio, lose heat faster, and so have higher rates per gram.

Equation: metabolic rate = ΔE/Δt, where ΔE is the energy used (J) and Δt is the time (s). The unit is the watt (W = J/s).

On the MCAT: A passage gives O₂ consumption against body mass or ambient temperature. You compare a mouse with an elephant, or predict how an ectotherm's rate changes as temperature rises, or how an endotherm's rate changes below its comfortable range.

Common confusion: Total and per-gram rates differ. An elephant uses more energy in total than a mouse, but the mouse uses more per gram of body mass. Check which quantity the axis shows.