An alternative model notes that metabolic rate does not solely serve to generate heat. Metabolic rate contributing solely to useful work should scale with power 1 (linearly), whereas metabolic rate contributing to heat generation should be limited by surface area and scale with power . Basal metabolic rate is then the convex combination of these two effects: if the proportion of useful work is , then the basal metabolic rate should scale asformula_4where and are constants of proportionality. in particular describes the surface area ratio of organisms and is approximately ; typical values for are 15-20%. The theoretical maximum value of is 21%, because the efficiency of glucose oxidation is only 42%, and half of the ATP so produced is wasted.