Exercise, exergy, and meateaters: Resolving the thermodynamic equilibrium of life
Life is about thermodynamic extremes. When scientists first began formulating the Laws of Thermodynamics and talking about disorder and entropy, it seemed that somehow living things were in breach of the laws. How could they be such self-contained ordered, non-chaotic entities? But, of course, the answer lies in the fact that they are not self-contained, they do not represent a closed system.
There is, of course, no decrease in entropy when we look at the complete system. Living creatures draw their energy from the sun and as the sun pours out its heart, it is the entropy of that body that increases. Life on Earth is essentially paying for its perceived order through the increased entropy of the solar system and the space that surrounds it.
Writing in the International Journal of Exergy, a team from Turkey discusses how organisms live far-from thermodynamic equilibrium with their surroundings. They import exergy, export entropy and maintain constancy of their vital internal physiological constituents via homeostasis. Exergy is a measure of energy with the capacity to do what is referred to technically as work in the thermodynamic sense.
Cennet Yildiz, Volkan Adem Bilgin, Bayram Yılmaz, and Mustafa Özilgen of Yeditepe University, in Istanbul, Turkey, have used data from the scientific literature to calculate how homeostasis helps organisms to save exergy when carrying out their life processes. Maintaining body temperature, by contrast, they show, costs exergy.
Intriguingly, the team has found that there are big differences between mammals and reptiles in terms of their exergy requirements. The daily exergy expenditure rate of an animal depends on metabolic rate, body mass, and nutritional exergy uptake, the team explains. Their calculations show that endothermic (homeothermic) mammal that spends about one-third of its time in active metabolic mode and the rest of the day at rest needs a mere 6 grams of meat per kilogram of body mass each day to satisfy its exergy requirements. In contrast, an ectothermic reptile with the same level of activity needs 500 grams of meat per kilogram of body mass.