It Takes Energy to Stay Alive 

And to explain that to the students

I always felt teaching energetics to our undergraduate biology students was no easy task. And yet all life depends on energy. So, trying to avoid as much as possible entropy, enthalpy, the law of the conservation of energy and the laws of thermodynamics with its not exactly simple equations, I started my physiology energetics lecture with a thought-provoking example. Imagine, I said, you want to compare the amounts of the required energy to heat up and bring to the boil a cup of water and a bucket of water. You need less energy to heat up the smaller amount, but it also cools down much faster compared with the water in the bucket which had needed a much greater amount of energy to reach boiling point. If you decided to re-heat the water and bring it back to 100°C every 30 minutes, which would have required more input of energy over a 10 or a 100 hour period: the cup or the bucket? An important variable to be considered is of course the temperature down to which the boiled water would be allowed to cool before being re-heated. But all that is calculable and can be expressed in mathematical equations.

Students all know that the reverse reaction of photosynthesis is the one that provides us and other animals with energy by ‘burning fuel’, i.e. the food ingested, and that some of the energy is for growth and work, but some is converted to heat; in fact, ultimately all is dissipated as heat. Daily rates of minimum standard heat production in animals are related to body temperatures, so that small 42°C warm birds have the highest rate followed by mammals with body temperatures of 35-37°C and ectothermic animals like reptiles, amphibians and fishes. Body sizes and weights are further complicating factors, because the slopes of the relationship between rate of minimum energy expenditure and body weight are nearly the same for ectotherm (cold-blooded) and endotherm (warm-blooded) animals. The slopes are not 1.0, which would indicate a direct proportion: the slopes are approximately 0.75 and that indicates that a doubling in body weight does not double minimum energy expenditure. What is, however, interesting is that when the minimum metabolic rate is expressed per gram of body weight, one notes that energy expenditure rates shoot up exponentially as body weight decreases: small animals need more energy per gram body weight than larger ones. This explains why small warm-blooded animals, e.g. mice, shrews and humming birds need to ingest food more frequently than bigger species and, of course, fish, amphibians and reptiles with their lower resting metabolic rates.

Although resting metabolic rate and maximum longevity have been regarded as not always being ideal to explain ageing, there is nevertheless an obvious relationship between an animal’s body size and longevity (humans being long-lived but not terribly huge and heavy are a bit of an exception). In species of bigger animals the latter generally enjoy longer possible life spans than species that contain smaller individuals, cf. mice, dogs, horses, elephants and whales. Small animals have a greater surface area than bigger ones and tend to lose heat more readily to the environment than the latter, but this alone apparently does not explain the slope of 0.75, mentioned above and an even lower one of 0.67 if body surface area and body weight are correlated to each other. To explain this discrepancy some researchers suggested entering time as a 4th dimension into the equation. Since longevity in mammals is related to weight, total metabolic capacity has to be subject to the time that an organism spends being alive.

Humans appear to be a special case as some studies have shown that shorter rather than taller people have a greater life expectancy! However, it needs to be pointed out that socioeconomic status, relative weight, regular exercise, gender and health practice styles can influence the outcome. It has, for example been suggested on the basis of Japanese and Dutch studies, the latter involving 7800 men and women, that the taller Dutch (but not Japanese) women could expect to live longer than shorter individuals, but that did not apply to men. With such uncertainties abound, I think it’s comforting to be just average.

© Dr V.B. Meyer-Rochow and, 2021. Unauthorized use and/or duplication of this material without express and written permission from this site’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to V.B Meyer-Rochow and with appropriate and specific direction to the original content.