A Biologist Tries to Think...About Energy in Animals

After our discussion about the penguin walking uphill, which I will have to make a post about (how did that not happen?), I've been thinking about endothermy ("warm-bloodedness") and ectothermy ("cold-bloodedness." Endothermic creatures work very hard to keep their body temperature at an ideal level for their metabolic activities; this means they can be active in a much wider variety of physical and temporal environments than ectotherms can, but they require several times as much food at the same body size. Ectotherms on the other hand are more vulnerable to environmental changes, but don't need to eat as much.

In flagrant violation of Energy Project principles, I started with definitions from the College Board's AP Biology Curriculum Framework:

Endothermy: The use of thermal energy generated by metabolism to maintain homeostatic body temperature 

Ectothermy: The use of external thermal energy to regulate and maintain body temperature

Then I tried to draw an "energy diagram" for an endothermic animal. As I've mentioned before, a lot of the transformations that biologists consider most important don't look like transformations to a physicist at all. There's a lot of chemical to chemical going on here. There is thermal energy "lost" at each step (by which I think I mean the chemical products contain less usable energy than the reactants...is that what I mean?) An endotherm is pretty good at retaining some of that thermal energy, but still a lot is "lost" to the environment. 

After this I was going to draw an ectotherm, but as I started to think about it, I realized that none of the transformations are actually different in an ectotherm. The difference is in the amount of food consumed, and the amount of thermal energy generated and maintained. Endotherms have body structures (fat, blubber, fur, feathers) and behaviors (shivering, sweating) that help them keep their body temperatures at optimal level. These structures and behaviors require energy, but also can provide a "payoff". 

My thinking here led me to question College Board's definition.

I'm wondering if this endothermy/ectothermy is one more example of a false dichotomy. I had a brainsplosion recently that ionic and covalent bonds are really points on a spectrum. Maybe endo and ectothermy are the same--an animal gets some proportion of its thermal energy from the environment and some proportion from metabolism. I bet those proportions could even change!!

One postscript; here is the footnote to the Glucose (C) to ATP (C) pathway.

I work a lot in my class to distinguish between where the matter in a reaction comes from vs where the energy comes from. Maybe a future blog post...