Energy is an essential ingredient for life. In fact, it's so important there's a whole field of study dedicated to examining how energy moves through living systems, and especially looking at how energy flow is related to an animal's size and its metabolism. Now metabolism is the sum of the chemical processes of an organism that sustain its life. A great example of one of these processes is cellular respiration, which is, of course, the process by which cells break down carbohydrates from proteins, sugars, and fats, and convert it into energy in the form of ATP through that amazing process known as oxidative phosphorylation. Go check out the videos on cellular respiration if you want a refresher, want to know more about this process. Now metabolism is often conceived of as a metabolic rate, or a rate of energy consumption. And there are different ways to measure metabolic rates, but I think this chart is a super neat example of metabolic rate. A specific type that we know as basal metabolic rate, which is basically the minimum rate of energy consumption of an endotherm at rest. Now, basically at rest means, you know, not exerting themselves physically, not stressed out, so just like, you know, chilling out. Kind of like what it sounds like—they're just relaxing, chilling out maxed, and relaxing all cool. Endotherm, of course, that's organisms like us that generate our body heat from internal processes. And what's so nifty about this chart is you can see how different types of food we eat actually sustain our energy in different ways. Carbohydrates, as you can see, give us a nice initial boost of energy, but they kind of don't last. Right? They plummet after a while. Proteins, on the other hand, don't give us as quick an initial boost of energy, but you can see that they sustain us much longer. Right? That curve goes way above the carbohydrate curve. They provide us with more long-term energy. Now, basal metabolic rate is looking at endotherms. There's another measure we know as standard metabolic rate, that is the minimum rate of energy consumption of an ectotherm at rest. And an ectotherm, you may recall, is an organism that absorbs most of their body heat from an external source. It doesn't mean they can't generate any heat internally, it's just that, their main source of it is coming from outside. Now, when you start comparing metabolisms of different animals, some really cool patterns come out. One of the more obvious ones is that warm-blooded organisms are going to have higher metabolic rates than cold-blooded organisms. And hopefully, that comes as no surprise. Warm-blooded organisms, and, you know, these terms warm-blooded, cold-blooded, very imprecise, kind of like common terms we throw around. We'll talk about the technicalities of all of that in a different lesson, so don't stress now. It's just, you know, warm-blooded, cold-blooded, you know, we can think of this in layman's terms for the sake of the explanation. So anyways, warm-blooded organisms have to consume energy to warm their bodies, whereas cold-blooded organisms are going to be, again, absorbing most, are going to be, well, absorbing heat for their bodies, but also not expending a ton of energy to warm their bodies. There are other sorts of strategies in there too. But the main point is that these guys are consuming energy and therefore adding to their metabolic rate in order to heat themselves, and these guys aren't so much. And hopefully, it comes as no surprise that unicellular organisms, which, of course, are very simplistic, have smaller energy requirements than these multicellular organisms, and therefore, will have even lower metabolic rates. That's what these lines are showing us, the metabolic rate increase of these particular types of organisms. Now, a really interesting pattern to note comes out when you compare larger animals and smaller animals. So looking at an elephant and a mouse, obviously, the elephant is far larger than a mouse, and in terms of total, like, tonnage of energy, sheer quantity of energy, they need more. I mean, obviously. Right? They are much bigger organisms. They have much larger muscles that need to be powered. So, of course, they are going to require more energy than a little mouse. Here's the thing. When you look at their metabolic rates compared to their body size, though, so essentially, you find the relative metabolic rate of metabolic rate, you know, look. When you look at the metabolic rate relative to the body size of the organism, what you see is that smaller animals, like the mouse, will actually have a larger relative metabolic rate than a larger animal, like an elephant. Essentially, pound for pound, the elephant's metabolic rate is lower than the mouse's. And this again has to do with those patterns of surface area to volume. Right? Larger organisms are going to be less prone to heat loss, for one thing, than smaller organisms. So smaller organisms are going to have to dedicate a greater percentage of their metabolic rate to warming themselves. Just lots of interesting patterns and things to look at when you start delving into the comparisons between energy use of different animals.
With that, let's flip the page.