Metabolism and Homeostasis: Study with Video Lessons, Practice Problems & Examples

Animal physiology encompasses metabolism and homeostasis, influenced by factors like body size and surface area-to-volume ratio. Larger animals require more energy but have lower metabolic rates per unit mass compared to smaller animals. Metabolic processes, such as glycolysis, are regulated by feedback mechanisms, primarily negative feedback, which conserves energy. Circadian rhythms, like cortisol and melatonin fluctuations, illustrate physiological regulation. Homeostasis maintains optimal conditions for enzyme function, with systems relying on sensors, integrators, and effectors to respond to environmental changes effectively.

Surface Area to Volume Ratio

Hi. In this video, we'll be talking about 2 very important aspects of animal physiology. Those are metabolism and homeostasis. Now before we get there, we need to understand some of the constraining factors on the animal form. Now, body size and functions are always going to be constrained by physics. For example, larger animals will weigh more and therefore, require thicker skeletons to support that weight, and also bigger muscles to move that weight around. Now one of my favorite examples of how physics can constrain the body size of animals has to do with terrifying giant insects. Believe it or not, millions of years ago there used to be some really scary big bugs out there. And fortunately for us, there are not today. Insects, in fact, basically have a much smaller maximum size than they did way back in prehistoric times when there were, you know, terrifyingly large insects. The reason for this, it's believed, has to do with the amount of oxygen in the atmosphere. There's a limit to how much oxygen can diffuse into an organism. We'll get into the details of all that when we talk about respiration, but, you know, just know that there's a physical limit on the availability of those gases to diffuse into tissues. Now back in these prehistoric times, there was a lot more oxygen in the atmosphere, which allowed for organisms like insects to grow larger than they can today. Because there's less oxygen in the atmosphere today, there's a smaller upper limit on bug size. So, never going to have to worry about that scenario, fortunately. Now, what this kind of gets into is this very important idea of surface area to volume ratio, which essentially, determines the physiology of an animal and its cells. Now, the reason for this is as organisms get bigger, this ratio of surface area to volume actually decreases. And we can see a nice example of that in this graph here that looks at area on the y axis and volume on the x axis. Now, as you can see, as our shapes get larger, if, you know, you look at the line from one shape, so to simulate a cell let's just look at the ball for argument's sake. So as this ball gets bigger you can see that as it gets bigger the line of its area versus volume curves. And it actually increases in volume at a faster rate than its area. So what does this mean? This means that as animals get bigger, they get, or they have less surface area compared to their volume. And this comes into play with ideas like molecular diffusion. Right? The more surface area you have, the more efficient your diffusion will be. It also relates to nutrient use and heat loss. It, organisms that are smaller basically use relatively more energy compared to organisms that are larger. We'll look at that in just a moment. Another way to think of this is that smaller organisms will actually lose more heat to the environment, relative to their larger counterpa

Metabolic Rate

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.

Homeostasis

Circadian rhythms are those daily cycles that result in regular physiological and metabolic fluctuations. One of the most famous and highly studied is the fluctuation between cortisol and melatonin over the course of the day. At night, cortisol is the main stress hormone, and as you can see, its concentration in the body is what this axis is supposed to represent. Think of it as the concentration of these hormones. I'm sorry for using a graph that doesn't have labeled axes. I know that's a real no-no, but it's a pretty picture. So, the point is that cortisol levels shoot up right as you're about to wake up, they peak early in the day, and then steadily drop over the course of the day and into the night, where in the middle of sleeping they start to increase again in preparation for the next morning. Now, cortisol is the main stress hormone, and it would make sense that you would want high levels of stress or alertness early in the day when you're getting up, when you have to look for food. Of course, we just go to the fridge these days, but we used to have to actually struggle to get our food. Anyway, melatonin is almost like a counter to that in a really interesting way. See, melatonin promotes sleep and sleepiness. And as you can see, while cortisol is shooting up in the morning, right here, melatonin is actually chilling out. Melatonin levels drop precipitously right as you are about to wake up, in part so that you don't feel super groggy in the morning. Though don't be thinking that melatonin is the main thing that makes you groggy in the morning. There's actually a lot of other stuff going on there, and it's just related to sleepiness and sleep. Now, levels of melatonin stay low throughout the day. You don't want to be sleeping during the day, but as nighttime sets in, as you should be getting to bed, as an organism that's not living in the age of electricity and works based off a day-night cycle, those levels shoot up to make you sleepy. So you go to bed, have a nice good night's sleep, and right when you're about to wake up, all that melatonin dries up, so that you're not all sleepy in the morning, and at the same time, your cortisol is popping up in order to make you nice and alert when you wake up.

Now, these are just daily cycles, but organisms can show interesting fluctuations in their metabolism and physiology over longer terms. You've probably heard of hibernation, but a lot of animals also do what's called torpor, or experience what's called torpor, which is a short-term state of decreased physiological activity and metabolic rate. It's not as long as hibernation, but you can think of it working to the same effect essentially. Hibernation, of course, I'm sure you're familiar with animals fattening up before winter, where they'll sleep for a really long time and wake up when it's spring again, so that they can kinda wait out the winter when there's not a lot of food and conserve energy. Well, that's not actually a sleep; it's hibernation. It's not a long nap. It's an actual state of depressed metabolic activity. And it's something that's specific to endotherms. We need lots of energy daily in order to sustain ourselves. And when energy in the environment, you know, food is really scarce like in the winter, this is a nice way to, you know, get it so that we can live until the spring, and then wake up, and start to eat again.

Now, organisms can't just let their metabolic, their metabolic, and physiological processes run wild. They have to be very tightly controlled and maintained. Regulation of physiological processes is super important in order to stay alive, because things are changing around us, things are changing within us, and our bodies need to be able to cope and to maintain ideal conditions for our survival. So this regulation of physiological properties is called homeostasis. And a great example of why this is so important is, for example, enzymes, right? Those proteins that basically do everything in our cells, they make the magic of life happen in a large part, function best in very specific physiological conditions. And in fact, proteins are very sensitive to temperature changes and changes in pH. And if you can't maintain these specific conditions for enzymes, they can actually cease functioning, which could obviously be very dangerous and potentially lead to death. That's, of course, just one example. There are other reasons why we need to maintain homeostasis.

Now, there are two strategies that animals will take when they are trying to maintain their internal environments. And those two strategies are conforming, like being a conformer, and regulating, like being a regulator. Conformers don't actively regulate what's going on. Instead, they'll kind of conform to their environmental conditions. It's more like making do with what's around them, instead of trying to fight against it like regulators, which actively control their internal environment, regardless of what fluctuations are occurring in the external environment. So one way you could think of this is in terms of body temperature. You'll have fluctuations in environmental temperature, and conformers will just kinda go along with that. They'll let their body temperature fluctuate more with the environment, whereas regulators will, you know, if it gets colder, for example, burn more energy to maintain that desired body temperature. So, you know, they're kind of fighting against what's happening in the environment instead of being like Zen with it.

Now homeostatic systems are often conceived of as having certain properties, and we're gonna talk about those properties in a very generic way right now. And depending on who your professor is or what book you look at, they might use different terms here. So if you see different terms come up in your course or something, don't worry. It's the same idea really. These are just generic terms. So, you know, don't worry about necessarily memorizing these names. Just kind of understand the ideas. That's what's really important. So a homeostatic system will be based off a set point. This is kind of like the temperature that you set your thermostat to. In your home, if you have a thermostat and you say like, okay, I want my house to be 70 degrees. Now, obviously, your house isn't going to be exactly 70 degrees all the time. That's the set point that your heating system is trying to get to. Right? It might go a little above sometimes, and then compensate, and go a little below, and back and forth. It's the ideal point in the system. Now, a sensor is going to detect stimuli related to the property of the homeostatic system. So, for example, if we're talking about temperature, there will be sensors that will pick up on body temperature cues. Now it's not always as direct as that. For example, the sensors in your brain that look at oxygen concentrations in your blood actually detect pH. They're looking for something, and they detect a property that's related to that. So it's not always so directly connected, but the point is they're looking at some particular property and using some sort of stimulus to keep track of that property. Now the integrator is going to evaluate the sensory information that comes in and determine the appropriate response. This is going to be like the wiring system in the thermostat that goes, uh-oh, I'm detecting that it's 2 degrees colder than the optimal temperature, and so here's what I need to do.

Lastly, you have the effector, which is the thing that actually generates a response to restore the homeostatic system to ideal conditions. And if I jump out of the way, as you can see, we have a nice example of body temperature.RELATED TABLEBehind me. You know body temperature, usually you want to keep it around 37 degrees Celsius. You have cells in your skin and your brain that can detect temperature, and you have a regulatory center in the brain for temperature that's going to decide what to do based on the information coming in from these sensors. And the response, let's say it's getting a little too hot. Well, you're gonna want to sweat. So it's going to stimulate those sweat glands throughout the bodies to secrete sweat, which will evaporate and cool you down. So that's just a nice generic example of a homeostatic system, and we will be looking into some more specific examples as we examine different physiological systems in the animal body. With that, let's turn the page.

Feedback Regulation

Regulation is super important to all metabolic and physiological processes. One of the best strategies for regulating is known as negative feedback, and this is a type of regulation where the output of a system will actually reduce the system's output. Now that's kind of a confusing general way to put it, so let me give you an example that kind of makes this a lot more clear. Here, we are looking at glycolysis, which is the first step of cellular respiration, and a super important process no matter what type of biology you're doing. Now, you don't need to worry about memorizing any of these names or technicalities, you'll have plenty of time for that, if and when you take biochemistry. I just want you to get a sense of how negative feedback works. So glycolysis begins with glucose and you wind up with pyruvate. In the process, you produce some ATP. Right? Now glycolysis is the first step of cellular respiration, which ultimately results in the production of a lot of ATP through oxidative phosphorylation. So ATP is not only a direct product of glycolysis, it's also the major downstream, like endgame product that this whole process is gunning for. So glycolysis is under negative feedback control. The way this works is one of the enzymes that catalyzes a reaction very early in the process, it's a very important step of the reaction, for reasons that you don't need to worry about. This enzyme is called phosphofructokinase, and it is negatively regulated by ATP. So ATP will feed back and shut off phosphofructokinase, shutting down this chemical pathway. So essentially, if there's too much ATP being produced, either directly from glycolysis, but more likely, through the downstream cellular respiration, oxidative sorry, the downstream oxidative phosphorylation process. If there's too much ATP being produced, it's going to negatively feedback and shut off the very beginning of this whole process to conserve resources, not waste energy, and just maintain the balance of generating just as much ATP as is needed. So you can see how powerful and eloquent a system negative feedback is, where a system's output will actually reduce this output of the system in order to control the levels in a nice passive way. Now, positive feedback is very much so a different beast. And it's also a lot rarer to see. And the reason for that is because with positive feedback, the output of a system actually increases the system's output. Right? So the most common example of positive feedback is in birth, where the infant's head pushes and sets off some receptors that send a signal, which induce greater labor contractions, which in turn are going to cause the infant's head to push harder against those receptors, which of course means more labor contractions. And so these two things just up the ante and feed back positively on each other, creating a bigger and a bigger effect. So you can see why something like that you wouldn't want to use in a lot of systems. It could very easily get out of control, which is why negative feedback is everywhere and positive feedback is a lot less common. Now, to look at an example of negative feedback that involves actual systems in the body, want to take a look at something known as the HPA axis. Now, this is going to involve the nervous system and the endocrine system. The nervous system is going to be a system responsible for transmitting information throughout the body, as well as receiving information from the body and the environment. It's going to transmit these signals via nerves, through those electric signals called action potentials. If you want to know more about this check out, the nervous system videos. The endocrine system is also a signaling system, but it functions differently than the nervous system. The endocrine system is a hormone signaling system. So it's going to involve glands that secrete hormones into the bloodstream, and those hormones are going to target and set off reactions at various organs that have their appropriate receptors. So both of these are signaling systems and they're actually connected by this really cool brain structure called the hypothalamus, which basically just means underneath the thalamus, which is where it's located. So very creative naming here. This structure coordinates the autonomic nervous system, which is going to be the part of the nervous system that we don't have direct control over. Right? Things like breathing, heart rate, that sort of stuff. We don't have direct conscious control over what I mean. You know, obviously our hypothalamus is controlling that. So we have control, we don't have conscious control. It's not like the part of the nervous system where I can say, alright, finger wants you to poke, and move, or whatever. So, the hypothalamus links the nervous and endocrine system by coordinating the autonomic nervous system, and also by coordinating the pituitary gland, which is a very important gland in the endocrine system, and in it has a variety of functions. I don't want to get ahead of myself because I could go off on tangents on all of this forever. So here's the important thing to note. You have the hypothalamus, that is a brain structure. Right? And in the HPA axis, which stands for hypothalamic pituitary adrenal axis. Right? HPA. Essentially, what you have is a stress hormone, you know, signaling system. So the hypothalamus can release something called corticotropin-releasing hormone. Don't worry about these names just now. This will stimulate the pituitary, the "P" in the HPA, to release ACTH or adrenocorticotropic hormone, again, like, don't worry about these names. These are just stress hormones, that's all you need to know, that will eventually lead the adrenal cortex to secrete, and let me jump out of the way here, cortisol, which is that main stress hormone. Now, the thing about cortisol is it actually feeds back negatively to the pituitary and the hypothalamus, as you can see here. This is standing for Cortisol, will actually have a negative feedback effect on the hypothalamus and the pituitary, to cause them to stop releasing corticotropin-releasing hormone, and adrenocorticotropic releasing hormone. Or as it's much easier to say, CRH and ACTH. Essentially, the downstream output of that system, cortisol, will go back to earlier points in the system and cause them to shut down that pathway. Again, this is known as the HPA axis and is a really nice example of negative feedback regulation. That's all I have for this video. I'll see you guys next time.

Here’s what students ask on this topic:

What is the relationship between body size and metabolic rate in animals?

The relationship between body size and metabolic rate in animals is inversely proportional when considering metabolic rate per unit mass. Larger animals, like elephants, have a lower metabolic rate per unit mass compared to smaller animals, like mice. This is because larger animals have a smaller surface area to volume ratio, which reduces heat loss and energy expenditure. Conversely, smaller animals lose heat more rapidly and thus have higher metabolic rates to maintain their body temperature. This relationship is crucial for understanding how different animals adapt their energy consumption to their size and environmental conditions.

How do circadian rhythms affect metabolism and physiological processes?

Circadian rhythms are daily cycles that regulate physiological and metabolic processes in organisms. These rhythms influence the fluctuation of hormones like cortisol and melatonin, which in turn affect alertness, stress levels, and sleep patterns. For example, cortisol levels peak in the morning to promote wakefulness and alertness, while melatonin levels rise in the evening to induce sleepiness. These cycles ensure that metabolic processes are optimized for different times of the day, helping organisms maintain energy balance and overall homeostasis.

What is the difference between basal metabolic rate (BMR) and standard metabolic rate (SMR)?

Basal Metabolic Rate (BMR) and Standard Metabolic Rate (SMR) are measures of energy consumption in animals at rest. BMR refers to the minimum rate of energy expenditure of endotherms (warm-blooded animals) at rest, under neutral temperature conditions. It reflects the energy required for basic physiological functions like breathing and circulation. SMR, on the other hand, is the minimum rate of energy expenditure of ectotherms (cold-blooded animals) at rest, also under neutral temperature conditions. Unlike endotherms, ectotherms rely on external sources of heat, so their energy requirements are generally lower than those of endotherms.

How does negative feedback regulate metabolic processes?

Negative feedback is a regulatory mechanism where the output of a system inhibits its own production to maintain balance. In metabolic processes, an example is the regulation of glycolysis. The enzyme phosphofructokinase, which catalyzes an early step in glycolysis, is inhibited by high levels of ATP. When ATP levels are sufficient, ATP binds to phosphofructokinase, reducing its activity and slowing down glycolysis. This prevents the overproduction of ATP and conserves resources, ensuring that energy production is matched to the cell's needs. Negative feedback thus helps maintain homeostasis by adjusting metabolic pathways based on the current state of the cell.

What are the main strategies animals use to maintain homeostasis?

Animals use two main strategies to maintain homeostasis: regulation and conformation. Regulators actively control their internal environment to maintain stable conditions despite external fluctuations. For example, mammals regulate their body temperature through metabolic processes like sweating or shivering. Conformers, on the other hand, allow their internal conditions to vary with the external environment. For instance, many fish conform to the temperature of the water they inhabit. Both strategies are essential for survival, allowing animals to adapt to their specific environments and maintain optimal physiological conditions.