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Introduction to energy storage

Video transcript
- [Instructor] Now, a very simple premise that we've built our discussion of metabolism on, is that we extract energy from food. And of course, this comes from the fact that we know that ATP is our body's main source of chemical energy, and the way we can produce this ATP is by breaking down nutrients, such as glucose, and fatty acids and proteins, which are all found in food. But of course, the question that you might wonder, is while we don't constantly eat food, right? So, how does our body produce a constant flow of ATP even though we only have food very intermittently? And of course the answer to that question is that our body has evolved to store fuel as well, so that we're not reliant on a immediate reflux of nutrients right after a meal. So in the remainder of the video, I actually wanna go ahead and compare and contrast the three main types of fuel that our body has evolved to be able to store. And then, touch on why one of these fuels is actually a much better storage fuel than the rest of them. So let's start off with glycogen. And glycogen is our body's way of storing carbohydrates. And essentially, it's just a long chain, or a polymer, of glucose molecules that are all attached to each other. And our body stores this mostly in the liver. But also there's some in our muscles, as well. And, if we were to tally up how much glycogen, how many grams of glycogen that we had, let's say, in an average 70 kilogram male, an average healthy 70 kilogram male, we would calculate that there would be approximately 480 grams of glycogen. And, just to give you some idea of how much energy we can extract from glycogen, we can extract approximately four kilocalories of energy per gram or glycogen. And kilocalories is just a unit of energy, and it's something that you might see on cereal boxes, or any type of food really, when you look at the nutrition label. And just to give you some perspective, the recommended average intake of energy for humans is somewhere around 2000 kilocalories per day, and of course that's a huge ballpark number. The exact number might fall above or below this, and really depend on how old you are, what your sex is, as well as how active you are during the day. Now a second type of fuel that our body stores is proteins. And, remember that proteins are nothing more than a long chain of amino acids. And, most of the protein in our body is in our muscles. And again, in an average 70 kilogram man, if we were to tally up how many grams of proteins, we would get around 6000 grams. And again, similar to glycogen, we would be able to extract about four kilocalories of energy per gram of protein. Now, finally, the third type of fuel that our body stores is in the form of fats. And these fats are actually stored up in specialized tissue in our body called adipose tissue. Now, if we were to tally up how many grams of adipose tissue a 70 kilogram man had, it would actually amount to a much higher amount than both proteins and glycogen combined. In fact, it's somewhere around 12,000 grams of fat in an average 70 kilogram healthy male. And, moreover, unlike glycogen and protein, we can actually extract a lot more energy per gram of fat. In fact, that comes out to be somewhere around nine kilocalories per gram of fat. Now to put this in perspective, let's kind of do a fun, simple math problem here. Let's assume that this 70 kilogram guy requires an intake of 2000 kilocalories per day. Now if this is the case, let's kind of ask ourselves a very theoretical question, because of course it would be unhealthy to starve oneself. But if one didn't have a intake of food, how long would this man be able to survive on each type of fuel? So, if we round this to about 500 grams of glycogen, times four kilocalories per gram, that would actually amount to about 2000 kilocalories that we could extract from glycogen, and so that would last him about a day, right? Now, let's move down to protein. So, protein, so we have about 6000 grams in our body, times 4 kilocalories per gram, which amounts to about 24,000 kilocalories, divided by 2000, that would last us about 12 days, right? So you're getting a little bit better. But now let's actually take a look at fats. We have substantially more amount of fats in the body than proteins and glycogen, right? So 12,000 times, let's round this up to ten, since we're just approximating anyways, so if we multiply 12,000 times ten, that's bout 120,000, right? Divided by 2000, that's about 60 days that one could go, theoretically of course, and probably may not even make it that far, but, theoretically we could make it 60 days on just our fat storage alone. Now that's pretty impressive, I think. So now the question I want to answer is, why has that become the major source of storage fuel in our body? And to do that, let's first actually remind ourselves what the chemical structure of fats are. So I've kind of, to save us some time, drawn out the chemical structure of a triacylglyceride. And I'll actually go ahead and write that out here, so tri acyl glyceride. And just a point of clarification, I am using the word fat and triacylglyceride interchangeably. And that's because fat is just kind of an everyday term that we use to refer to the type of fat, the triacylglyceride, that we store in our body. And since we're talking about the chemical structure, it probably makes sense in this case, to refer to it from its chemical name, which is triacylglyceride. And in fact, its chemical name tells us a lot about its structure, so let's take a look. So this tri acyl refers to these three acyl groups, these acyl side chains. So what is an acyl side chain? So, you know, an acyl is just a reference to a type of organic chemistry functional group. So, there are some functional groups you might be familiar with, like hydroxyl groups, or phosphate groups, and in this case, the acyl group is anything that has a carbon double bond oxygen attached to a long chain of carbons and hydrogens. And actually, I should say, the chain doesn't have to be long, it just has to be some type of organic functional group, but in this case, the chain happens to be very very long. And of course, I've kind of just gone ahead and drawn three acyl groups that have come pretty much randomly to my mind, because the idea here is that these chains can vary immensely, depending on the type of triacylglyceride in our body, depending on the type of fats that we ingest. And so if some of them might have single bonds, which we refer to as them being saturated with hydrogens, and some of them might have double bonds, in which case we refer to these triacylglycerides, or these side chains, as being unsaturated. So that's kind of just some nomenclature that you might see. And then, finally, this glyceride refers to the backbone of this molecule, these kind of three carbons that are hanging out down here. And, they also have a oxygen attached to them as well, and of course they link with these acyl side chains through this ester linkage, that I'm kind of highlighting in green here. So, that's the big picture of this molecule. Now, the reason why I think it's important to be familiar with this structure of a fat, when trying to understand why fat is such a prominent type of storage fuel in our body, is because now you can visually see where all of that energy, where all of that nine kilocalories per gram of energy, is coming from. Because looking at this molecule, you can see the bulk of it is formed by these long carbon hydrogen chains. And, these carbon hydrogen chains are referred to as being very high energy, because they have a lot of electrons kind of stored up in these bonds. And we know, if we remind ourselves, back to kind of our general principle, is that if you have a reduced organic molecule, like this, like we can see here, we are able to extract energy by oxidizing it in kind of subsequent steps, and this flow of electrons can be harnessed by something like the electron transfer chain to allow us to produce ATP. So that's kind of my first point here, and I'll scroll down here, and I'll go ahead and write that, which is that the triacylglyceride is a very energy-rich molecule, has lots of these carbon-hydrogen bonds, that can be oxidized to produce ATP. Now a second reason why these triacylglycerides are such a good form of storage energy is because they are relatively chemically inert. So, what I mean by this, is that they're unlikely to react with other things in the body. And this is of course in contrast to things like glucose and proteins, which are quite polar, they have many polar functional groups, like hydroxyl groups, for example, and they can react with a lot of things in the aqueous environment of the body. But triacylglycerides are very, remember, they're not soluble in water, and just remind yourself, if you've ever made salad dressing, it's so hard to mix the oil and water together, right? And so because this isn't going to dissolve in water, and it has a lot of these carbon-hydrogen chains that are considered not to be very reactive, it serves as a great form of storage energy, because it won't randomly react, or be wasted in side reactions. A third reason why fats are good for energy storage is because they have no large or prominent functional role inside the body, in contrast to proteins, for example, which are used to make enzymes. And enzymes, are of course, of paramount importance in our body, and so, we wouldn't want to rely too heavily on proteins, because it would kind of be a conflict of interest for our body, right? Because if we use up too many proteins, we wouldn't be able to make enzymes, and that's why fats end up being kind of a good compromise, because we're able to kind of essentially just store up fats for one major purpose alone, which is to produce energy. Of course, that's a blanket statement, fats are important in other ways as well, but largely speaking, their main role is to store energy. So I've touched on why triacylglycerides are a good form of energy storage and why proteins might not be such a good form of energy storage. But you might be wondering, what about carbohydrates? What about glycogen, why didn't our body evolve to make glycogen the major storage fuel? And that brings me to my fourth point, which is that unlike proteins or carbohydrates, triacylglycerides fats are very hydrophobic. And, a benefit of being hydrophobic, as we kind of touched on earlier, so not only does it make it more inert, chemically, because it can't react with a lot of things that water, in an aqueous environment, but it also means that it won't be kind of weighed down by water. So you might be wondering, why is that beneficial? Why is that beneficial not to be weighed down by water? And to answer that question we can actually just go ahead and do kind of a quick math problem here, to kind of give some insight into that. So the math problem that I want to solve is, how many grams, of glycogen, including, I'm gonna say this, including its water weight, because we know that all of these polar molecules attract some water, and that contributes, of course, to how much weight that they have. So how many grams of glycogen, including the water weight, will our body need, let's say, if we want to have the same number of calories, so I'm gonna say, if we're equal to the same number of calories, kilocalories, that, the 12,000 grams of fat, in the average 70 kilogram man that we talked about above, can produce. So in order to answer this question, we have the information in the table above. But we also just need to know one more thing, which is, how many grams of water are associated with each gram of glycogen? And the answer to that is, there's about three grams of water weight, so let's say H2O weight, associated with one gram of glycogen, or protein actually, for that matter. So I'll just put that in parentheses for our reference. Alright, so to answer this question, we first need to find out how many kilocalories can 12,000 grams of fat produce? And that's simply 12,000 grams of fat, right, times, from the table above, I'll remind you, we said that, for every gram of fat, we can burn about nine kilocalories of energy, and because we're just trying to get a ballpark number, I'm gonna go ahead and round that up to about ten kilocalories, okay? And that is going to be equal to 120,000 kilocalories. So that's how many calories, kilocalories we need to be able to produce with glycogen. Now, with glycogen, we know that for every four kilocalories, we're able to essentially utilize, or for every four kilocalories, we're able to get that out of one gram of glycogen, and that's without accounting for its water weight. But if we do account for its water weight, and we know from this ratio above, right here, that for every gram of glycogen, there is three grams of water weight. I'll just put H2O here to remind us, okay? So, our units are gonna cancel out here, and here, and altogether, if we scroll down, we end up getting 120,000 divided by four, ends up being 30,000 times three, which ends up being a whopping 90,000 grams of glycogen that would weigh us down, in order to produce the same number of calories as only 12,000 grams of fats. So nearly a seven to eight times fold difference in the amount of weight that our bodies would have to carry. And of course, this is not practical, right? Because, we were talking about a 70 kilogram man, and here we have 90,000 grams of glycogen. That's equal to 90 kilograms, which is more than 100% of body weight just in glycogen alone. And so, that's not practical because we haven't evolved a skeletal structure, or enough muscle mass, to be able to handle that extra weight. And so that's probably why fats has evolved to be the most prominent type of energy storage molecule in our body. So just to kind of summarize, we return to our list up here, we notice that not only it's that itself, just by its structure, energy-rich, it's also chemically inert, it plays no functional role, unlike proteins which are important in enzymatic function, and because fats are very hydrophobic, we're able to pack a lot in our body without carrying any extra water weight.