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Current time:0:00Total duration:16:27

Introduction to energy storage

Video transcript

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 AGP 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 as well 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 influx of nutrients right after a meal so in the remainder of the video I actually want to 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 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 extract approximately 4 kilocalories of energy per gram of glycogen and kilocalories is just a unit of gene 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 2,000 kilocalories per day and of course is a huge ballpark net worth 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 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 6,000 grams and again similar to glycogen we would be able to extract about 4 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 meal and moreover unlike glycogen and protein we can actually extract a lot more energy per gram of fat in fact it's some that comes out to be somewhere around 9 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 acquires an intake of 2,000 kilocalories per day now if this is the case that's kind of ask yourself a very theoretical question because of course it would be 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 4 kilocalories per gram that would actually amount to about 2,000 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 proteins do we have about 6,000 grams in our body times 4 kilocalories per gram which amounts to about 24,000 kilocalories divided by 2,000 that would last us about 12 days right so we're getting a little bit better but now let's actually take a look about 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 10 since we're just approximating anyways and so if we multiply 12,000 times 10 that's about 120,000 right divided by 2,000 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 tri ethyl glyceride and I'll actually go ahead and write that out here so tri acyl glyceride now just a point of clarification I am using the word fat and tri acyl glyceride interchangeably and that's because fat is just kind of an everyday term that we use to refer to the type of fat the triethyl glyceride that we store in our bodies we're talking about the chemical structure it probably makes sense in this case to refer to it from its chemical name which is tryi so let's arrive and in fact its chemical name tells us a lot about its structures let's take a look this triethyl refers to these three acyl groups these acyl side-chains so what is an acyl sidechain so you know an acyl is just a reference to each type of organic chemistry functional groups 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 hydrogen's 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 triglyceride in our body depending on the type of fats that we ingest and so some of them might have single bonds which we refer to as them being saturated with hydrogen's and some of them might have double bonds in which we in which case we refer to these triacylglycerols 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 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 the structure of a fad when trying to understand why fad is such a prominent type of storage fuel in our bodies because now you can visually see where all of that energy well that 9 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 transport 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 tri acyl glyceride 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 tri acyl glycerides are such a good form of storage energies because they're 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 as a body but try acyl glycerides are very remember they're they're not soluble in water and just you know remind yourself of you know 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 you know going to dissolve in water and it has a lot of these you know carbon hydrogen chains that are considered not to be very reactive it serves a great form of storage energy because it won't you know 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 of 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 a body right because if we use up too many proteins we wouldn't be able to make enzymes and that's why fat's end up being kind of a good copper 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 the 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 triethyl glycerides 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 evolved to make like egde in the major storage fuel and that brings me to my fourth point which is that unlike proteins or carbohydrates try acyl glycerides fats are very hydrophobic and a benefit of being hydrophobic as we kind of touched on earlier so not only does that make it more inert chemically because it can't react with a lot of things in the 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 it beneficial not to be weighed down by water and to answer that question we can actually just go ahead and to 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 going to 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 I'm going to say if we're equal to the same number of calories kilocalories that the 12,000 grams of fat in the average 70 kilogram and 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 3 grams of water weight so say each to a weight associated with one gram of glycogen or protein actually for that matter so I'll just put that in parenthesis for our reference all right so Dan Jones question we first need to find out how many kilocalories can 12000 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 9 kilocalories of energy and because we're just trying to get a ballpark number I'm going to go ahead and round that up to about 10 kilocalories okay and that is going to be equal to a hundred and twenty thousand kilocalories so that's how many calories kilocalories and we need to be able to produce with glycogen now with glycogen we know that for every 4 kilocalories we're able to essentially utilize for every 4 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 we know from this ratio above right here that for every gram of glycogen there is 3 grams of water weight I'll just put h2o here to remind us ok so our units are going to cancel out here and here and all together if we scroll down we end up getting 120,000 divided by 4 ends up being 30,000 times 3 which ends up being a whopping 90 thousand grams of glycogen that would weigh us down in order to produce the same number of calories is only 12,000 grams of fat so nearly a 7 to 8 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're we're 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 a hundred percent 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 fat 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 is fat 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