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Video transcript

- [Instructor] You may know that athletes and sports people have a lot more stamina compared to non-athletes like me, right, because of the practice that they have. But, did you know that if you were to look inside their muscle cells, because of all the practice, they have developed something more in number. Their cells have something more in number compared to that of non-athletes. What exactly is that? And why do they have that more in number? How does that help them get more stamina? To answer this question, we need to find out exactly where inside the cell respiration takes place, why? Because it's the respiration that gives us energy. Now in a previous video, you've seen how cells can take glucose and break it apart to give us energy which is called respiration, right? And this can be done in the presence of oxygen which is uses a lot of energy. We call this as aerobic. And it can be done in the absence of oxygen too where we get little bit of energy. This is called anaerobic respiration or fermentation. But if this seems new to you or if you feel you need a refresher, it'll be a great idea to go back and watch that video on aerobic and anaerobic respiration. This is explained in detail over there, and I've also explained how to remember these equations. And so if you feel you are comfortable with this, great, let's go ahead. So these reactions are happening inside our cells, right? The question now is in exactly which part of the cell are these reactions taking place? The problem in trying to answer that question is these are not one-step reactions. Okay, it takes multiple steps to do this. For example, over here, it's not that oxygen just gets added to glucose and we get carbon dioxide directly. There are multiple things happening in between which is not shown over here, okay. And guess what? It turns out that some of these steps will take place in one part of the cell, and other steps will take place in some different part of the cell. So should we learn all the steps in between? Well, no, we don't have to. There are many steps in between and we don't have to learn all of them. We will just learn the first step of respiration, okay. It turns out that regardless of which respiration you want to do, whether you wanna do aerobic or lactic acid fermentation or ethanol fermentation, whichever you wanna do, the first step is common. Let me just move it a little bit to the right more. And already that first step you know. In the first step, glucose gets broken into two pieces, that's how I like to think about it, glucose gets broken in two molecules and each of them are called pyruvate, two molecules of pyruvate. All right and in this process, again because glucose is getting broken, we get a little bit of energy. So some energy is released. Again, I'm writing that energy in small just to showcase that little bit of energy is released over here. Now, pyruvate might be a very new word, right? So what exactly is this? It's just a molecule, just like how we have glucose, pyruvate is another molecule and we will not look at what its formula is and all of that, not needed. Just a little bit, to give you a little bit of insight what it is, glucose is a six carbon molecule, it has six carbons inside it. It turns out that pyruvate has three carbons inside it. And that's why I like to think that glucose is broken into two parts but it's not exactly broken into two halves. There are other reactants in between as well. We don't have to worry about that in detail but this is the first step, okay? And we also give a name to this step. This particular step is called glycolysis. Glycolysis. You might hear this word a lot in respiration, okay? It seems like a fancy word but you know what it's telling us? The word glyco is referring to glucose. So this, this over here means glucose. And lysis means breaking. So you can now kind of see what this means. Breaking of glucose and that's basically what's happening. Glucose is being broken into two molecules of pyruvate. And when this pyruvate gets further broken in the presence of oxygen, that's when we get this aerobic respiration. And that's mostly what's happening inside your cells right now. On the other hand, if the pyruvate gets broken without oxygen, then we'll get these reactions. But one important difference between these reactions are when you break pyruvate in the presence of oxygen, these reactions also release energy and that's why aerobic respiration releases a lot of energy. On the other hand, if we break pyruvate without oxygen, that does not release energy. So to release energy from pyruvate, it has to be broken in the presence of oxygen. And so now you can see that if you take the aerobic pathway, we get energy when glucose is broken into pyruvate during glycolysis and we again get a lot of energy when pyruvate is further broken down and it's for that reason aerobic respiration is awesome because it gives you a lot of energy. On the other hand, if you take the fermentation pathway, you see that energy's only released during the glycolysis part. Further, when you break down pyruvate, now more energy's released and that's why fermentation only gives you little bit of energy that was released during glycolysis. And so now we can answer original question. Where do these reactions take place? Well, it turns out that the ones that do not require oxygen, they happen in the cytoplasm of the cell. And just to remind you what cytoplasm is, if this is an animal cell, let's say, then this dotted stuff represents the cytoplasm, this whole thing. The stuff that is outside of the nucleus but within the cell is the cytoplasm. And that's where all the anaerobic stuff is happening, the glycolysis happens in the cytoplasm and the fermentation are also happening in the cytoplasm. On the other hand, if we want aerobic respiration, this part, then that happens inside the mitochondria. So if you want aerobic respiration, once the glycolysis happens in the cytoplasm, the pyruvate has to enter into the mitochondria and then oxygen also has to enter into the mitochondria and that's where this reaction takes place and after the reaction takes place, it's the mitochondria that will release all that energy. All that energy and that is the reason why they say mitochondria is the powerhouse of the cell because it's the mitochondria that's releasing most of the energy needed for the cell by performing aerobic respiration. And so now can you guess what organelles do the muscles cells of athletes have more? Well, if you guessed mitochondria, then you are absolutely right because the more mitochondria you have, the more you can carry out these reactions simultaneously and the more energy that can be released whenever you want. And so scientists have found that on average, the cells, the muscle cells of athletes have about twice as much mitochondria compared to the muscle cells of a non-athlete which means when you do regular practice, one of the changes that you find inside your body is you'll start developing, your muscle cells will start developing more and more mitochondria. That's pretty cool, right? And so the energy which is released by these mitochondria can now be used up by the cell to perform all the activities, right? That makes sense, right? But guess what? That's not what happens. The energy released by breaking glucose is not used up by the cell. Then what happens? Well, it turns out that that energy is stored in yet another molecule, okay? And that molecule is a pretty famous molecule, it's called ATP and it stands for adenosine triphosphate. Now you don't have to worry too much about that lengthy name, we'll just call it as ATP but that energy gets stored in that ATP and when I first learned this, it didn't make any sense to me because let's go to a different screen to summarize what respiration is. See, in respiration we have glucose. Let's say that this is our glucose, what do we do, what do the cells do? They break it open and release that energy but instead of using that energy, what do they do? They take that energy and store it in yet another molecule called ATP. So basically they're transferring energy from glucose into ATPs, into smaller packets but why, why are they doing that? Well, that's because breaking glucose releases a lot of energy and cells do not need that much energy to carry out their function. This is too much for them and it's for that reason they store it into smaller packets and then whenever cells want to perform any energy, they just break ATP open and ATP will now have just the right amount of energy they need to perform their functions. Now, to understand this even better, think of energy as money. Then glucose would be like a large denomination, like a 2,000 rupees notes and ATP is like the smaller denomination, like 10 or five rupees note. Now, if I want to eat panipuri or ice cream or chocolates, it just makes a lot of sense to carry 10 rupees notes, right? I mean, the panipuri guy outside my house doesn't even accept 2,000 rupees note. On the other hand, if I'm traveling from one place to another and let's say I'm not buying anything, just want to carry money from one place to another, now it makes a lot of sense to carry higher denominations. If I just carry 10 rupees note, my wallet will get filled up and I will have no space. In a similar manner, the functions needed to carry out by the cell only require little bit of energy and so ATP's perfect for them. On the other hand, when energy needs to be transported from one place to another, we will transport it in terms of glucose because it's compact. If everything was in terms of ATP, it would just take up a lot of space and turns out, ATP is also a bulky molecule. And so when we're transferring energy from one part of the body to another or from one cell to another, usually it is in glucose, it's compact, it carries a lot of energy but when the cell wants to use that energy, first it'll store it into ATP and then will use ATPs. And another important reason to use ATP is releasing energy from glucose is not a one-step process. We just saw that glucose has to first broken into pyruvate, then the pyruvate has to enter into mitochondria, the mitochondria then carries out a series of reactions to give energy. Right, so this is not instant. On the other hand, ATPs, well, to break ATP open, it's just one-step process. So it gives us instant energy whenever a cell wants it, so it just makes a lot of sense to use ATPs. And so this means if we go back, the mitochondria does not just release that energy, mitochondria puts that energy into the ATP and then it gives ATP out and then that ATP will be used up by the cell whenever it wants to do any work and just to give you some numbers, the ATPs produced by the mitochondria per glucose, so for every glucose molecule, we get about 36 ATPs over here and when that glucose gets broken into pyruvate in the cytoplasm, we get about two ATPs over here. This also is given into ATP. So you can see in the aerobic respiration, you get total two plus 36, about 38 ATPs, although that's a textbook number, in reality, it may vary. It could definitely be less than that. But during the fermentation process, we only get two ATPs in total and that's because remember, if you break pyruvate without oxygen, we do not get any further energy. And so in the entire fermentation process, only two ATPs and which is why most of our energy comes due to aerobic respiration for which mitochondrias are super important to keep us alive. All right, so what did we learn in this video? We saw the first step of respiration is glycolysis where glucose is broken down into pyruvate molecules releasing a little bit of energy and then we saw that if the pyruvate gets broken in the absence of oxygen, we get fermentation process and all of this anaerobic stuff happens in the cytoplasm. On the other hand, if the pyruvate enters into the mitochondria in the presence of oxygen, then we get aerobic respiration where a lot of energy's released. So aerobic happens inside the mitochondrias and finally, we saw that the energy released in the respiration process is stored in ATP molecules. They act like tiny packets of instant energy whenever the cells want it.