 Have you wondered why you need to breathe oxygen to stay alive? How does oxygen help our bodies? And what's the connection of this oxygen with the pain that you get sometimes when you are lifting weights or when you are running? The cramps that we get. How are these things connected? The answer lies in cellular respiration. So what is this? Let me start with what it is not. Respiration reminds me of breathing. But let me tell you, respiration is not breathing. Even though in our daily terms we might associate respiration with breathing, but in biology there are two different things. So what exactly is respiration then? Well, respiration is a process in which the cells of your body, your cells, release energy from glucose. And in our daily terms we usually say burning calories, right? That's what we mean. Burning calories is respiration. So this is where your cells are releasing that energy from the glucose. Now to be more clear, what does this mean? Here's how I like to think about it. So imagine this is your body. The white dots represent the glucose inside your cells. Why do you need glucose? Because glucose has energy inside of it. But that energy is trapped. You can't use it now. So to release that energy, your cells carry out some chemical reactions. They break down that glucose. And that's what respiration is. So respiration is where your cells break down that glucose and release this energy. Think of these yellow dots as energy. And then your body can now use that energy for all the various activities. It's that same energy that keeps your heart pumping and makes your brain work. Almost everything that you do comes from that energy. And now we can answer why do we need oxygen? If we go back, these are glucose molecules. Guess what? Cells need oxygen to break that glucose. So this is where oxygen comes into picture. So when we breathe oxygen, the oxygen goes to all the cells. Then the cells use oxygen to break that glucose and release energy. Okay, so why do we need to breathe oxygen? Because cells for respiration, they require that oxygen to break the glucose. So let's write this down as a chemical reaction now. So what do you think are the reactants for this reaction? Well, we saw that we need glucose. So let's write that. Glucose is C6H12O6. This is the molecule of glucose. This is the molecule of glucose. And we saw that cells also require oxygen. They also need oxygen. So this is what they're taking. And then what did we see? What happens? We saw that the oxygen reacts with glucose. It gets broken down and energy is released. Right? So what do we get when we break glucose in the presence of oxygen? Well, we get carbon dioxide. Carbon dioxide is the gas that we throw out of our body. We exhale that because we don't need it. And we also get water. Water is useful for the body and all the cells. So water is retained. And most importantly, we get energy. This is the energy that you saw in the animation. And you may have to balance this equation, which I'm pretty sure you can do all by yourself. But anyways, this is our equation of respiration. Now, how do you remember this? Well, here's a trick. If you look at this equation carefully, it's a little familiar. This is the reverse of photosynthesis. Remember photosynthesis? That was where plants and all the green things will take up carbon dioxide and water and the energy from the sunlight and use it to manufacture glucose and give out oxygen. Right? So respiration is exact opposite of that. So in photosynthesis, plants trap the energy from the sun into glucose and respiration is the exact opposite. They release that energy from glucose. And we've talked a lot about how to remember the photosynthesis equation in a previous video on photosynthesis. So if you need a refresher, great idea to go back and watch that. Anyways, plants also need oxygen to release that energy. I should think plants only need carbon dioxide, but they also need oxygen, remember. And similarly, your cells also need oxygen to release that energy. And this is happening right now. It's that same energy that's allowing your body to function and allows you to do all the work. And it's the same energy that's allowing me to walk right now by contracting and relaxing the muscles of my body. Now, since walking is slow, this energy is sufficient. But what happens when I start running? Well, you can imagine when I start running, I spend this energy faster. That means these reactions now have to be carried out faster to give me more energy, right? So that requires a lot of glucose and a lot more of oxygen. Now, glucose is fine because we have a lot of glucose in our body. So there is no problem with it because I ate a lot of food. But what about oxygen? Oxygen comes from outside our body, right? So our cells start demanding now. They say, we need oxygen faster, faster, faster. I need more oxygen, faster. And therefore, the brain will say, okay, okay, fine, I'll give it to you. I'll give it to you. And brain forces you to breathe shallow. Make you breathe faster, right? And that's why we do the shallow breathing. Why do we do that? Because I want to get that oxygen fast and I want to release that carbon dioxide from my body very fast. And so because of the shallow breathing, because of this, the shallow breathing starts. But guess what? Even that is not enough for the core cells over here. You see, the core muscles of my legs are doing the maximum amount of work in running. And so the cells over there need to release a lot of energy. And even with that shallow breathing, the cells are saying, no, this is not enough. I need oxygen even faster, even faster. And the brain says, I can't give you that oxygen that faster. We can't breathe that fast. That's not possible. So what to do because they need that energy? So the cells now start doing that a desperate attempt of getting energy. They start conducting a different kind of respiration. They start breaking down that glucose without oxygen. So let's look at the reaction where glucose is broken down without oxygen. And so when there is lack of supply of oxygen, some of the glucose molecules get broken without oxygen. And you know what we'll get? This time we will not get this because this needs oxygen. We will get one product, lactic acid. That's what we'll get. Lactic acid. And it releases some energy. Energy. And you might see I'm writing energy very small over here. You may not be able to see that also. And the reason is the energy released is very small compared to over here. Okay. And so that's why this kind of respiration definitely is not all that great. But it's a desperate situation. Oxygen supply is very low. And so some glucose molecules are broken this way to give that extra energy. This is what gives you that boost. Okay. And if you keep on working out, then this reaction keeps on happening. And as a result, lactic acid starts getting built up in your muscles. So when I'm running, a lot of lactic acid gets built up in my legs. And that lactic acid that makes those muscles acidic and causes that burn and causes that pain. And sometimes it also causes cramps. So it's the lactic acid that does that. Okay. Now, of course, recent studies show that lactic acid may not be the culprit. But our syllabus has not caught up with the recent studies. So in your exams, if they ask you what causes cramps in athletes, our answer is going to be lactic acid. And so now you can see your cells can break down glucose in two different ways. One by using oxygen and one without oxygen. So we need to give them names. The one where oxygen is used, we call that as aerobic respiration. So this is called aerobic. And the word aerobic kind of means in the presence of air and air here refers to oxygen. And since this respiration does not involve oxygen in it, it is called anaerobic. So it's called anaerobic without oxygen. Or it's also called, it's also given another name, it's called fermentation. They're both same things. Fermentation. So this needs no oxygen. That's a great thing. But it builds up lactic acid and very little energy is produced. Now lastly, there is another possibility for anaerobic respiration which we, our muscle cells, our cells can't do it. But some microorganisms and fungi, the most famous one is yeast. So this happens in yeast. They can also perform anaerobic respiration meaning without oxygen. But they don't get lactic acid. You know what they give when they break glucose? They give out carbon dioxide. So they give out carbon dioxide. Plus a very important product called ethanol. Ethanol. And again, plus energy. If it's respiration, it has to release energy. Again, very tiny amounts of energy. You may not be able to see this, but that's energy, okay? Very small compared to this. Now what is this ethanol? Now you learn a lot about this in chemistry that ethanol is a kind of an alcohol that are different kinds. And this is the one that people consume. And so you can see that yeast has all of importance in alcohol industry. And how do we remember this reaction now? Well, I like to imagine a picture of a bottle of beer. Now we do not promote drinking or alcohol or anything, but the picture helps to remember the fermentation of yeast. So they put yeast in this beer, right? And that's what produces the ethanol. And the carbon dioxide produces this froth. But a more mind-blowing example of this is when I learned that yeast is used in bread and that's what makes it fluffy. So when they prepare the dove of that bread, they add some sugar and yeast to it. And then they let it sit for a while. The yeast starts breaking down that glucose from the sugar and causes carbon dioxide to be released from inside. And that carbon dioxide, since it's a gas, it makes that dove rise. Then they start baking it maybe by putting it in an oven. And once you start heating it, the carbon dioxide gas starts expanding even more and eventually at a particular temperature, both of them start escaping from that, leaving behind air spaces. And that's the air space that you see in that bread, which makes it spongy and fluffy. Beautiful, right? And so both of these are fermentation or anaerobic reactions. This one is lactic acid fermentation and this one is called alcohol fermentation. So that's pretty much it. So let's summarize. What did we learn in this video? We saw that respiration in biology is not the same thing as breathing. It is process where cells release energy from glucose. And one way of doing that is in the presence of oxygen, which we call it aerobic respiration, which gives the maximum amount of energy. And how do we remember this? I remember this as the opposite or reverse of photosynthesis equation. And then we saw when there is shortage of oxygen, sometimes our cells can break down glucose without oxygen. This time we get lactic acid and a little bit of energy. Because this is without oxygen, we'll call this anaerobic or fermentation. And the way I remember this equation is that I remember that I'm working out that pain is caused, that cramps are caused by lactic acid. Even though it may not be really true for the sake of syllabus, we'll use this. And finally, some other microorganisms like yeast can undergo a different kind of fermentation, where they give us carbon dioxide and ethanol. And again, a little bit of energy. And the way I remember this is I know yeast is used in alcohol industries and it's also used in bread making and that bread dove rises because of the carbon dioxide gas that gets trapped.