Introduction to cellular respiration | Cellular respiration | Biology | Khan Academy

In my humble opinion, the single most important biochemical reaction, especially to us, is cellular respiration. And the reason why I feel so strongly about that is because this is how we derive energy from what we eat, or from our fuel. Or if we want to be specific, from glucose.

At the end of the day, most of what we eat, or at least carbohydrates, end up as glucose. In future videos I'll talk about how we derive energy from fats or proteins. But cellular respiration, let's us go from glucose to energy and some other byproducts.

And to be a little bit more specific about it, let me write the chemical reaction right here. So the chemical formula for glucose, you're going to have six carbons, twelve hydrogens and six oxygens. So that's your glucose right there.

So if you had one mole of glucose-- let me write that, that's your glucose right there-- and then to that one mole of glucose, if you had six moles of molecular oxygen running around the cell, then-- and this is kind of a gross simplification for cellular respiration. I think you're going to appreciate over the course of the next few videos, that one can get as involved into this mechanism as possible. But I think it's nice to get the big picture.

But if you give me some glucose, if you have one mole of glucose and six moles of oxygen, through the process of cellular respiration-- and so I'm just writing it as kind of a big black box right now, let me pick a nice color. So this is cellular respiration. Which we'll see is quite involved.

But I guess anything can be, if you want to be particular enough about it. Through cellular respiration we're going to produce six moles of carbon dioxide. Six moles of water.

And-- this is the super-important part-- we're going to produce energy. We're going to produce energy. And this is the energy that can be used to do useful work, to heat our bodies, to provide electrical impulses in our brains.

Whatever energy, especially a human body needs, but it's not just humans, is provided by this cellular respiration mechanism. And when you say energy, you might say, hey Sal, on the last video didn't you just-- well, if that was the last video you watched, you probably saw that I said ATP is the energy currency for biological systems. And so you might say, hey, well it looks like glucose is the energy currency for biological systems.

And to some degree, both answers would be correct. But to just see how it fits together is that the process of cellular respiration, it does produce energy directly. But that energy is used to produce ATP.

So if I were to break down this energy portion of cellular respiration right there, some of it would just be heat. You know, it just warms up the cell. And then some of it is used-- and this is what the textbooks will tell you.

The textbooks will say it produces 38 ATPs. It can be more readily used by cells to contract muscles or to generate nerve impulses or do whatever else-- grow, or divide, or whatever else the cell might need. So really, cellular respiration, to say it produces energy, a little disingenuous.

It's really the process of taking glucose and producing ATPs, with maybe heat as a byproduct. But it's probably nice to have that heat around. We need to be reasonably warm in order for our cells to operate correctly.

So the whole point is really to go from glucose, from one mole of glucose-- and the textbooks will tell you-- to 38 ATPs. And the reality is, this is in the ideal circumstances that you'll produce 38 ATPs. I was reading up a little bit before doing this video.

And the reality is, depending on the efficiency of the cell in performing cellular respiration, it'll probably be more on the order of 29 to 30 ATPs. But there's a huge variation here and people are really still studying this idea. But this is all cellular respiration is.

In the next few videos we're going to break it down into its kind of constituent parts. And I'm going to introduce them to you right now, just so you realize that these are parts of cellular respiration. The first stage is called glycolysis.

Which literally means breaking up glucose. And just so you know, this part, the glyco for glucose and then lysis means to break up. When you saw hydrolysis, it means using water to break up a molecule.

Glycolysis means we're going to be breaking up glucose. And in case you care about things like word origins, glucose comes from, the gluc part of glucose comes from Greek for sweet. And glucose is indeed sweet.

And then all sugars, we put this ose ending. So that just means sugar. So you might think it's kind of a redundant statement to say sweet sugar.

But there are some sugars that aren't sweet. For example, lactose. Milk, it might be a little bit, but when you actually digest lactose then you can turn it into an actual sweet sugar, but it doesn't taste sweet like glucose or fructose or sucrose would taste.

But anyway, that's an aside. But the first step of cellular respiration is glycolysis, breaking up of glucose. What it does is, it breaks up the glucose from a 6-carbon molecule-- so it literally takes it from a 6-carbon molecule-- let me draw it like this-- a 6-carbon molecule that looks like this.

And it's actually a cycle. Let me show you what glucose actually looks like. This is glucose right here.

And notice you have one, two, three, four, five, six carbons. I got this off of Wikipedia. Just look up glucose and you can see this diagram if you want to kind of see the details.

You can see you have six carbons, six oxygens. That's one, two, three, four, five, six. And then all these little small blue things are my hydrogens.

So that's what glucose actually looks like. But the process of glycolysis, you're essentially just taking-- I'm writing it out as a string, but you could imagine it as a chain-- and it has oxygens and hydrogens added to each of these carbons. But it has a carbon backbone.

And it breaks that carbon backbone in two. That's what glycolysis does, right there. So you've kind of lysed the glucose and each of these things.

And I haven't drawn all the other stuff that's added on to that. You know, these things are all bonded to other things, with oxygens and hydrogens and whatever. But each of these 3-carbon backbone molecules are called pyruvate.

We'll go into a lot more detail on that. But glycolysis, it by itself generates-- well, it needs two ATPs. And it generates four ATPs.

So on a net basis, it generates two-- let me write this in a different color-- it generates two net ATPs. So that's the first stage. And this can occur completely in the absence of oxygen.

I'll do a whole video on glycolysis in the future. Then these byproducts, they get re-engineered a little bit. And then they enter into what's called the Krebs cycle.

Which generates another two ATPs. And then, and this is kind of the interesting point, there's another process that you can say happens after the Krebs cycle. But we're in a cell and everything's bumping into everything all of the time.

But it's normally viewed to be after glycolysis and the Krebs cycle. And this requires oxygen. So let me be clear, glycolysis, this first step, no oxygen required.

Doesn't need oxygen. It can occur with oxygen or without it. Oxygen not needed.

Or you could say this is called an anaerobic process. This is the anaerobic part of the respiration. Let me write that down too.

Anaerobic. Maybe I'll write that down here. Glycolysis, since it doesn't need oxygen, we can say it's anaerobic.

You might be familiar with the idea of aerobic exercise. The whole idea of aerobic exercise is to make you breathe hard because you need a lot of oxygen to do aerobic exercise. So anaerobic means you don't need oxygen.

Aerobic means it needs oxygen. Anaerobic means the opposite. You don't need oxygen.

So, glycolysis anaerobic. And it produces two ATPs net. And then you go to the Krebs cycle, there's a little bit of setup involved here.

And we'll do the detail of that in the future. But then you move over to the Krebs cycle, which is aerobic. It is aerobic.

It requires oxygen to be around. And then this produces two ATPs. And then this is the part that, frankly, when I first learned it, confused me a lot.

But I'll just write it in order the way it's traditionally written. Then you have something called-- we're using the same colors too much-- you have something called the electron transport chain. And this part gets credit for producing the bulk of the ATPs.

34 ATPs. And this is also aerobic. It requires oxygen.

So you can see, if you had no oxygen, if the cells weren't getting enough oxygen, you can produce a little bit of energy. But it's nowhere near as much as you can produce once you have the oxygen. And actually when you start running out of oxygen, this can't proceed forward, so what happens is some of these byproducts of glycolysis, instead of going into the Krebs cycle and the electron transport chain, where they need oxygen, instead they go through a side process called fermentation.

For some organisms, this process of fermentation takes your byproducts of glycolysis and literally produces alcohol. That's where alcohol comes from. That's called alcohol fermentation.

And we, as human beings, I guess fortunately or unfortunately, our muscles do not directly produce alcohol. They produce lactic acid. So we do lactic acid fermentation.

Let me write that down. Lactic acid. That's humans and probably other mammals.

But other things like yeast will do alcohol fermentation. So this is when you don't have oxygen. It's actually this lactic acid that if I were to sprint really hard and not be able to get enough oxygen, that my muscles start to ache because this lactic acid starts to build up.

But that's just a side thing. If we have oxygen we can move to the Krebs cycle, get our two ATPs, and then go on to the electron transport chain and produce 34 ATPs, which is really the bulk of what happens in respiration. Now I said this as an aside, that to some degree this isn't fair.

Because while these guys are operating they're also producing these other molecules. They're not producing them entirely, but what they're doing is, they're taking-- and I know this gets complicated here, but I think over the course of the next few videos we'll get an intuition for it-- in these two parts of the reaction, glycolysis and the Krebs cycle, we're constantly taking NAD-- I'll write it as NAD plus-- and we're adding hydrogens to it to form NADH. And this actually happens for one molecule of glucose, this happens to 10 NADs.

Or 10 NAD plusses to become NADHs. And those are actually what drive the electron transport chain. And I'll talk a lot more about it and kind of how that happens and why is energy being derived and how is this an oxidative reaction and all of that.

And what's getting oxidized and what's being reduced. But I just wanted to give due credit. These guys aren't just producing two ATPs in each of these stages.

They're also producing, actually combined, 10 NADHs, which each produce three ATPs in an ideal situation, the electron transport chain. And they're also doing it to this other molecule, FAD, which is very similar. But they're producing FADH.

Now I know all of this is very complicated. I'll make videos on this in the future. But the important thing to remember is cellular respiration, all it is is taking glucose and kind of repackaging the energy in glucose, and repackaging it in the form of, your textbooks will tell you, 38 ATPs.

If you're doing an exam, that's a good number to write. It tends to, in reality be a smaller number. It's also going to produce heat.

Actually most of it is going to be heat. But 38 ATPs, and it does it through three stages. The first stage is glycolysis, where you're just literally splitting the glucose into two.

You're generating some ATPs. But the more important thing is, you're generating some NADHs that are going to be used later in the electron transport chain. Then those byproducts are split even more in the Krebs cycle, directly producing two ATPs.

But that produces a lot more NADHs. And all of those NADHs are used in the electron transport chain to produce the bulk of your energy currency, or your 34 ATPs.