Cellular respiration | Energy and matter in biological systems | High school biology | Khan Academy

In this video, we're going to talk about cellular respiration, which sounds like a very fancy thing, but it's really just about the biochemical processes that can take things that we find in food and convert it into forms of energy that we can use to do things like run and maintain our body temperature and maintain body homeostasis.

So to get into the chemistry of cellular respiration, and this really is an overview video, we will start with the chemical equation for respiration. What it's all about is a series of steps that, when you look at them in aggregate, you're starting with the glucose right over here, using oxygen. And that's why we have to breathe really hard in order to do our cellular respiration.

It's going to yield some carbon dioxide, which we also need to breathe hard to exhale, some water, and some energy. Now that energy is in the form of heat, which can help us maintain our body temperature, especially if it's cold outside, but also ATP. Now, you might be wondering, what is ATP? To help us answer that question, I will show you a picture of ATP, and I will also show you a picture of glucose or a visualization of it.

Now, you don't have to memorize what these structures are, but what's really going on here is that glucose, as you are able to shape it into other things, as you're able to break the bonds in glucose and having its constituents form bonds with other things, has a net release of energy. That energy can be used to stake what's known as ADP, which has two phosphate groups, and add a third phosphate group onto it right over here.

Now, you might say, why is that useful? Why is that a more readily usable form of energy? Well, as you go forward in your biological journey or your understanding of biology, you'll see that ATP molecules like this, by losing that phosphate and allowing that phosphate to bond to other things, can actually release energy and can fuel muscles and can fuel other biological processes.

Now, to understand the steps of it, we will start in the cytosol of a cell, where a process known as glycolysis takes place. Glycolysis literally means the breaking of glucose, so let me write it down: glycolysis. What it does is break each glucose molecule into two molecules known as pyruvate.

Now, just that process alone—and we'll go into much more depth in other videos—does start to produce some ATPs and also helps produce some molecules known as NADH. I know this is all sounding very complex, but you will have a molecule known as NAD⁺. If you want to know what it looks like, it looks like this. Once again, don't get too bogged down in the details. It is worth noting NAD stands for nicotinamide adenine dinucleotide, and we also notice that ATP stands for adenosine triphosphate.

So, you have these very similar molecular components that are showing up in different places in biology. You might also recognize that adenosine is involved in the formation of DNA as well. So, once again, these molecules are adapted and reused all over the place. But going back to our journey of cellular respiration, an NAD⁺ molecule, if you were to add to that two hydrogen protons— and this is the important part—two electrons, it will be reduced to NADH.

Remember, reduction is the gaining of electrons, which is happening right over here. The reason why these two electrons are really interesting is in NADH; they're at a fairly high energy state. As we'll see, as they're able to go to other molecules and go to lower and lower energy states, they're able to do useful things that can eventually end up in the production of ATP. It's essentially a transfer of energy.

So, glycolysis is directly producing some ATPs, and it's also reducing NAD⁺ in this way to produce NADH for the next stage. We have to go into the mitochondria, which is often known as the powerhouse of the cell, where now our pyruvate will enter into the mitochondrial matrix right over here. That's where the citric acid cycle occurs.

The citric acid cycle is going to use a derivative of the pyruvate, which we got from glycolysis. You don't have to know all the details; it's called acetyl CoA. But that's going to go through a series of transformations, and the reason why it's called a cycle is there are some molecules that react with the acetyl CoA and then, through a series of transformations, get back to where they started.

The reason why it's called the citric acid cycle is one of those intermediaries is citrate. But this process, once again, produces more ATPs directly, but it also produces more NADHs, and it also is able to do something similar with another molecule. Once again, I'm not going to go into all of the details, but we are able to go from another molecule known as flavin adenine dinucleotide (FAD)⁺, plus two hydrogen protons, plus two electrons, to get to FADH₂.

Once again, this is an interesting molecule because it has these electrons in a higher energy level, which, through a series of molecular processes—which you'll go into much detail in future courses on—you are able to do useful work. You are able to transfer energy.

So, once you have a few ATPs and a bunch of NADHs and FADHs, you then go into something known as the electron transport chain. This is essentially where those electrons go from a high energy state and get transferred from one molecule to another, actually along this membrane right over here.

As they do, the proteins that they are interacting with can use that energy in order to pump hydrogen protons into this intermembrane space of the mitochondria. Then, that concentration gradient of hydrogen protons is released through another enzyme, which is actually able to produce the actual ATP.

So, I know that is a lot to process, but this is meant to be just an overview. I know you might have a lot of questions, as I did the first time that I learned this. The important thing to realize is that glucose does store energy, but we don't use it directly. We have to go through cellular respiration to convert that glucose into ATPs, which are more readily used by cells.

Now, the various steps are also going to produce heat as they release energy, which could also be useful for the cell. Now, one final point: you might say glucose, well that's only one form of food. You showed a picture of bread initially. Well, carbohydrates are made up of chains of simple sugars like glucose. If we're thinking about things like protein or fats, which you could also use for energy, those are going to be adapted and enter at different phases of cellular respiration.

At some point, you're going to have a very similar process. So, I lied—I actually have one last, last point. We talked a lot about glucose as it enters into glycolysis, and then the pyruvate enters into the mitochondria. But what about the oxygen? Where is that involvement as an input?

Well, the oxygen is the eventual electron acceptor at the end of the electron transport chain. Not only is it accepting electrons, it's accepting hydrogen protons. So the oxygen is an input into the electron transport chain. Then, once it gets those electrons and those hydrogen protons, well, you can imagine what the output is. You add some oxygen to some hydrogen protons and electrons; you're going to get water.

You're going to get this output right over here. And what about the carbon dioxide? Well, the carbon dioxide is going to be an output of the citric acid cycle.