If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

Cellular respiration

Cellular respiration is a chemical process in which the bonds of food molecules and oxygen molecules are broken and new compounds are formed that can transport energy to muscles. Cellular respiration also releases the energy needed to maintain body temperature despite ongoing energy transfer to the surrounding environment. Created by Sal Khan.

Want to join the conversation?

  • piceratops seedling style avatar for user Draven
    There is SO MUCH info being given at once that it becomes a little overwhelming to grasp it all. I know he's really succinct, but when you're first learning about these concepts, it's a LOT. Does anyone know of any videos that break this down into more understandable steps?
    (36 votes)
    Default Khan Academy avatar avatar for user
  • male robot hal style avatar for user AGLUTENFREEFOOD.
    HE SAID THE THING AT ! "Mitochondria, which is often know as 'the powerhouse of the cell'" HE FLIPPING SAID IT!
    (12 votes)
    Default Khan Academy avatar avatar for user
  • aqualine ultimate style avatar for user Chrisdude2
    How do these processes produce ATP? He mentions when glucose is broken down ATP is released and other processes and cycles that happen in the mitochondria with pyruvate to also produce ATP. From my understanding, ATP is a compound made up of C10H16N5O13P3. If glucose is broken down into Pyruvate, then there shouldn't be extra atoms to bond with Nitrogen and Phosphorus to make ATP. The same with the things that happen in the mitochondria, if cycles have other atoms interact with pyruvate but eventually ends up where it started, then there shouldn't be extra atoms to create ATP either.
    (5 votes)
    Default Khan Academy avatar avatar for user
    • stelly yellow style avatar for user Quantum Cat
      The atoms in glucose and pyruvate are not directly used to form ATP. When these molecules are broken down, they release energy. This energy is used to stick a phosphate group (Pi) onto another molecule called ADP (ATP missing a phosphate) to produce ATP. So ATP isn't made from scratch, but uses existing molecules in the cell and combines them using the energy from glucose.
      (3 votes)
  • piceratops ultimate style avatar for user JJ999
    How does cellular respiration result in a net release of energy?
    (6 votes)
    Default Khan Academy avatar avatar for user
  • old spice man blue style avatar for user Levi
    so MUCH INFO
    (5 votes)
    Default Khan Academy avatar avatar for user
  • primosaur ultimate style avatar for user Yellow Venusaur
    Sal said that a third phosphate group makes ATP how does it work
    (3 votes)
    Default Khan Academy avatar avatar for user
    • sneak peak green style avatar for user The Butler
      "ATP is able to make cellular processes by transferring a phosphate group to another molecule (a process called phosphorylation). This change is carried out by specific enzymes that couple the release of energy from ATP to cellular activities that require energy."
      (4 votes)
  • leaf yellow style avatar for user 1122223
    Why is it different in humans and plants? Plants make ATP too but also that comes from ADP and NADPH comes from NADP+ and 2 electrons.
    (2 votes)
    Default Khan Academy avatar avatar for user
  • leafers sapling style avatar for user Justin
    Anybody super confused about the sheer number of abbreviations in photosynthesis and cellular respiration? Because from this video this is sort of how I invision cellular respiration. Please correct me if I'm wrong.

    Ok, so there's glucose, and we split it into two pyruvates. And then that pyruvate goes through the mitochondria in a process known as ETC where it mixes and whatevers with a bunch of other stuff like FAB and things I don't quite remember. And then basically there's something about protons and transferring energy, and then glucose turns into ATP and that ATP can be used for energy.
    (1 vote)
    Default Khan Academy avatar avatar for user
  • aqualine ultimate style avatar for user Chrisdude2
    Where do the hydrogen and electrons that are added to pyruvate and the compounds written at the bottom of the video come from?
    (1 vote)
    Default Khan Academy avatar avatar for user
  • area 52 purple style avatar for user Takunda Borerwe
    Is breathing and respiration the same thing?
    (1 vote)
    Default Khan Academy avatar avatar for user

Video transcript

- [Instructor] 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. And 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. And 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 he is in the form of heat, which can help us maintain our body temperature, especially if it's cold outside side. But also ATP. Now you might be wondering, what is ATP? And 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, that has a net release of energy. And that energy can be used to take what's known as ADP that has two phosphate groups, and add a third phosphate group onto it right over here. Now you see it 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, 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. And glycolysis literally means the breaking of glucose. So let me write it down, glycolysis. And what it does is it breaks 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. And 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 noticed that ATP stands for adenosine triphosphate. And so you have these very molecular components that are showing up in different places in biology, and 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. And remember, reduction is the gaining of electrons, which is happening right over here. And the reason why these two electrons are really interesting is, in NADH, they're in a fairly high energy state. And 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. And that's where the citric acid cycle occurs. And 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 acetylcholine. But that's going to go through a series of transformations. And the reason why it's called a cycle, there's some molecules that react with the acetyl-CoA, and then through a series of transformations, get back to where they started. And the reason why it's called a 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 to another molecule. And once again, I'm not going to go into all of the details. But we're able to go from another molecule, known as flavin adenine dinucleotide, FAD, plus two hydrogen protons, plus two electrons to get to FADH2, which once again 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're 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. And this is essentially where those electrons go from a high energy state and they get transferred from one molecule to another, actually along this membrane right over here. And as they do, the proteins that they are interacting with are able to use that energy in order to pump hydrogen protons into this intermembrane space of the mitochondria. And then that concentration gradient of hydrogen protons, it's 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. But the important thing to realize, 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 is 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. And 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. But 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's that involvement as an input? Well, the oxygen is the eventual electronic sector at the end of the electron transport chain. And not only is it accepting electrons, it's accepting hydrogen protons. So the oxygen is an input into the electron transport chain. And 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.