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

Overview of cellular respiration

Cellular respiration is how cells get energy from glucose. The process has three main parts: Glycolysis happens in the cytosol and breaks glucose into two pyruvate, producing 2 ATPs and 2 NADHs. The Krebs cycle occurs in the mitochondrial matrix, where pyruvate is turned into acetyl-CoA, which then goes through a series of reactions, producing ATP, NADH, and FADH2. Finally, oxidative phosphorylation uses NADH and FADH2 to create a proton gradient, which helps make more ATP.

Want to join the conversation?

  • blobby purple style avatar for user SN
    What is the QH2 that Sal was talking about? In my classes, we only learned about FADH2, not about QH2. Is it important that we know this?
    (32 votes)
    Default Khan Academy avatar avatar for user
    • female robot ada style avatar for user Lidiya
      Q is the coenzyme Q, an antioxidant called ubiquinone with vitamin-like properties, and QH2 is the reduced form of the coenzyme. I have to acknowledge that Sal is right in using QH2 as the electron carrier. Although textbooks teach that FADH2 does this, it is not correct. FADH2 remains bound to the Succinate dehydrogenase (complex II) and as such it does not carry the electrons to the next complex III (cytochrome bc1); QH2 does. Therefore, QH2 is the "true" electron carrier. Regarding what you have to learn? Keep in mind that you need to pass the exam, thus, it would be better to use FADH2 as your answer, but remember that it is wrong until the AP college board changes its thinking. There is an excellent discussion on using QH2 instead of FADH2: http://sandwalk.blogspot.com/2007/06/cellular-respiration-ninja-enzymes.html
      (60 votes)
  • leaf blue style avatar for user Chris
    So a muscle cell in an environment with a large amount of oxygen available would produce a number of ATP closer to the maximum 38 ATP versus a skin cell in an environment with less oxygen available?
    (9 votes)
    Default Khan Academy avatar avatar for user
  • starky tree style avatar for user Stephen Bannister
    If cellular respiration produces 10 x NADH molecules then where do the NAD+ come from to reduce into NADH?
    (6 votes)
    Default Khan Academy avatar avatar for user
  • blobby green style avatar for user JC
    how can you have 1.5 atp's?
    (9 votes)
    Default Khan Academy avatar avatar for user
  • spunky sam red style avatar for user Melody Kimbrough
    At , NAD+ --> NADH. Where does the proton come from? Is it just floating around in the cytosol?
    (6 votes)
    Default Khan Academy avatar avatar for user
    • leafers ultimate style avatar for user M
      The hydrogen comes from the glucose that is broken down to produce ATP. This is the source of the hydrogen ions that are floating around. Energy is released when bonds are broken, so when hydrogen is removed from the glucose, this releases energy (in the form of electrons) that is incorporated into the reduced NADH molecule and subsequently ATP.

      If you add only a proton to NAD+ you won't get NADH. A proton is positive charge. NADH is neutral (no charge). You cannot add a hydrogen ion to NAD+ to get NADH.

      The actual reaction is NAD+ and 2H ----> NADH and H+
      So a hydrogen molecule (proton + electron) and an additional electron are added to NAD+ to form NADH. The leftover hydrogen ion (H+) is now free to float around and participate in the electron transport chain.
      (3 votes)
  • marcimus orange style avatar for user Mango
    This is complicated...
    (6 votes)
    Default Khan Academy avatar avatar for user
  • blobby green style avatar for user Freckledfrog
    I know sal already taught about NAD+ and NADH but what is the difference? could someone help me understand, please
    (4 votes)
    Default Khan Academy avatar avatar for user
  • duskpin ultimate style avatar for user Rio.Peer
    What is the difference between NADH and NADPH?
    (3 votes)
    Default Khan Academy avatar avatar for user
    • male robot hal style avatar for user NJ33
      The difference between NADH and NADPH is that NADPH has an extra phosphate group. While NADH is used to power cellular reactions such as glycolysis, during which molecules are broken down, NADPH is used to power photosynthesis.
      (6 votes)
  • blobby green style avatar for user Freckledfrog
    what exactly does net mean?
    (3 votes)
    Default Khan Academy avatar avatar for user
    • scuttlebug purple style avatar for user San-dwich
      Net is the resulting amount of something including the deductions. For example, if I spend $10 on supplies to start a lemonade stand, and my stand makes $30 dollars, my net gain is $20 (30-10). In the case of glycolysis, we need 2 ATP to get the reaction started, and the process makes 4 ATP, making a net of 2 ATP (4-2).
      Hope this helps!
      (5 votes)
  • duskpin ultimate style avatar for user Katherine
    Is there such a thing as a phosphorylated nucleotide with Thymine or Cytosine as a nitrogenous base? If only Adenine and Guanine can function as nitrogenous bases in energy baring molecules (i.e. ATP and GTP), then why is that the case?
    (4 votes)
    Default Khan Academy avatar avatar for user
    • female robot grace style avatar for user tyersome
      You appear to be asking about the energetic roles for ribonucleotides other than ATP and GTP — all four standard ribonucleotides function as energy bearing molecules (though ATP is by far the most commonly used), they are also used in the synthesis of ribonucleic acids (RNA molecules).

      The standard ribonucleotides are:
      • Adenosine triphosphate — adenine bonded to ribose bonded to 3 phosphoryl groups.

      • Guanosine triphosphate — guanine bonded to ribose bonded to 3 phosphoryl groups.

      • Cytidine triphosphate — cytosine bonded to ribose bonded to 3 phosphoryl groups.

      Finally, there is a fourth ribonucleotide, but it isn't based on thymine — remember these are ribonucleotides not deoxyribonucleotides!
      • Uridine triphosphate — uracil bonded to ribose bonded to 3 phosphoryl groups.

      The four corresponding deoxyribonucleotides are almost exclusively used for the synthesis of DNA.
      They are:
      deoxyadenosine triphosphate,
      deoxyguanosine triphosphate,
      deoxycytidine triphosphate, and
      deoxythymidine triphosphate — confusingly this is often written as thymidine triphosphate.

      There are reasonable wikipedia articles on each of these compounds if you wish to learn more.
      (3 votes)

Video transcript

- [Voiceover] So what I wanna do in this video is give ourselves an overview of cellular respiration. It can be a pretty involved process, and even the way I'm gonna do it, as messy as it looks, is going to be cleaner than actually what goes on inside of your cells, and other organs themselves, because I'm going to show clearly from going from glucose, and then see how we can produce ATP through glycolysis, and the Krebs cycle, and oxidative phosphorylation, but in reality, all sorts of molecules can jump in at different parts of the chain, and then jump out at different parts of the chain, to go along other pathways. But I'll show, kind of the traditional narrative. So we're gonna start off, for this narrative, we're gonna start off with glucose. We have a six-carbon-chain right over here. And we have the process of glycosis, which is occurring in the cytosol, the cytosol of our cells. So if this is a cell right over here, you can imagine, well the glycolysis, the glycolysis could be occurring right over there. And that process of glycolysis is essentially splitting up this six-carbon glucose molecule into two three-carbon molecules, and these three-carbon molecules, we go into detail in another video, we call these pyruvate. Pyruvate. And in the process of doing so, and this is, I guess you could say, the point of glycolysis, we're able to, on a net basis, produce two ATP's. We actually produce four, but we have to use two, so on a net basis, we produce two ATP's. I'm gonna keep a little table here, to keep track. So we produce two ATP's, and we are also, we're also, in the process of that, we reduce two NAD molecules to NADH. Remember, reduction is gaining of electrons. And you see over here, this is positively charged, this is neutrally charged, it essentially gains a hydride. So this is reduction. Reduction. And if we go all the way through the pathway, all the way to oxidative phosphorylation, the electronic transport chain, these NADH's, the reduced form of NAD, they can be, then, oxidized, and in doing so, more energy is provided to produce even more ATP's, but we'll get to that. So you're also gonna get two NADH's. Two NADH's get produced. Now at that point, you could kind of think of it as a little bit of a decision point. If there's no oxygen around, or if you're the type of organism that doesn't want to continue, for some reason, with cellular respiration, or doesn't know how, this pyruvate can be used for fermentation. We have videos on fermentation, lactic acid fermentation, alcohol fermentation, and fermentation is all about using the pyruvates to oxidize your NADH back into NAD, so it could be re-used again, for glycolysis. So even though the NADH has energy that could eventually be converted into ATP, and even though pyruvates have energy that could eventually be converted into ATP, when you do fermentation, you kinda give up on that, and you just view them as waste products, and you use the pyruvate to convert the NADH back into NAD, And then, glycolysis can occur again. But let's assume we're not gonna go down the fermentation pathway, and we're gonna continue with traditional aerobic cellular respiration, using oxygen. Well, the next thing that's going to happen, is that the carboxyl group, and and everything I'm going to show now, it's going to happen for each of these pyruvates. So, you can imagine these things all happening twice. So I'm gonna multiply a bunch of things, times two. But what happens in the next step, is this carboxyl group, this carboxyl group is stripped off of the pyruvate, and it, essentially, is going to be released as carbon dioxide. So this is our carbon dioxide being released here, and then the rest of our pyruvate, which is, essentially, an acetyl group, that latches onto coenzyme A. And you'll hear a lot about coenzyme A. Sometimes I'll write just CoA, like this. Sometimes I'll do CoA, and then the sulfur, bonded to the hyrdrogen. And the reason why they'll draw the sulfur part, is because the sulfur is what bonds with the acetyl group, right over here. So, you have the carbon dioxide being released, and then the acetyl group, bonding with that sulfur, and by doing that, you form acetyl-CoA. And acetyl-CoA, just so you know, you only see three letters here, but this is actually a fairly involved molecule. This is actually a picture of acetyl-CoA, I know it's really small, but hopefully you'll appreciate that it's a more involved molecule. That, the acetyl group that we're talking about is just this part, right over here, and it's a coenzyme. It's really acting to transfer that acetyl group, and we'll see that in a second. But it's also fun to look at these molecules, because once again, we see these patterns over and over again in biology or biochemistry. Acetyl-CoA, you have an adenine right over here. It's hard to see, but you have a ribose, and you also have two phosphate groups. So this end of the acetyl-CoA is essentially, is essentially an ADP. But it's used as a coenzyme. Everything that I'm talking about, this is all going to be facilitated by enzymes, and the enzymes will have cofactors, coenzymes, if we're talking about organic cofactors, that are gonna help facilitate things along. And as we see, the acetyl group joins on to the coenzyme A, forming acetyl-CoA, but that's just a temporary attachment. The acetyl-CoA is, essentially, gonna transfer the acetyl group over to, and now we're going to enter into the citric acid cycle. It's gonna transfer these two carbons over to oxaloacetic acid, to form citric acid. So it's gonna transfer these two carbons to this one, two, three, four carbon molecule, to form a one, two, three, four, five, six carbon molecule. But before we go into the depths of the citric acid cycle, I wanna make sure that I don't lose track of my accounting, because, even that step right over here, where we decarboxylated the pyruvate, we went from pyruvate to acetyl-CoA, that also reduced some NAD to NADH. Now, this is gonna happen once for each pyruvate, but we're gonna- all the accounting we're gonna say, is for one glucose molecule. So for one glucose molecule, it's gonna happen for each of the pyruvates. So this is going to be times- This is going to be times two. So we're gonna produce two, two NADH's in this step, going from pyruvate to acetyl-CoA. Now, the bulk of, I guess you could say, the catabolism, of the carbons, or the things that are eventually going to produce our ATP's, are going to happen in what we call the citric acid, or the Krebs cycle. It's called the citric acid cycle because, when we transferred the acetyl group from the coenzyme A to the oxaloacetic acid, we formed citric acid. And citric acid, this is the thing that you have in lemons, or orange juice. It is this molecule right over here. And the citric acid cycle, it's also called the Krebs cycle, when you first learn it, seems very, very complex, and some could argue that it is quite complex. But I'm just gonna give you an overview of what's going on. The citric acid, once again, six-carboned, it keeps getting broken down, through multiple steps, and I'm really not showing all of the detail here, all the way back to oxaloacetic acid, where, then, it can accept the two carbons again. And just to be clear, once the two carbons are released by the coenzyme A, then that coenzyme A can be used again, to decarboxylate some pyruvates. So there's a bunch of cycles going on. But the important take-away, is as we go through the citric acid cycle, as we go from one intermediary to the next, we keep reducing NAD to NADH, in fact, we do this three times for each cycle of the citric acid cycle, but remember, we're gonna do this for each acetyl-CoA. For each pyruvate. So all of this stuff is going to happen twice. So we're going to go through it twice for each original glucose molecule. So, here we have one, two, three NADH's being produced, but since we're going to go through it twice, and we're gonna be accounting for the original glucose molecule, we could say that we have six six NADH's, or you could say, six NAD's get reduced to NADH. Now, you also, in the process, as you're breaking down, going from the six-carbon molecule to four-carbon molecule, you're releasing carbon, as carbon dioxide, and you also have, traditionally GDP being converted into GTP, or sometimes ADP converted into ATP, but functionally, it's equivalent to ATP, either way. So, we could also say that we're gonna directly- Remember, we're gonna do all of this stuff twice. So, we could say that two, I'll just say two ATP's, to make it simple. We could say GTP, but I'll say two ATP's. Because once again, this happens once in each cycle, but we're gonna do two cycles, for each glucose. And then, we have this other coenzyme right over here, FAD, that gets reduced to FADH2, but that stays covalently attached to the enzymes that are facilitating it, so eventually, that's being used to reduce . coenzyme Q to QH2. So I'm just gonna write the QH2 here, but once again, you're gonna get two of these. So two QH2's. Now let's think about what the net product, over here, is going to be. And to think about it, we should just, we'll just- I'll do a little bit of a shorthand. We'll go into more detail in future videos. These coenzymes, the NADH, the QH2, these are going to be oxidized, during oxidative phosphorylation, and the electron transport chain, to create a proton gradient across the inner membrane of mitochondria. We're gonna go into much more detail in the future, but that proton gradient is going to be used to produce more ATP. And one way to think about it, is each NADH is going to produce, and I've seen accounts, it depends on the efficiency, and where the NADH is actually going to be produced, but it's going to produce anywhere between two and three ATP's. Each of the reduced coenzyme Q's, so QH2, that's going to each produce about one and a half ATP's. And people are still getting a good handle on exactly how this is happening. It depends on the efficiency of the cell, and what the cell is actually trying to do. So, using these ranges, actually I'll say one and a half to two ATP's. And these are approximate numbers. So let's think about what our total accounting is. So if we just count up the ATP or the GTP's, we're gonna get two there, two there. So we're gonna have four direct, or very close to direct, ATP's net, being created. And then how many NADH's? We have two, four, and then we add six. We have ten NADH's. Ten NADH's. And then, we have two of the coenzyme Q's. Two QH2's. So that's gonna be four ATP's, this is going to be between- this is going to be between 20 and 30 NADH's. Sorry, 20 and 30 ATP's. 20 to 30 ATP's. And then, this is going to be three to four. Three to four ATP's. So if you add them all together, if you add the low ends of the range, you get, let's see, 20 plus three, plus four. That's 27 ATP's. 27 ATP's. And the high end of the range, let's see. You have four plus 30 plus four. You have 38. 38 ATP's. And 38 ATP's is currently considered to become the theoretical maximum, but when we actually observe things in cells, it looks like it comes right at around 29 to 30 ATP's. And once again, it depends what the cell's trying to do, the type of cells, and the type of efficiency. But all of this is happening through cellular respiration. And just to get a better sense of where all of this is occurring. Where all of this is occuring, we said glycolosis is occurring in the cytosol. The citric acid cycle, this is occurring in the matrix of the mytochondria. So this space right over here, that is the citric acid cycle, in that little magenta space that I've drawn. So that's the matrix. In the video on mitochondria, we go into much more detail on that. And then, the actual conversion of the coenzymes, of the electron transport chain, that's occurring across the membrane of the crista. And the crista are these folds, these kind of, inner membrane folds, of our mitochondria. So, it's occurring across the membranes of those, actually the plural is cristae. Crista is the singular of the cristae. And we'll go into more detail into that, in other videos.