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  • - [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,