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  • So we already know that if we start off with a glucose

  • molecule, which is a 6-carbon molecule, that this

  • essentially gets split in half by glycolysis and we end up 2

  • pyruvic acids or two pyruvate molecules.

  • So glycolysis literally splits this in half.

  • It lyses the glucose.

  • We end up with two pyruvates or pyruvic acids.

  • ruby And these are 3-carbon molecules.

  • There's obviously a lot of other stuff

  • going on in the carbons.

  • You saw it in the past. And you could look up their

  • chemical structures on the internet or on Wikipedia and

  • see them in detail.

  • But this is kind of the important thing.

  • Is that it was lysed, it was cut in half.

  • And this is what happened in glycolysis.

  • And this happened in the absence of oxygen.

  • Or not necessarily.

  • It can happen in the presence or in the absence of oxygen.

  • It doesn't need oxygen.

  • And we got a net payoff of two ATPs.

  • And I always say the net there, because remember, it

  • used two ATPs in that investment stage, and then it

  • generated four.

  • So on a net basis, it generated four, used two, it

  • gave us two ATPs.

  • And it also produced two NADHs.

  • That's what we got out of glycolysis.

  • And just so you can visualize this a little bit better, let

  • me draw a cell right here.

  • Maybe I'll draw it down here.

  • Let's say I have a cell.

  • That's its outer membrane.

  • Maybe its nucleus, we're dealing with

  • a eukaryotic cell.

  • That doesn't have to be the case.

  • It has its DNA and its chromatin form all spread

  • around like that.

  • And then you have mitochondria.

  • And there's a reason why people call it the power

  • centers of the cell.

  • We'll look at that in a second.

  • So there's a mitochondria.

  • It has an outer membrane and an inner

  • membrane just like that.

  • I'll do more detail on the structure of a mitochondria,

  • maybe later in this video or maybe I'll do a

  • whole video on them.

  • That's another mitochondria right there.

  • And then all of this fluid, this space out here that's

  • between the organelles-- and the organelles, you kind of

  • view them as parts of the cell that do specific things.

  • Kind of like organs do specific things

  • within our own bodies.

  • So this-- so between all of the organelles you have this

  • fluidic space.

  • This is just fluid of the cell.

  • And that's called the cytoplasm.

  • And that's where glycolysis occurs.

  • So glycolysis occurs in the cytoplasm.

  • Now we all know-- in the overview video-- we know what

  • the next step is.

  • The Krebs cycle, or the citric acid cycle.

  • And that actually takes place in the inner membrane, or I

  • should say the inner space of these mitochondria.

  • Let me draw it a little bit bigger.

  • Let me draw a mitochondria here.

  • So this is a mitochondria.

  • It has an outer membrane.

  • It has an inner membrane.

  • If I have just one inner membrane we call it a crista.

  • If we have many, we call them cristae.

  • This little convoluted inner membrane, let

  • me give it a label.

  • So they are cristae, plural.

  • And then it has two compartments.

  • Because it's divided by these two membranes.

  • This compartment right here is called the outer compartment.

  • This whole thing right there, that's the outer compartment.

  • And then this inner compartment in here, is called

  • the matrix.

  • Now you have these pyruvates, they're not quite just ready

  • for the Krebs cycle, but I guess-- well that's a good

  • intro into how do you make them ready

  • for the Krebs cycle?

  • They actually get oxidized.

  • And I'll just focus on one of these pyruvates.

  • We just have to remember that the pyruvate, that this

  • happens twice for every molecule of glucose.

  • So we have this kind of preparation step

  • for the Krebs Cycle.

  • We call that pyruvate oxidation.

  • And essentially what it does is it cleaves one of these

  • carbons off of the pyruvate.

  • And so you end up with a 2-carbon compound.

  • You don't have just two carbons, but its backbone of

  • carbons is just two carbons.

  • Called acetyl-CoA.

  • And if these names are confusing, because what is

  • acetyl coenzyme A?

  • These are very bizarre.

  • You could do a web search on them But I'm just going to use

  • the words right now, because it will keep things simple and

  • we'llget the big picture.

  • So it generates acetyl-CoA, which is

  • this 2-carbon compound.

  • And it also reduces some NAD plus to NADH.

  • And this process right here is often given credit-- or the

  • Krebs cycle or the citric acid cycle gets

  • credit for this step.

  • But it's really a preparation step for the Krebs cycle.

  • Now once you have this 2-carbon chain, acetyl-Co-A

  • right here.

  • you are ready to jump into the Krebs cycle.

  • This long talked-about Krebs cycle.

  • And you'll see in a second why it's called a cycle.

  • Acetyl-CoA, and all of this is catalyzed by enzymes.

  • And enzymes are just proteins that bring together the

  • constituent things that need to react in the right way so

  • that they do react.

  • So catalyzed by enzymes.

  • This acetyl-CoA merges with some oxaloacetic acid.

  • A very fancy word.

  • But this is a 4-carbon molecule.

  • These two guys are kind of reacted together, or merged

  • together, depending on how you want to view it.

  • I'll draw it like that.

  • It's all catalyzed by enzymes.

  • And this is important.

  • Some texts will say, is this an enzyme catalyzed reaction?

  • Yes.

  • Everything in the Krebs cycle is an

  • enzyme catalyzed reaction.

  • And they form citrate, or citric acid.

  • Which is the same stuff in your lemonade

  • or your orange juice.

  • And this is a 6-carbon molecule.

  • Which makes sense.

  • You have a 2-carbon and a 4-carbon.

  • You get a 6-carbon molecule.

  • And then the citric acid is then oxidized

  • over a bunch of steps.

  • And this is a huge simplification here.

  • But it's just oxidized over a bunch of steps.

  • Again, the carbons are cleaved off.

  • Both 2-carbons are cleaved off of it to get back to

  • oxaloacetic acid.

  • And you might be saying, when these carbons are cleaved off,

  • like when this carbon is cleaved off,

  • what happens to it?

  • It becomes CO2.

  • It gets put onto some oxygen and leaves the system.

  • So this is where the oxygen or the carbons, or the carbon

  • dioxide actually gets formed.

  • And similarly, when these carbons get cleaved

  • off, it forms CO2.

  • And actually, for every molecule of glucose you have

  • six carbons.

  • When you do this whole process once, you are generating three

  • molecules of carbon dioxide.

  • But you're going to do it twice.

  • You're going to have six carbon dioxides produced.

  • Which accounts for all of the carbons.

  • You get rid of three carbons for every turn of this.

  • Well, two for every turn.

  • But really, for the steps after glycolysis you get rid

  • of three carbons.

  • But you're going to do it for each of the pyruvates.

  • You're going to get rid of all six carbons, which will have

  • to exhale eventually.

  • But this cycle, it doesn't just generate carbons.

  • The whole idea is to generate NADHs and FADH2s and ATPs.

  • So we'll write that here.

  • And this is a huge simplification.

  • I'll show you the detailed picture in a second.

  • We'll reduce some NAD plus into NADH.

  • We'll do it again.

  • And of course, these are in separate steps.

  • There's intermediate compounds.

  • I'll show you those in a second.

  • Another NAD plus molecule will be reduced to NADH.

  • It will produce some ATP.

  • Some ADP will turn into ATP.

  • Maybe we have some-- and not maybe, this is what happens--

  • some FAD gets-- let me write it this way-- some FAD gets

  • oxidized into FADH2.

  • And the whole reason why we even pay attention to these,

  • you might think, hey cellular respiration is all about ATP.

  • Why do we even pay attention to these NADHs and these

  • FADH2s that get produced as part of the process?

  • The reason why we care is that these are the inputs into the

  • electron transport chain.

  • These get oxidized, or they lose their hydrogens in the

  • electron transport chain, and that's where the bulk of the

  • ATP is actually produced.

  • And then maybe we'll have another NAD get reduced, or

  • gain in hydrogen.

  • Reduction is gaining an electron.

  • Or gaining a hydrogen whose electron you can hog.

  • NADH.

  • And then we end up back at oxaloacetic acid.

  • And we can perform the whole citric acid cycle over again.

  • So now that we've written it all out, let's account for

  • what we have. So depending on-- let me draw some dividing

  • lines so we know what's what.

  • So this right here, everything to the left of that line right

  • there is glycolysis.

  • We learned that already.

  • And then most-- especially introductory-- textbooks will

  • give the Krebs cycle credit for this pyruvate oxidation,

  • but that's really a preparatory stage.

  • The Krebs cycle is really formally this part where you

  • start with acetyl-CoA, you merge it

  • with oxaloacetic acid.

  • And then you go and you form citric acid, which essentially

  • gets oxidized and produces all of these things that will need

  • to either directly produce ATP or will do it indirectly in

  • the electron transport chain.

  • But let's account for everything that we have. Let's

  • account for everything that we have so far.

  • We already accounted for the glycolysis right there.

  • Two net ATPs, two NADHs.

  • Now, in the citric acid cycle, or in the Krebs cycle, well

  • first we have our pyruvate oxidation.

  • That produced one NADH.

  • But remember, if we want to say, what are we producing for

  • every glucose?

  • This is what we produced for each of the pyruvates.

  • This NADH was from just this pyruvate.

  • But glycolysis produced two pyruvates.

  • So everything after this, we're going to multiply by two

  • for every molecule of glucose.

  • So I'll say, for the pyruvate oxidation times two means that

  • we got two NADHs.

  • And then when we look at this side, the formal Krebs cycle,

  • what do we get?

  • We have, how many NADHs?

  • One, two, three NADHs.

  • So three NADHs times two, because we're going to perform

  • this cycle for each of the pyruvates produced from

  • glycolysis.

  • So that gives us six NADHs.

  • We have one ATP per turn of the cycle.

  • That's going to happen twice.

  • Once for each pyruvic acid.

  • So we get two ATPs.

  • And then we have one FADH2.

  • But it's good, we're going to do this cycle twice.

  • This is per cycle.

  • So times two.

  • We have two FADHs.

  • Now, sometimes in a lot of books these two NADHs, or per

  • turn of the Krebs cycle, or per pyruvate this one NADH,

  • they'll give credit to the Krebs cycle for that.

  • So sometimes instead of having this intermediate step,

  • they'll just write four NADHs right here.

  • And you'll do it twice.

  • Once for each puruvate.

  • So they'll say eight NADHs get produced from the Krebs cycle.

  • But the reality is, six from the Krebs cycle two from the

  • preparatory stage.

  • Now the interesting thing is we can account whether we get

  • to the 38 ATPs promised by cellular respiration.

  • We've directly already produced, for every molecule

  • of glucose, two ATPs and then two more ATPs.

  • So we have four ATPs.

  • Four ATPs.

  • How many NADHs do we have?

  • 2, 4, and then 4 plus 6 10.

  • We have 10 NADHs.

  • And then we have 2 FADH2s.

  • I think in the first video on cellular

  • respiration I said FADH.

  • It should be FADH2, just to be particular about things.

  • And these, so you might say, hey, where are our 38 ATPs?

  • We only have four ATPs right now.

  • But these are actually the inputs in the electron

  • transport chain.

  • These molecules right here get oxidized in the electron

  • transport chain.

  • Every NADH in the electron transport chain

  • produces three ATPs.

  • So these 10 NADHs are going to produce 30 ATPs in the

  • electron transport chain.

  • And each FADH2, when it gets oxidized and gets turned back

  • into FAD in the electron transport chain,

  • will produce two ATPs.

  • So two of them are going to produce four ATPs in the

  • electron transport chain.

  • So we now see, we get four from just what

  • we've done so far.

  • Glycolysis, the preparatory stage and the Krebs or citric

  • acid cycle.

  • And then eventually, these outputs from glycolysis and

  • the citric acid cycle, when they get into the electron

  • transport chain, are going to produce another 34.

  • So 34 plus 4, it does get us to the promised 38 ATP that

  • you would expect in a super-efficient cell.

  • This is kind of your theoretical maximum.

  • In most cells they really don't get quite there.

  • But this is a good number to know if you're going to take

  • the AP bio test or in most introductory biology courses.

  • There's one other point I want to make here.

  • Everything we've talked about so far, this is carbohydrate

  • metabolism.

  • Or sugar catabolism, we could call it.

  • We're breaking down sugars to produce ATP.

  • Glucose was our starting point.

  • But animals, including us, we can catabolize other things.

  • We can catabolize proteins.

  • We can catabolize fats.

  • If you have any fat on your body, you have energy.

  • In theory, your body should be able to take that fat and you

  • should be able to do things with that.

  • You should be able to generate ATP.

  • And the interesting thing, the reason why I bring it up here,

  • is obviously glycolysis is of no use to these things.

  • Although fats can be turned into glucose in the liver.

  • But the interesting thing is that the Krebs cycle is the

  • entry point for these other catabolic mechanisms. Proteins

  • can be broken down into amino acids, which can be broken

  • down into acetyl-CoA.

  • Fats can be turned into glucose, which actually could

  • then go the whole cellular respiration.

  • But the big picture here is acetyl-CoA is the general

  • catabolic intermediary that can then enter the Krebs cycle

  • and generate ATP regardless of whether our fuel is

  • carbohydrates, sugars, proteins or fats.

  • Now, we have a good sense of how everything works out right

  • now, I think.

  • Now I'm going to show you a diagram that you might see in

  • your biology textbook.

  • Or I'll actually show you the actual diagram from Wikipedia.

  • I just want to show you, this looks very

  • daunting and very confusing.

  • And I think that's why many of us have trouble with cellular

  • respiration initially.

  • Because there's just so much information.

  • It's hard to process what's important.

  • But I want to just highlight the important steps here.

  • Just so you see it's the same thing that we talked about.

  • From glycolysis you produce two pyruvates.

  • That's the pyruvate right there.

  • They actually show its molecular structure.

  • This is the pyruvate oxidation step that I talked about.

  • The preparatory step.

  • And you see we produce a carbon dioxide.

  • And we reduce NAD plus into NADH.

  • Then we're ready to enter the Krebs cycle.

  • The acetyl-CoA and the oxaloacetate or oxaloacetic

  • acid, they are reacted together to

  • create citric acid.

  • They've actually drawn the molecule there.

  • And then the citric acid is oxidized through the Krebs

  • cycle right there.

  • All of these steps, each of these steps are

  • facilitated by enzymes.

  • And it gets oxidized.

  • But I want to highlight the interesting parts.

  • Here we have an NAD get reduced to NADH.

  • We have another NAD get reduced to NADH.

  • And then over here, another NAD gets reduced to NADH.

  • So, so far, if you include the preparatory step, we've had

  • four NADHs formed, three directly from the Krebs cycle.

  • That's just what I told you.

  • Now we have, in this diagram they say GDP.

  • GTP gets formed from GDP.

  • The GTP is just guanosine triphosphate.

  • It's another purine that can be a source of energy.

  • But then that later can be used to form an ATP.

  • So this is just the way they happen to draw it.

  • But this is the actual ATP that I drew in the

  • diagram on the top.

  • And then they have this Q group.

  • And I won't go into it.

  • And then it gets reduced over here.

  • It gets those two hydrogens.

  • But that essentially ends up reducing the FADH2s.

  • So this is where the FADH2 gets produced.

  • So as promised, we produced, for each pyruvate that

  • inputted-- remember, so we're going to do it twice-- for

  • each pyruvate we produced one, two, three, four NADHs.

  • We produced one ATP and one FADH2.

  • That's exactly what we saw up here.

  • I'll see you in the next video.

So we already know that if we start off with a glucose

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