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  • In my humble opinion, the single most important

  • biochemical reaction, especially to us, is cellular

  • respiration.

  • And the reason why I feel so strongly about that is because

  • this is how we derive energy from what we

  • eat, or from our fuel.

  • Or if we want to be specific, from glucose.

  • At the end of the day, most of what we eat, or at least

  • carbohydrates, end up as glucose.

  • In future videos I'll talk about how we derive energy

  • from fats or proteins.

  • But cellular respiration, let's us go from glucose to

  • energy and some other byproducts.

  • And to be a little bit more specific about it, let me

  • write the chemical reaction right here.

  • So the chemical formula for glucose, you're going to have

  • six carbons, twelve hydrogens and six oxygens.

  • So that's your glucose right there.

  • So if you had one mole of glucose-- let me write that,

  • that's your glucose right there-- and then to that one

  • mole of glucose, if you had six moles of molecular oxygen

  • running around the cell, then-- and this is kind of a

  • gross simplification for cellular respiration.

  • I think you're going to appreciate over the course of

  • the next few videos, that one can get as involved into this

  • mechanism as possible.

  • But I think it's nice to get the big picture.

  • But if you give me some glucose, if you have one mole

  • of glucose and six moles of oxygen, through the process of

  • cellular respiration-- and so I'm just writing it as kind of

  • a big black box right now, let me pick a nice color.

  • So this is cellular respiration.

  • Which we'll see is quite involved.

  • But I guess anything can be, if you want to be particular

  • enough about it.

  • Through cellular respiration we're going to produce six

  • moles of carbon dioxide.

  • Six moles of water.

  • And-- this is the super-important part-- we're

  • going to produce energy.

  • We're going to produce energy.

  • And this is the energy that can be used to do useful work,

  • to heat our bodies, to provide electrical

  • impulses in our brains.

  • Whatever energy, especially a human body needs, but it's not

  • just humans, is provided by this

  • cellular respiration mechanism.

  • And when you say energy, you might say, hey Sal, on the

  • last video didn't you just-- well, if that was the last

  • video you watched, you probably saw that I said ATP

  • is the energy currency for biological systems. And so you

  • might say, hey, well it looks like glucose is the energy

  • currency for biological systems. And to some degree,

  • both answers would be correct.

  • But to just see how it fits together is that the process

  • of cellular respiration, it does produce energy directly.

  • But that energy is used to produce ATP.

  • So if I were to break down this energy portion of

  • cellular respiration right there, some of it

  • would just be heat.

  • You know, it just warms up the cell.

  • And then some of it is used-- and this is what the textbooks

  • will tell you.

  • The textbooks will say it produces 38 ATPs.

  • It can be more readily used by cells to contract muscles or

  • to generate nerve impulses or do whatever else-- grow, or

  • divide, or whatever else the cell might need.

  • So really, cellular respiration, to say it

  • produces energy, a little disingenuous.

  • It's really the process of taking glucose and producing

  • ATPs, with maybe heat as a byproduct.

  • But it's probably nice to have that heat around.

  • We need to be reasonably warm in order for our cells to

  • operate correctly.

  • So the whole point is really to go from glucose, from one

  • mole of glucose-- and the textbooks will

  • tell you-- to 38 ATPs.

  • And the reality is, this is in the ideal circumstances that

  • you'll produce 38 ATPs.

  • I was reading up a little bit before doing this video.

  • And the reality is, depending on the efficiency of the cell

  • in performing cellular respiration, it'll probably be

  • more on the order of 29 to 30 ATPs.

  • But there's a huge variation here and people are really

  • still studying this idea.

  • But this is all cellular respiration is.

  • In the next few videos we're going to break it down into

  • its kind of constituent parts.

  • And I'm going to introduce them to you right now, just so

  • you realize that these are parts of cellular respiration.

  • The first stage is called glycolysis.

  • Which literally means breaking up glucose.

  • And just so you know, this part, the glyco for glucose

  • and then lysis means to break up.

  • When you saw hydrolysis, it means using water to break up

  • a molecule.

  • Glycolysis means we're going to be breaking up glucose.

  • And in case you care about things like word origins,

  • glucose comes from, the gluc part of glucose comes from

  • Greek for sweet.

  • And glucose is indeed sweet.

  • And then all sugars, we put this ose ending.

  • So that just means sugar.

  • So you might think it's kind of a redundant statement to

  • say sweet sugar.

  • But there are some sugars that aren't sweet.

  • For example, lactose.

  • Milk, it might be a little bit, but when you actually

  • digest lactose then you can turn it into an actual sweet

  • sugar, but it doesn't taste sweet like glucose or fructose

  • or sucrose would taste.

  • But anyway, that's an aside.

  • But the first step of cellular respiration is glycolysis,

  • breaking up of glucose.

  • What it does is, it breaks up the glucose from a 6-carbon

  • molecule-- so it literally takes it from a 6-carbon

  • molecule-- let me draw it like this-- a 6-carbon molecule

  • that looks like this.

  • And it's actually a cycle.

  • Let me show you what glucose actually looks like.

  • This is glucose right here.

  • And notice you have one, two, three,

  • four, five, six carbons.

  • I got this off of Wikipedia.

  • Just look up glucose and you can see this diagram if you

  • want to kind of see the details.

  • You can see you have six carbons, six oxygens.

  • That's one, two, three, four, five, six.

  • And then all these little small blue

  • things are my hydrogens.

  • So that's what glucose actually looks like.

  • But the process of glycolysis, you're essentially just

  • taking-- I'm writing it out as a string, but you could

  • imagine it as a chain-- and it has oxygens and hydrogens

  • added to each of these carbons.

  • But it has a carbon backbone.

  • And it breaks that carbon backbone in two.

  • That's what glycolysis does, right there.

  • So you've kind of lysed the glucose and

  • each of these things.

  • And I haven't drawn all the other stuff

  • that's added on to that.

  • You know, these things are all bonded to other things, with

  • oxygens and hydrogens and whatever.

  • But each of these 3-carbon backbone

  • molecules are called pyruvate.

  • We'll go into a lot more detail on that.

  • But glycolysis, it by itself generates-- well,

  • it needs two ATPs.

  • And it generates four ATPs.

  • So on a net basis, it generates two-- let me write

  • this in a different color-- it generates two net ATPs.

  • So that's the first stage.

  • And this can occur completely in the absence of oxygen.

  • I'll do a whole video on glycolysis in the future.

  • Then these byproducts, they get

  • re-engineered a little bit.

  • And then they enter into what's called the Krebs cycle.

  • Which generates another two ATPs.

  • And then, and this is kind of the interesting point, there's

  • another process that you can say happens

  • after the Krebs cycle.

  • But we're in a cell and everything's bumping into

  • everything all of the time.

  • But it's normally viewed to be after glycolysis

  • and the Krebs cycle.

  • And this requires oxygen.

  • So let me be clear, glycolysis, this first step,

  • no oxygen required.

  • Doesn't need oxygen.

  • It can occur with oxygen or without it.

  • Oxygen not needed.

  • Or you could say this is called an anaerobic process.

  • This is the anaerobic part of the respiration.

  • Let me write that down too.

  • Anaerobic.

  • Maybe I'll write that down here.

  • Glycolysis, since it doesn't need oxygen, we

  • can say it's anaerobic.

  • You might be familiar with the idea of aerobic exercise.

  • The whole idea of aerobic exercise is to make you

  • breathe hard because you need a lot of oxygen

  • to do aerobic exercise.

  • So anaerobic means you don't need oxygen.

  • Aerobic means it needs oxygen.

  • Anaerobic means the opposite.

  • You don't need oxygen.

  • So, glycolysis anaerobic.

  • And it produces two ATPs net.

  • And then you go to the Krebs cycle, there's a little bit of

  • setup involved here.

  • And we'll do the detail of that in the future.

  • But then you move over to the Krebs cycle, which is aerobic.

  • It is aerobic.

  • It requires oxygen to be around.

  • And then this produces two ATPs.

  • And then this is the part that, frankly, when I first

  • learned it, confused me a lot.

  • But I'll just write it in order the way it's

  • traditionally written.

  • Then you have something called-- we're using the same

  • colors too much-- you have something called the electron

  • transport chain.

  • And this part gets credit for producing

  • the bulk of the ATPs.

  • 34 ATPs.

  • And this is also aerobic.

  • It requires oxygen.

  • So you can see, if you had no oxygen, if the cells weren't

  • getting enough oxygen, you can produce a

  • little bit of energy.

  • But it's nowhere near as much as you can produce once you

  • have the oxygen.

  • And actually when you start running out of oxygen, this

  • can't proceed forward, so what happens is some of these

  • byproducts of glycolysis, instead of going into the

  • Krebs cycle and the electron transport chain, where they

  • need oxygen, instead they go through a side process called

  • fermentation.

  • For some organisms, this process of fermentation takes

  • your byproducts of glycolysis and

  • literally produces alcohol.

  • That's where alcohol comes from.

  • That's called alcohol fermentation.

  • And we, as human beings, I guess fortunately or

  • unfortunately, our muscles do not directly produce alcohol.

  • They produce lactic acid.

  • So we do lactic acid fermentation.

  • Let me write that down.

  • Lactic acid.

  • That's humans and probably other mammals.

  • But other things like yeast will do alcohol fermentation.

  • So this is when you don't have oxygen.

  • It's actually this lactic acid that if I were to sprint

  • really hard and not be able to get enough oxygen, that my

  • muscles start to ache because this lactic acid

  • starts to build up.

  • But that's just a side thing.

  • If we have oxygen we can move to the Krebs cycle, get our

  • two ATPs, and then go on to the electron transport chain

  • and produce 34 ATPs, which is really the bulk of what

  • happens in respiration.

  • Now I said this as an aside, that to some

  • degree this isn't fair.

  • Because while these guys are operating they're also

  • producing these other molecules.

  • They're not producing them entirely, but what they're

  • doing is, they're taking-- and I know this gets complicated

  • here, but I think over the course of the next few videos

  • we'll get an intuition for it-- in these two parts of the

  • reaction, glycolysis and the Krebs cycle, we're constantly

  • taking NAD-- I'll write it as NAD plus-- and we're adding

  • hydrogens to it to form NADH.

  • And this actually happens for one molecule of glucose, this

  • happens to 10 NADs.

  • Or 10 NAD plusses to become NADHs.

  • And those are actually what drive the

  • electron transport chain.

  • And I'll talk a lot more about it and kind of how that

  • happens and why is energy being derived and how is this