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- We've already learned that cellular respiration can be
- broken down into roughly three phases.
- The first is glycolysis, which literally means the breaking
- down of glucose.
- And then this can occur with or without oxygen.
- If we don't have oxygen, then we go over to fermentation.
- We'll talk about that in the future.
- Go over to fermentation and in humans it
- produces lactic acid.
- In other types of organisms it might
- produce alcohol or ethanol.
- But if we have oxygen-- and for the most part we're going
- to assume that we can proceed forward with oxygen-- if there
- is oxygen, then we could proceed forward
- to the Krebs cycle.
- Sometimes called the citric acid cycle because it deals
- with citric acid.
- The same thing that's in orange juice or lemons.
- And then from there we proceed to the
- electron transport chain.
- And we learned in the first overview video of cellular
- respiration that this is where the bulk of the ATP is
- actually produced.
- Although it uses raw materials that came out of
- these phases up here.
- Now what I want to do in this video is just focus on
- glycolysis.
- And this is kind of-- it's sometimes a challenging task
- because you can really get stuck in the weeds.
- And I'll show you the weeds in a little bit,
- and the actual mechanism.
- And it can be very daunting.
- But what I want to do is simplify it for you so you can
- have the big take-aways.
- And then we can appreciate, and then maybe when we look at
- the weeds of glycolysis we can make a little bit
- more sense of it.
- So glycolysis, or really cellular respiration, it
- starts off with glucose.
- And glucose, we know its formula.
- It's C6H12O6.
- And I could draw its whole structure; it would take a
- little time.
- But I'm just going to focus on the carbon backbone.
- So it is a ring, or can be a ring.
- But I'm just going to draw it as six carbons in a row.
- Now there's two kind of important phases of glycolysis
- that are good to know.
- One, I call the investment phase.
- And the investment phase actually uses two ATPs.
- So you know, the whole purpose of cellular respiration is to
- generate ATPs, but right from the get-go I actually have to
- use two ATPs.
- But I use two ATPs and then I'm essentially going to break
- up the glucose into two 3-carbon compounds right here
- that actually also have a phosphate group on them.
- The phosphate groups are coming from those ATPs.
- They also have a phosphate group on them and this is
- often called-- well, there's a lot of names for it.
- Sometimes it's called PGAL.
- You really don't have to know this.
- Or phosphoglyceraldehyde, really challenging my spelling
- skills right here.
- That's not that important to know.
- All you have to know is in this first
- phase you use two ATPs.
- That's why I call it the investment phase.
- If we use a business analogy, investment phase.
- And then each of these two PGAL molecules can then go
- into the payoff phase.
- So in the payoff phase, each of these
- PGALs turn into pyruvate.
- Which is another 3-carbon, but it's reconfigured.
- But the process of it going to pyruvate-- and let me write
- pyruvate in blue, because this is something that, at least
- it's good to know the word.
- And I'll show you the structure in a second.
- Pyruvate.
- Sometimes it's called pyruvic acid.
- Same thing.
- And that's essentially the end product of glycolysis.
- So you start off with glucose in the investment phase.
- You end up in this phosphoglyceraldehyde, which
- essentially you broke up your glucose and you put a
- phosphate on either end of it.
- And then those each independently go through the
- payoff phase.
- So you end up with two molecules of pyruvate for
- every molecule of glucose you started off with.
- Now you're saying, hey, Sal, there was a payoff phase, what
- was our payoff?
- Well our payoff, we got, for each-- let me write this down
- as a payoff phase.
- This is our payoff phase.
- And I apologize for the white background.
- I did it because, the mechanism I'm showing you, I
- copy-and-pasted it from Wikipedia, and they had a
- white background so I just ran with the white background for
- this video.
- But I, personally at least, like the black background a
- lot better.
- But this is the payoff phase right here.
- And so when we go from the phosphoglyceraldehyde to the
- pyruvate or the pyruvic acid, we produce two things.
- Or I guess we could say we produce three things.
- We produce, each of these PGALs to
- pyruvates produce two ATPs.
- So I'm going to produce two ATPs there, I'm going to
- produce two ATPs there.
- And then they each produce an NADH.
- And I'll do it in a darker color.
- NADH.
- And of course they're not producing the whole molecule
- in a vacuum.
- Essentially what they're doing is they're starting with the
- raw material of an NAD plus-- so they start off with an NAD
- plus-- and they essentially reduce
- it by adding a hydrogen.
- Remember, we learned a couple of videos ago that you could
- view reduction as a gain in hydrogen.
- So the NAD gets reduced to NADH.
- And then later on, these NADHs are used in electron transport
- chain to actually produce ATPs.
- So the big take-away here, if I were to write the reaction
- that we get for glycolysis, is that you
- start off with a glucose.
- And you need some NAD plus.
- And actually, for every mole of glucose, you're going to
- need two NAD plusses.
- You're going to need two ATPs.
- So I'm just writing all the ingredients that we need to
- start off with.
- And then you're going to need-- well, let me say, these
- guys are going to be ADPs before we turn them to ATPs.
- So I'll write plus four ADPs.
- And then, after performing glycolysis-- and
- let me write it here.
- Let me write also-- sorry that was ADPs.
- Let me just rewrite that part right there.
- Four ADPs.
- And then you maybe need two phosphate groups.
- Because we're going to need four phosphate groups.
- Plus four-- I'll just call them, sometimes they're
- written like that.
- But maybe I'll write it like this.
- Four phosphate groups.
- And then once you perform glycolysis, you have two
- pyruvates, you have two NADHs.
- The NAD has been reduced.
- It gained a hydrogen.
- RIG.
- OIL RIG.
- Reduction is gain an electron.
- But in the biological sense, we think of
- it gaining the hydrogen.
- Because hydrogen is very non-electronegative, so you're
- hogging its electrons.
- You've gained its electrons.
- So two NADHs and then plus these two ATPs get used in the
- investment phase.
- That's why I kind of wrote them a little separately.
- So these two get used.
- So then you're left with two ADPs.
- And then these guys, essentially,
- get turned into ATPs.
- So plus four ATPs.
- I guess we didn't need four.
- We only needed a net of two phosphate groups.
- Because two jump off of here.
- And then we need a total of two more to get
- four jumping on there.
- But the big picture is, you start with a glucose, you end
- up with two pyruvates.
- You use up two ATPs.
- You get four ATPs.
- So you have a net of two ATPs formed.
- Let me write that very big.
- Net, what you get out of glycolysis, is two ATPs.
- You get two NADHs that can each later be used in the
- electron transport chain to produce three ATPs.
- You get two NADHs and you get two pyruvates, which are going
- to be re-engineered into acetyl-CoAs that are going to
- be the raw materials for the Krebs cycle.
- But these are the outputs of glycolysis.
- So now that we have that big picture, let's actually look
- at the mechanism.
- Because this is a little bit more daunting
- when you see it here.
- But we'll see the same themes that I just talked about.
- We're starting with a glucose right there.
- It is a six chain.
- It's in a circle, in a ring.
- One, two, three, four, five, six carbons.
- I could write it like that, just to make a huge
- oversimplification.
- It goes through a few steps.
- I use an ATP here.
- So let me do that in a color.
- Let me do it in orange whenever I use an ATP.
- I use one ATP there.
- I use one ATP there.
- And just like I told you, they have a slightly
- different name for it.
- But this is the
- phosphoglyceraldehyde right here.
- They call it glyceraldehyde 3-phosphate.
- It's the exact same molecule.
- But as you can see, just when I drew it very roughly before,
- you've got one, two three carbons there.
- And it also has a phosphate group on it.
- The phosphate group's actually attached to the oxygen.
- But for just for simplification I draw the
- phosphate group just like that.
- And I showed that right here.
- This was the
- phosphoglyceraldehyde right here.
- This is the actual structure up here.
- But I think sometimes when you look at the structure it's
- easy to miss the big picture.
- And there are two of these.
- They kind of say that you can go back and forth with this,
- with this other kind of isomer of this.
- But the important thing is that you have two of these
- compounds that are now 3-carbon compounds.
- Glucose has been split.
- And now we're ready to enter the payoff phase.
- Remember you have two of these compounds right here.
- That's why, when they drew this mechanism, they wrote
- times two right there.
- Because the glucose has been split into
- two of these molecules.
- So each of the molecules are now going to
- do this right here.
- And for each of the glyceraldehyde 3-phosphates,
- or PGALs, or phosphoglyceraldehyde, we can
- look at the mechanism and say, OK look here, there's going to
- be an ADP turning into an ATP there.
- So this is plus one ATP.
- And then we see it again happening here
- on our way to pyruvate.
- On our way to pyruvate right, there then we have another
- plus one ATP.
- So for each of the PGALs, or the phosphoglyceraldehydes
- that were produced, we're producing two ATPs in the
- payoff phase.
- Now there were two of these.
- So total for one glucose, we're going to produce four
- ATPs in the payoff phase.
- So in the payoff phase, four ATPs.
- In the investment phase we used one, two ATPs.
- So total net ATPs directly generated from
- glycolysis is two ATPs.
- Four, gross produced.
- But we had to invest two in the investment phase.
- And then the NADs and the NADHs, we see right here.
- For each phosphoglyceraldehyde, or
- glyceraldehyde 3-phosphates or PGALs or whatever you want to
- call them, at this stage right here you see that we are
- reducing NAD plus to NADH.
- So this happens once for each of these compounds.
- And obviously there are two of these.
- Glucose got split into two of these guys.
- So two NADHs are going to be produced.
- And later these are going to be used in the electron
- transport chain to actually each produce three ATPs.
- And then finally, when everything is said and done,
- we're left with the pyruvates.
- And it's nice, at least that they made it nice and big.
- We can take a look at what a pyruvate looks like.
- And just as promised, we can look at all the oxygen bonds
- and all that.
- But it's a 3-carbon structure.
- It has a 3-carbon backbone.
- So the end result is that the carbon, that the glucose got
- split in half.
- It got oxidized.
- Some of the hydrogens got stripped off of it.
- As you can see there's only three hydrogens here.
- We started off with 12 hydrogens in glucose.
- And now it has its carbons bonding more
- strongly with oxygen.
- So it's essentially having its electrons stolen by the
- oxygens, or hogged by the oxygens.
- So carbon has gotten oxidized in this process.
- There's going to be more oxidation left to do.
- And in the process we were able to generate two net ATPs
- and two NADHs that can later be used to produce ATPs.
- Anyway, hopefully you found that helpful.