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  • - [Voiceover] So let's give ourselves

  • an overview of glycolysis.

  • and glycolysis is an incredibly

  • important biochemical pathway.

  • It occures in practically all life as we know it

  • and it's all about taking glucose as a fuel and,

  • in the process of breaking it up,

  • lycing the glucose, glycolysis,

  • breaking it up into two pyruvate molecules.

  • Glucose is a six carbon molecule.

  • Each of the pyruvates are three carbon molecules.

  • In the process of doing that,

  • you produce two ATPs net.

  • It actually turns out that you need to use two ATPs

  • and then you produce four.

  • So you use two ATPs.

  • That's often called the investment phase

  • and we'll talk about that in a second.

  • And then you produce four ATPs

  • for a net of

  • plus two ATPs

  • and that's what we see right over here.

  • You see a net of two ATPs being produced directly

  • by glycolysis, and then you also have

  • the reduction of NAD to NADH.

  • Remember, reduction is all about gaining electrons,

  • and over here, NAD, that's nicotinamide

  • adinine dinucleotide, we have other videos on that,

  • it's an interesting molecule, it's actually a

  • fairly decent-sized molecule, we see this

  • positive charge, but then we see that not only

  • does it gain a hydrogen, but it loses its positive charge.

  • It gains a hydrogen and an electron.

  • You can think on a net basis it's gaining

  • a hydride.

  • Now a hydride anion's not going to typically be

  • all by itself, but on a net basis, you can think about

  • that's what's happening.

  • And so it's gaining a hydrogen and an extra electron

  • and so this, the NAD+, this is going to get reduced.

  • That is going to get reduced to NADH.

  • So this is getting reduced to NADH.

  • And that NADH, it can then

  • be oxidized in the electron transport chain.

  • We'll study that later on when we think about

  • oxidative phosphorylation, to produce

  • even more ATPs.

  • But on a very high-level, simple basis.

  • Glucose being broken down in pyruvate,

  • six carbons, three carbons each of these

  • pyruvates, now there's other things attached

  • to the carbons, and we'll see that in a little bit.

  • Two ATPs net generated, and you have the reduction

  • of two NADs to two NADHs, and those can be used

  • later on to produce more ATPs.

  • Now, glycolysis is typically just the beginning

  • of cellular respiration.

  • If oxygen is around, then you have these products,

  • some of these moving into the mitochondria

  • where you can have the citric acid cycle,

  • Krebs cycle, and the oxidative phosphorylation occur.

  • If you don't have oxygen around, then you're

  • going to do anaerobic respiration, or you're

  • going to go into fermentation.

  • We'll talk about that in a future video, and that's

  • really about figuring out what to do with these

  • products, and especially replenishing your NAD+.

  • Now that we have a very high-level overview

  • of glycolysis, let's get a better appreciation

  • for exactly what's going on.

  • And whenever I look at these more detailed

  • processes, the one thing to just appreciate

  • is how much complexity is occurring

  • in all of your cells right now.

  • This is fairly abstract,

  • to even imagine these things, but this

  • is happening throughout your body

  • gazillions of times, right now.

  • This isn't something that is somehow

  • distant from you.

  • And it's also fun to appreciate, well how

  • all of this was discovered by scientists.

  • That's a whole other fascinating discussion.

  • But the whole point of this video is just to

  • give us an appreciation for the actual mechanism

  • or the reaction by which it occurs.

  • I'm not gonna go into the detailed

  • organic chemistry mechanism.

  • So over here, this is a glucose molecule over here,

  • you see one, two, three, four, five, six carbons.

  • And then the first step is, it gets phosphorylated

  • and we have a whole video on the phosphorylation

  • of glucose, and all of these steps are facilitated

  • with enzymes.

  • The phosphorylation is facilitated with the hexokinase.

  • Kinase is a general term for an enzyme

  • that either facilitates phosphorylation or

  • dephosphorylates, it's dealing with

  • phosphorylation, I guess you could say.

  • And enzymes are all about lowering

  • the activation energy.

  • And the way that hexokinases do, or part of

  • how they do it, is they involve the cofactor,

  • a magnesium ion.

  • And we've talked about that in other videos,

  • how cofactors can help an enzyme lower the

  • activation energy.

  • And to do the phosphorylation, we use an ATP.

  • So this is minus one ATP.

  • So we are in the investment phase.

  • But this reaction strongly goes from

  • left to right, it's a coupled reaction that,

  • phosphorylating the glucose, that

  • requires free energy, but the ATP releases free energy

  • you couple these reactions, it strongly goes from

  • left to right.

  • Now, and just to be clear what happened,

  • this over here got replaced, or maybe

  • I should say this over here got replaced

  • with that over there.

  • Just to keep track of what's happening.

  • Now, another enzyme-catalyzed reaction,

  • this one is actually an equilibrium, it can

  • go both ways, but as we'll see, the right

  • side or the things that are further into the

  • glycolysis process, these are constantly

  • being turned into further products, so their

  • concentrations are going to go down, and so

  • the reaction will tend to go that way.

  • Although this particular reaction, going from

  • glucose 6-phosphate to fructose 6-phosphate,

  • this could be an equilibrium.

  • But the enzyme that facilitates this,

  • phosphoglucose isomerase, these are

  • enzymes that help go from one isomer of

  • a molecule to another isomer.

  • And that's what's happening here.

  • Instead of this oxygen being bound to this carbon,

  • this bond forms with this carbon.

  • So you have fructose, you have the five-carbon

  • ring over here, or you have the five-element

  • ring, you have four carbons in it,

  • versus a six-element ring where right over here

  • you have five carbons.

  • So this bond goes to this carbon right over here

  • and that's the main difference.

  • And then you have another, very strong

  • forward reaction, once again facilitated

  • by ATP, and this is done by phosphofructokinase.

  • It has the word kinase in it.

  • And it's using up the ATP, you can guess

  • what's going to happen.

  • We're going to attach another phosphate

  • group to the fructose 6-phosphate, and now

  • you have two of these phosphate groups.

  • So this hydrogen right over here is now

  • replaced with another phosphate group.

  • And once again it's facilitated by the

  • magnesium cofactor, it helps stabilize

  • some of the negative charge associated with

  • the phosphate groups, we talk about that in other videos.

  • But the important thing is, it uses another ATP.

  • We're still in the investment phase,

  • negative one ATP.

  • And every time I look at this it's just fascinating

  • that all of this stuff is happening in your cells

  • as we speak.

  • In fact, in order for me to speak this has to happen,

  • because my body needs to take glucose and come up

  • with some energy to turn into ATPs so that my muscles

  • can actually move and I can actually inhale and exhale

  • and all the things that I need to do for speech.

  • So appreciate what's going on over here.

  • Now the next step we talk about, the whole

  • process of glycolysis is lysing glucose.

  • And over here this is derived from

  • glucose and some phosphates, and the

  • next step, we're actually going to break it up.

  • And we're going to break it up using the enzyme

  • fructose biphosphate aldolase.

  • Aldolase enzymes facilitate the aldol reaction.

  • And this one, the aldol reaction could be

  • to merge two molecules or in this case,

  • we're going to break them up.

  • And we break them up into two three-carbon chains.

  • Now these two three-carbon chains,

  • glyceraldehyde 3-phosphate, and this character

  • right over here, they can be converted between

  • the two with another isomerase, this

  • triosephosphate isomerase right over here.

  • So at this point in glycloysis, we can think of ourselves

  • as really having two of these.

  • So let's say two times glyceraldehyde 3-phosphate.

  • So as we go further on, just imagining

  • this happening twice for every glucose molecule.

  • And any time you get confused, I encourage you

  • to pause the video.

  • See how these pieces and these pieces

  • put together, can form that over there.

  • So now we have another reaction,

  • it's facilitated by a dehydrogenase.

  • Dehydrogenases usually are involved in

  • this case, this is the reduction of NAD.

  • We saw that in the overview video.

  • So NAD is being reduced.

  • And this can be used, this NADH later on

  • can be used in the electron transport chain

  • to potentially produce some more ATP,

  • but in that process we also add

  • another phosphate group to the

  • glyceraladehyde 3-phosphate.

  • So you see this phosphate group

  • right over here that wasn't there before.

  • And actually this right over here is

  • I should have arrows on both sides,

  • this right over here, that reaction

  • could actually go both directions.

  • Actually, that reaction can as well.

  • And then, we are now

  • going to be in the payoff phase.

  • So this right over here, we're starting

  • with this molecule that has these

  • two phosphate groups, and then

  • using the phosphoglycerate kinase, we're

  • able to pop one of those phosphate groups off

  • and in the process, produce ATP.

  • Now we might want to say plus one ATP,

  • but we have to remember, this is now

  • happening twice.

  • Cuz we had two of those glyceraldehyde 3-phosphates,

  • so now we could say, if we're talking about

  • this happening twice, plus two ATPs.

  • We are now in the payoff phase.

  • Then you have, facilitated by the

  • phosphoglycerate mutase, a mutase is a

  • class of isomerases.

  • I have trouble saying that.

  • That'll take a functional group from one place

  • to another, or take one part of a molecule

  • to another part, and you see this phosphate group

  • moving on from this carbon to the middle carbon.

  • And so that's what that's doing.

  • Then we use an enloase to get over here

  • and then the pyruvate kinase, and here

  • the kinase is going to be used to

  • dephosphorylate this molecule right over here,

  • and it gets us to the way I've drawn it is

  • pyruvic acid, since I've drawn the hydrogen here,

  • and if the hydrogen is let go and this oxygen

  • hogs the electron, we would call this pyruvate.

  • And this is considered to be the end of,

  • I guess you could say, mainstream glycolysis.

  • But what happened, and I don't want to

  • glaze over what happened over here,

  • this ADP got converted to another ATP, but it's

  • going to happen twice.

  • So this is another plus two ATPs.

  • So hopefully you see the investment phase,

  • we use an ATP right over here to phosphorylate

  • the glucose, we use another ATP right over here

  • to throw that second phosphate group

  • on what was the fructose 6-phosphate,

  • but then we get the payoff phase.

  • So we're able to produce this NADH, and

  • this is actually going to be two NADHs,

  • because everything here's going to happen

  • twice now, we can assume that this character

  • over here also gets converted to a

  • glyceraldehyde 3-phosphate, and now

  • we've produced two ATPs, cuz this is happening twice,

  • and we've produced two ATPs right over there.

  • So hopefully everything we talked about in the beginning

  • actually makes sense.

- [Voiceover] So let's give ourselves

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糖酵解的步驟|細胞呼吸|生物|可汗學院 (Steps of glycolysis | Cellular respiration | Biology | Khan Academy)

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    林易昌 發佈於 2021 年 01 月 14 日
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