Biology 1A - Lecture 8: Glycolysis, fermentation

Uploaded by UCBerkeley on 12.09.2012

>>INSTRUCTOR: Good morning everybody. Good morning. So as you can see, we have a
slight technical difficulty. The large screen isn't working right now.
But the technical assistance are working on this, so hopefully this works, otherwise we're
using, so far only the small screen. I do not know if this t small screen is broadcasted
to 10 Evans. >> [Indiscernible]
>>INSTRUCTOR: Okay so you're going to be fine? Okay.
So, I hope this works and I hope you have your handouts because some of the slides have
very small text today. So you might not be able to see it all, my apologies for that.
So, today, we're going to discuss cellular respiration, but before we start I would like
you to be aware that we have a special guest today.
My wife. [Indiscernible]. Sitting in the back. [Applause]
So, quiet please. So she is a psychologist, she's an industrial
and organizational psychologist so she's here to monitor me to tell me how I can improve
my teaching. So, I apologize if I'm slightly more nervous
because you know it's going to be interesting dinner discussion today, right?
Oh, well, we will see. Okay.
Cellular respiration. So, cellular respiration is used by the cell
to produce energy. And in particular, things like glucose are oxidized by oxygen to carbon
dioxide. And then the oxygen accepts electrons to form
water. And this reaction is exergonic and therefore energy is released and this energy
is captured by the cell in form of ATP and there's also some heat that is released through
this process. And so that's what we're going to discuss
in the next two hours. This reaction is oxidation reduction reaction. And so very briefly, review
of redox reactions so you should all know this from chemistry. If you have a in this
case, an elemental sodium and the sodium loses one electron it becomes positively charge
and then this is called: Oxidation, the electron is lost.
Sodium is oxidized. If chlorine here is elemental form gains an
electron it becomes negatively charged so it becomes reduced.
And so, what one would say then is that the sodium that loses the electron would be the
reducing agent because it reduces the other compound even though it's being oxidized. And chlorine which
accepts the electron is an oxidizing agent. Okay. So we have an oxidation, and we have
a reduction. And both of them have to occur at the same time, so the electron is not just
going to sit there in solution somewhere, the electron is transferred to a molecule
and that's why these reactions are called: Redox reactions. So the transfer of an electron
from one compound to the next. The electron does not have to be completely
transferred, necessarily. It can also be a change in electron sharing
and that depends on the electronegativity of the atom that's responsible. So we have
methane, two electrons shared by the hydrogen and the carbon and they're approximately equal,
so if methane becomes oxidized ate becomes CO2 and CO2 because it's the stronger electron
negative compound you see that oxygen takes over the electron but not completely that's
why the carbon isn't charged the oxygen is also not charged but that means that carbon
in a sense lost electrons, it didn't really lose them little bit because it still has
four electrons but its in a hire oxidative state. At the same time you have oxygen, it
becomes reduces it accept it is electrons. And it accepts the protons.
So the oxidation is not a transfer of protons, it's the transfer of protons. The electrons
happen to come with it in this case. So the oxygen is reduced to water, the electrons
are transferred, and so in this nomenclature again, methane loses it electrons it reduces
the other compounds so this is again, the reducing agent. And oxygen which accepts the
electrons is the oxidizing agent. So, this will all not be new to you.
So here's back you go back to the formula for the cellular respiration. We have glucose
+ oxygen becomes CO2 and that's what happens, the carbon in the water gets oxidized it loses
electrons and forms CO2 and the oxygen accepts electrons and it becomes water.
This is what's happening here. So, this is the, and the energy that is released
through out this process is captured at ATP as we will learn and this oxidation reaction
is very exergonic and the delta G O for this is 2,800 and 40 kilojoule per mol. So that's
a lot of energy. So if this reaction were to occur as it is
here, you would release a lot of energy and if you don't have molecules that capture that
energy, the cell would heat up quite a bit. To the extent that it might be destroyed and
therefore the cell needs a way to capture this large amount of energy into portions.
And to multiple ATPs and other agents into portions so that it can so that it basically
doesn't explode. So, one way to capture electrons and in a
biological systems, are these electron capture systems so one of them is NAD +. NAD stands
for nick tin you see how it's spelled correctly, nick tin amide dinucleotide.
And so here you see the structure of this, you the nick tin, you the amide, you have
the alanine and then you have the nucleotide there's always a nitrogenous base a sugar
and phosphate group and you have two of those. And now this molecule has a positive charge
here on the nitrogen and that's why commonly it's referred to as NAD +.
But please note that this plus, describes the oxidation state it does not describe the
charge of the molecules so if you look here, if you look here at the phosphates, it has
two phosphates and they're negatively charge, so the overall charge of the molecule would
be negative. One positive and two negatives would be a negative. So the overall charge
of the molecule is negative. But, still, oh, thank you.
Yes, we're back in business up here. So please, I'm not doing this alone, Anna
she's sitting in the back, she's our little helper she makes sure everything runs so big
her a big hand. [Applause].
Thank you Anna. So, NAD +, so it's plus because of the oxidation
state it's not plus because it's charged. So this molecule can accept electrons.
So the electrons come from hydrogen. So hydrogen is a proton and electrons. So it can take
two, so it has two electrons and two protons, but the molecule itself can accept both electrons,
so these are two electrons and one proton. This is called: A hydride. If you have one
proton but two electrons you a hydride, so you have a hydride transfer and what's left
is one proton. And so, this is a reduction, right, it accepts
electrons, so the reduced form of NAD + is NADH, no charge. + H +.
Okay. So, again, NAD can be a reducing agent. An
oxidizing agent, geez, oxidizing agent is that it can abstract electrons and gets reduced
an then these electrons can be used by something. This so this is a hydrite transfer.
And this reaction, this transfer of electrons is usually governed by enzymes that belong
to the dehydrogenase family so here you have another example of a dehydrogenase.
You have here part of a carbohydrate, HCO H + you have [Indiscernible] and the protons
are removed, you get a carboxyl group and then you can + H + out of this reaction. So
this is a way of how the electrons can be moved from the sugars.
So, let's see if you're up to speed on reduction and oxidation. First question for today:
Get your iClickers out. What is the reducing agent in the following reaction? Isocitrate
+ NAD +... (Reading). Now, we haven't discussed what the structures
of these are it doesn't matter you don't really need to know because it's only a transfer
of electrons so here are the five possible components and only one of them is the reducing
agent. So, please go.
Okay. 10 more seconds. Okay.
2, 1. Stop.
So, what happens in this reaction? NAD gets reduced to NADH + H +, so it's going
to accept electrons. And so the electrons have to come then from
this molecule because it's the only one that's around and therefore the molecule is the reducing
agent because it donates the electrons to reduce NAD. And so most of you have that right.
So the 7% that have that wrong, please review oxidation and reduction.
You should have had that in chemistry somewhere. Again, this is an example of a type of question
I wouldn't ask that particular question in the exam, but it demonstrates that we'll always
tell you do not actually need to learn structures this will be names of structures but you can
still get the answer even though by thinking about relatively simple issues here so you
don't need to know the structure of isocitrate to get the answer to this really.
So cellular respiration overview: And you do not need to understand everything
right now because it's what we're going go through in the next 90 minutes.
So we start with glucose. And the glucose is converted to carbon dioxide
in 4 steps. There is glycolysis, pyruvate oxidation, the
citric acid cycle and then oxidative phosphorylation and through all of these four processes we
get ATP and that's the whole purpose of this. You can see there's two organelles that are
involved, two organelles that I involve, one is the cytosol and one is the mitochondrion.
Yeah. So you need both of them to go through respiration and we're going to start with
the details of glycolysis. So in glycolysis, glucose, which is a C 6
carbon 6 compound, oops, uhuh. Here we go. (Lost sound)
Okay. Sorry about that. Good. We are back in business.
Hm. Sorry about that, but I can't move this thing.
I can also not write. It's the PC, what can I do, right?
[Laughter]. Something is happening, it finally woke up.
Can I write? No.
Okay. I'm going write on the board then.
You're going to see the slides, and you them in your handouts, so whatever is going to
be put in the blanks I'm going to write on the board.
So, gibing sis, as you can see, takes glucose and glucose is a C 6 compound, and that gets
converted to pyruvate and pyruvate is a C 3 compounds and because no carbon is released
we're going to make two of these. Two pyruvates.
And through this reaction, through glycolysis, you're going to obtain ATP, and you'll also
transfer electrons to NAD to make NADH + H +.
So, if we go back, you see below the ATP it says, this reaction represents what is called
a substrate level phosphorylation. So substrate level is phosphorylation looks
like this you have an enzyme, the enzyme has a substrate that is phosphorylated so it contains
a phosphate and this phosphate is transferred to ADP, that's on the left side, to release
the substrate that is not phosphorylated because ATP.
Yeah, so the phosphate is transferred from the substrate to ADP to form ATP that substrate
level phosphorylation. This reaction obviously occurs also in equilibrium, so we can also
be the reverse. You can take ATP in the phosphate from ATP
the gamma phosphate the last one is transferred to the substrate to make a phosphorylated
substrate. Which direction this reaction goes, depends on the delta G on the free energy.
So, here you see now the glycolysis in reactions in detail.
There are 10 enzymes involved. To make glycolysis work. And we just going
to go through them. So glycolysis first of all occurs in the cytosol and it only occurs
in the cytosol to no mitochondria required get.
So we have glucose, and then there's an enzyme called hexokinases which does what I describes
the phosphorylation, it transfers a phosphate to ATP to glucose so it makes glucose 6 phosphate.
So this green box here should be a phosphate group. Yeah.
So it makes glucose 6 phosphate. So as you know from this reaction this is
not the production of ATP, in fact, ATP needs to be utilized to jump start glycolysis. So
this is why this first part is called: The investment phase.
So ATP is required to start glycolysis. You do not produce yet ATP.
So, with this we have the glucose 6 phosphate that is I summarize to fructose 6 phosphate,
and the advantage of the fructose is, that you open up a second hydroxyl group up here.
And when you do this, this molecule can accept a second phosphate group.
So a second ATP is utilized the phosphate is transferred to fructose 6 phosphate and
it makes a bisphosphate so we have a sugar molecule that has two phosphate groups. One
on C 1 and one on C 6 so it has two phosphate groups there.
Then the sugar that's a hex sew, so 6 carbon sugar is split by an aldolase and then you
get a C 3 compound and each of the C 3 compounds has one phosphate so it's split in the middle
so you have a C 3 phosphate. And you have two of these you started with
carbon 6, now you have carbon 3. And these can be interconverted.
So that was the investment phase. So far what has happened 2 ATPs were needed to phosphorylate
the sugar and then the sugar is broken down. So, now comes the pay off phase. So that's
a the phase where ATP is earned and so the most important in glucose is that first one,
No.6, the triose phosphate dehydrogenases reactions that means it's a reduction, oxidation
reaction so in this case, electrons are taken andNAD + becomes NADH + H +.
Oh. Thank you. Yes.
So, here NAD becomes NADH + H + and the compound is because the electrons are removed it's
oxidized. So what we had before was an aldehyde and
what we have now is a carboxyl group an acid so that's an oxidation reaction so, the aldehyde
is converted to a carboxyl group it's oxidized the electrons are transferred to NAD and we
have our NADH + H +. And we have 2 of those because when you start
with glucose, you will have two C 3 compounds therefore you will create 2 NADH.
So once this is done, once this is oxidized there will also be phosphate accepted inorganic
phosphate now does not come from ATP. It's just inorganic phosphate in the solutions
and through this oxidation reaction the phosphate can be used to phosphorylate the substrate
so now we have a C 3 compound that has 2 phosphate groups again.
Yeah so we're going to make C 3 but diphosphate. And ATP is not required for the oxidation
not required for transferring the phosphate onto this compound, because this is on ax
addition reaction and therefore releases so much energy that you can do this just with
unorganic phosphate ATP is not required for this.
So now we have a C 3 bisphosphate, and now we have again oxidative substrate level phosphorylation
but the reverse. So, now the phosphates are transferred from
the C 3 compound to ADP to form ATP and we have this twice.
So, all four phosphates, so 2 x diphosphate here, are all moved to form ATP, and at the
end we have this compound called pyruvate and the pyruvate is a C 3 compound and it
doesn't contain any phosphate groups anymore. Okay.
>>STUDENT: [Indiscernible]. >>INSTRUCTOR: Of course.
No, you don't. So, the question is right, do you need to
know all of this? No, you don't. All you need to know is this: Okay. So exclamation mark
that's all you need to know about glycolysis. So, here's a summary of this: So you have
investment phase you start with glucose C 6 compounds, 2 ATP are used up to phosphorylate
the glucose, and then in the pay off phase, you've performed the oxidation, NADH is reduced
so you get two NADHs and you release 4 ATPs [Indiscernible] and so few of you look at
the net, glucose is converted to pyruvate, to 2 pyruvates so that would be a number 2
in this blank, so it's 2 pyruvates, you have 4 ATPs form two reduced to the net affect
is the formation of two ATP, so that would be a 2.
And you have 2 NAD that are used to form NADH + H +.
And so that what you should know about glycolysis, what goes in what comes out and what do you
get in between. But not the structures and not the names of
the structures. If you ever take MCD 102 and then you will
have to learn it all. So, we learned now when you think about the
energetics of this reaction, we learned that ATP has a delta G 0 of 30 kilojoules per mol.
The gibing sis the energy we recoup rate is 2 ATPs the net is 2 ATPs, so the delta G is60,
right, so we get energy of 60245 we can use for cellular work, but we learn that had the
energy content of glucose is 2,840 kilojoule so what is our energy capture here? If you
divide the 2 no if you divide the 60 by the 2,840 you get an energy efficiency here of
about 2%. So it's very low.
So where are the other 98%? >>STUDENT: [Indiscernible]
>>INSTRUCTOR: Space. Universe? >>STUDENT: [Indiscernible]
>>INSTRUCTOR: Heat. So there's some heat, yes.
What else? >>STUDENT: [Indiscernible]
>>INSTRUCTOR: Pyruvate. Yes. We go pyruvate out and pyruvate is a high-energy compound.
Yes. And? >>STUDENT: [Indiscernible]
>>INSTRUCTOR: Yes. NADH + H + so there's a high-energy compound
because it captured the electrons. So the energy that was in glucose is now split up
into the ATPs, but only 2%. The NADH is very high and the pyruvate, which
can be further converted to capture more energy. Okay. Any questions to glycolysis?
Yes? >>STUDENT: [Indiscernible]
>>INSTRUCTOR: Why the, you mean, how the protons are the electrons are added to NADH + H +?
>>STUDENT: [Indiscernible] [Indiscernible] >>INSTRUCTOR: Yeah.
>>STUDENT: [Indiscernible] >>INSTRUCTOR: Ah. So where the energy goes?
The 98% of the energy. So, every molecule has an inert an energy, itself, and so pyruvate
happens to be still a high energy molecule. >>STUDENT: [Indiscernible]
>>INSTRUCTOR: That's what it is. I mean, you know, it can further react to
the liver energy. I would need to look up the delta G for the
oxidation of pyruvate. But it's quite high. Yes?
>>STUDENT: [Indiscernible] >>INSTRUCTOR: Okay. Yes?
>>STUDENT: [Indiscernible] >>INSTRUCTOR: So the question is what does
the hexokinases actually do? It binds the glucose and binds the ATP, and the phosphate
is transferred to glucose. So that would be the perfect example of the enzyme scheme that
I showed you in the slide before. So that would be no, the one on the right. That would
be an example of a hexokinase. Yes? Now, so, the whole reason why the cellular
goes through this process is to get ATP. Because the cell needs energy to maintain
itself. So, it can create ATP, so that's great, so
it can take the glucose, convert it to pyruvate and make ATP. That's all very nice. However
for glycolysis to occur you don't just need sugar in your cell, you also need ADP and
phosphate in your cell. Otherwise glycolysis wouldn't occur because you wouldn't make ATP.
But that's relatively straightforward, if all of your ADP + phosphate would have been
converted to ATP you have such a high energy in your cell that it doesn't need anymore
ATP and therefore glycolysis will stop. So that's easy. Only if ADP is available this
would indicate the cell I need to produce more ATP so please go ahead with glycolysis,
so the ADP and phosphate is no problem, and it will regulate whether glycolysis occurs
or not. Day digs to that you need NADH +, so you can envision a scenario where lots
of ATP is made and suddenly your ADP + pool is reduced to NADH +, and then what? Glycolysis
will not continue anymore and you will not be able to produce more ATP. So the cell needs
to come up with a way of getting rid of these electrons that are captured by NADH and H
+ and there are various ways of how the cell can do this and the first one, how are the
electron captures so you recycle NAD + so you can start and continue with glycolysis
so you make ADP and the first resection the fermentation reaction to alcohol.
So in the alcohol fermentation reaction is what happens is pyruvate, the product of glycolysis
is decarboxylated. So what you release is two carbon dioxides so in this field you should
have two carbon dioxides. And you make out of these C 3 compounds, pyruvate,
you make a C 2 compound, acetaldehyde and then the acetaldehyde is the molecule that
accepts the electrons again to accept ethanol so, they're moved to the carbonyl group and
then you make ethanol so grow think of an organism making ethanol it's a recycle NAD
so it can perform much more glycolysis and to the ethanol is a waste product. And of
course we have been able to use utilize this reaction in our lives, with yeast, yeast is
a very famous fermenter that produces alcohol in the beer brewing industry and in the wine
brewing industry. So one of the ethanol production has been
increased in the last few decades, as a way not only to make Tequila and whiskey but also
to make bioethanol to fuel your cars so you can see here there's a substantial amount
of bioethanol produced about 59 billion liters so they're all based on that fermentation
reaction, sugar is fed is then decarboxylated and then turns into ethanol. And to get this,
this is used from crop, plants about 2.2% of the land on this planet is used to make
this much bioethanol. In the U.S., bioethanol production is [Indiscernible]
and. But as you can see the consumption of gasoline is quite high so this is only 4%
so there's still a long way to go through this.
So here you see the distribution of bioethanol formation in the U.S., that's No.1 in the
world, they use corn kernels, starch from the corn kernels, actually the first country
to commercialize this process was Brazil in the '70s as a result of the oil crisis so
they use sugar cane, they use the sucrose in the sugar cane to make bioethanol and so
here you see the process of awe thou Brazilians do this you have the sugar cane, you burn
off the leaves this is going to be banned by 2014, and you need the feed the sugar juice
to yeast, and then fermentation happen. Yeast is going to do gibing sis and then fermentation
to ethanol. So you have to distill it, one of the problems
is it's a waste product, and eventually the yeast will die if you produce too much ethanol
in your broth. Does anybody know what the percentage is and then the yeast will die?
12, 13, they go up to 15% so you can only do 15%, very good of ethanol production and
then the yeast will die. So you need to distill off the ethanol to make it sure ethanol, get
rid of the water basically and that's what you can put in your car.
Now, the Brazilians were smart they take the left over stalk where is the sugar has been
removed, that's the over left stock and they burn it and when they burn it they can use
the heat to run the distillation column and have a turbine next to it to form electricity
so this factory is selfsufficient it doesn't need energy in. So in the U.S. we use the
corn kernels before you can do fermentation you need to convert the corn starch into monosaccharides
so into sucrose so, the corn kernels are cooked the [Indiscernible] and you have enzymes to
degrade the sugar and the sugars are given to the yeast it's the same process, yeast
is going to go through glycolysis, fermentation to make ethanol and you can put the ethanol
in your car. So here, what is left, after the starch is gone so the remains of the kernels
is called: Distillers grain and that's a good source of energy and protein for feed for
cattle feed. And so also here, some of the remaining residual
is used, actually most of it is utilized for cattle feed. And so this is a way to make
sure that you can make bioethanol which has the advantage that we don't use so much fossil
fuels and therefore we don't release so much CO2 in the atmosphere. Another advantage of
this U.S. is in last 5 years, since this process has really ramped up in the U.S., the oil
imports in the U.S. have been reduced to 65% to 40% because it's domestic source of fuel
basically, however there's one major problem, and that problem is that as you saw, the material
that is used this starch and sugar and both of those are food for us, as a direct competition
with food even though I would like to mention that we don't actually eat a lot of corn it's
cattle that eat a lot of corn so if you are if you like meat that's where the corn goes
into the cattle if you're a vegetarian you will get very little in touch with corn, most
of the corn is just fed to cattle. And so that's one of the research goes that we have
to is replace the food stuff, the starch and the glucose with plant cell walls so we don'thave
a direct competition with food and then go through the same process basically.
Okay. This is side track. Back to fermentation.
So again, same problem, glucose, glycolysis occurring we made energy, but ATP is required.
A second way is the pyruvate accepts the electrons directly.
And when it does so, you make the pyruvate is reduced the electrons are transferred and
then pyruvate is reduced it forms lactic acid. And so here you see the structure of lactic
acid, so again, lactic acid is a waste product, just to ensure that NAD + is recycled so you
can continue with glycolysis and make more ATP.
And this is utilized by bacteria that we use in yogurt formation or cheese making but this
reaction is also performed in our muscles, so in our muscles, this is performed when
you have no oxygen. Or very low oxygen so if you go for a run
and your oxygen diffusion through your blood doesn't work properly that you have low oxygen
diffusing into the muscles the muscles still requires energy so it's still requires ATP
so it will try to push through glycolysis as much as possible, but it doesn't know what
to do with the NADH and as a side product, it's going to make lactic acid which leads
to acidification of your muscle. So as you know in our muscle we go this reaction,
we do not do alcohol fermentation and there's a reason for this, the reason why we don't
do alcohol fermentation because we do not have that first enzyme, this pyruvate decarboxylase,
which decarboxylase pyruvate, can you imagine if we have to enzyme in our muscles? You would
run and would produce ethanol and you would get drunk.
[Laughter]. Would be very cheap way to get drunk.
It would also be a very good excuse when you're stopped by the police, oh I just went for
a jog. But we do not have that enzyme, we do not have the pyruvate decarboxylase and
we do not produce ethanol we produce lactic acid instead and that can be metabolized by
the liver for example. Okay.
So that was glycolysis. Glycolysis to pyruvate, around if oxygen is
not available, the pyruvate goes off that's not shone on this picture to fermentation.
So that NAD + is recycled so is there any question to fermentation?
All right. Everything's clear?
>>STUDENT: [Indiscernible] >>INSTRUCTOR: If there's no oxygen fermentation
will happen. If there is oxygen, what we're going to discuss
now will happen. And we get back to that question when we're
done with the remaining pathways of cellular respiration.
So, we've got pyruvate. Now, the pyruvate will translocate into the
mitochondria. So this now the first reaction where the mitochondria is involved.
The pyruvate will go into the mitochondria, it will become converted to [Indiscernible]
and I will show you the structure in a minute and it will go through the citric acid cycle
where it gets completely [Indiscernible] to carbon dioxide. So we have glucose taken with
the reaction and the carbon is gone and the carbon dioxide you breathe out, basically.
And again you will see in the TCA cycle we have ATP production, that's again by substrate
phosphorylation and electrons aretured by NAD, and there's another one FADH and we're
going go through that. So the first part of this is the import of
pyruvate into the mitochondria and it is ported into the matrix of the mitochondria. So it
has to go through two membranes, remember the mitochondria is two membranes one outer
and one inner the pyruvate has to go through both membranes.
And so this is shown here. This enzyme does this reaction and this enzyme
is called pyruvate dehydrogenase complex. So it's the pyruvate dehydrogenase complex.
So what do you find out from that name already? Well it was found with pyruvate, there is
a dehydrogenase reaction that means electrons are shifted and it's complex so there's not
just one proteins, one polypeptide that's responsible for this reaction there are multiple
ones that play a role in this. There are 5 electrons in this complex that are not shown
here. Here we go. So, what happens in this reaction is No. 1,
1 carbon is released as carbon dioxide. So up in this green field, you should write
carbon dioxide. So, this carbon dioxide is released out of
a C 3 compound we make a C 2 compound. Right?
The C 2 compound is an acetate, methyl group and carboxyl group. And so doing this reaction
you will also capture electrons so NAD + gets reduced to NADL + H + the dehydrogenase so
we have another NAD H + H + that's reduced and we have another molecule that's called
coenzyme A and this molecule coenzyme A will covalently to form an acetyl coA and this
is o molecule is not a protein it's organic molecule and the only interesting thing you
might or might not want to remember is that this coenzyme A has a thio group so it has
an SH group and this SH group is oxidized to form this [Indiscernible] ester, that's
the sulfur it forms an ester with this carboxyl group so as a thioester. And this gets the
reduction of the NADH and one of the reasons why they're form this coA complex is because
the thioester bond is energy rich and they can utilize that later on.
Yeah. So, again summary in this first step, pyruvate
is important, one carbon is released as carbon dioxide, and NAD + is reduced to NADH and
you can get out acetyl coA, which is a C 2 compound in this.
And then this acetyl coA moves to the TCA cycle and we're going to discuss that on Friday.
So have a nice day and apologize for the quality of the presentation.