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Course: Biology library > Unit 12
Lesson 1: Introduction to cellular respiration- ATP: Adenosine triphosphate
- ATP hydrolysis mechanism
- Cellular respiration introduction
- Oxidation and reduction review from biological point-of-view
- Oxidation and reduction in cellular respiration
- Introduction to cellular respiration and redox
- Introduction to cellular respiration
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Oxidation and reduction in cellular respiration
Oxidation and reduction in cellular respiration. Reconciling the biology and chemistry definitions of oxidation and reduction. Created by Sal Khan.
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- What does Sal mean by "hogging electrons" ??(9 votes)
- When atoms are covalently bonded, they share electrons. Some atoms don't share as evenly as others, and when there's a big enough electronegativity difference they draw the shared electrons toward them more.
There's a few ways of describing this, and hogging the electrons is a nice, simple way to do it. You can also say the bond has some ionic character, as one atom is partially stealing the electron. You can also say the more electronegative atom is withdrawing the charge from the less electronegative atom, exposing its nucleus and making it effectively a little bit positive.(12 votes)
- Is respiration kinda like breathing like we do or is it oxygen consumed?(7 votes)
- Respiration, as in respiratory system, means inhaling and exhaling. Respiration, as in cellular respiration, means different thing. Cellular respiration is using glucose (or other food material) to produce ATP.(8 votes)
- I have a question about where the 38 ATPs come from in energy. I thought the general numbers of ATPs produced by cellular respiration was 30-32: a net gain of 2 in glycolysis, 2 from the citric cycle (Kreb's cycle), and 28 from oxidative phosphorylation.(5 votes)
- It is 36-38.
The net gain from glycolysis is two molecules of ATP and two molecules of NADH. The conversion of pyruvate to acetyl CoA and its metabolism via the citric acid cycle yields two additional molecules of ATP, eight of NADH, and two of FADH2. Assuming that three molecules of ATP are derived from the oxidation of each NADH and two from each FADH2, the total yield is 38 molecules of ATP per molecule of glucose.
Depending on the system used, this transfer may result in these electrons entering the electron transport chain at the level of FADH2. In such cases, the two molecules of NADH derived from glycolysis give rise to two rather than three molecules of ATP, reducing the total yield to 36 rather than 38 ATPs per molecule of glucose.
https://www.ncbi.nlm.nih.gov/books/NBK9903/(2 votes)
- So around10:00, hydrogen loses 12 electrons like carbon loses 24 electrons to oxygen?(4 votes)
- Hydrogen remains with same amount of electrons, essentially. It is more like the hydrogen atom itself is shifted as a result of cellular respiration. That is why the biological definition of oxidation and reduction hold up in this example.(2 votes)
- What type of bonding occurs in glucose?(2 votes)
- In the molecule itself it is Polar-Covalent bonding.
Mostly single bonds but one double bond if it is in the open-chain form.
Ring:
http://upload.wikimedia.org/wikipedia/commons/5/5d/Beta-D-glucose-3D-balls.png
Chain:
http://upload.wikimedia.org/wikipedia/commons/5/5a/D-glucose-chain-3D-balls.png
Between molecules of glucose you have a special kind of bond called a Glycosidic bond, which forms the disaccharide Maltose(4 votes)
- Sal says that when carbon gains hydrogen, it is reduced, because carbon is much more electronegative than hydrogen, so it hogs the electrons. But I thought that hydrogen and carbon have quite similar electronegativities, which is why hydrocarbons are nonpolar and hydrophobic. Aren't these two facts contradictory?(3 votes)
- If you put it that way it sounds contradictive but it is not.
In the context of redox reactions, Carbon is more electronegative than hydrogen, therefore, it hogs electrons.
While C-H the covalent bond is non-polar and hydrophobic. The difference is electronegativity is not enough to make a polar bond.(2 votes)
- At7:19, it says the carbon has a "+1" oxidation state. I thought it is always written as "1+". Which way is correct?(3 votes)
- 1+ is normally the standard convention, but it can be written as +1 occasionally.(2 votes)
- Um... I thought that Glucose has covalent bonds .
I think non of covalent bonded substances can have some - or + charge for each atom ( It supposes to be ionic bond then)(3 votes)- Glucose is neutral, i.e. without any charge (+ or -), it has covalent bonds.
A molecule with covalent bonds still can have a local charge, for example deprotonated acids can be negative (CH3COO⁻). This is not an ionic bond, it is more a polarized covalent bond.
You are right, there is no molecule with a charge on every atom.(2 votes)
- I'm in 9th grade and havn't taken Chemistry yet so I'm not sure if I should be worrying, but How do you know how much electrons are gaining or loosing in Oxidation? You say like 1+ and 2- or 24+ etc. How do you figure out how many you add or loose?(2 votes)
- good question. check out REDOX reactions in the Chemistry section:
http://www.khanacademy.org/science/chemistry/oxidation-reduction
I'm sure your doubts will be clarrified.
Hope that helps! :)(4 votes)
- When writing the oxidation states, isn't is incorrect to write out the molecular forms of an atom along with the oxidation state, such as O₆ ⁻² instead of writing it out as 6 O⁻² ? Wouldn't O₆ ⁻² indicate that a molecule of 6 Oxygens has an overall oxidation state of -2 instead of indicating that each Oxygen atom has an oxidation state of -2? To me, writing O₆ ⁻² indicated that each oxygen would have an oxidation state of -1/3 and not that each oxygen has an oxidation state of -2.(3 votes)
- Sal sometimes takes an idiosyncratic approach to explaining things that isn't formally correct — I try to accept his quirks and let go of details like this, but if it really bothers you I suggest posting a "Clarification" under "Tips and Thanks".(2 votes)
Video transcript
Now that we have a little bit
of a review of oxidation and reduction under our belts, let's
see if we can apply what we now, maybe, re-understand
to cellular respiration. So cellular respiration, for
every mole of a glucose, C6H12O6, we combine that--
and maybe that's in an aqueous state. It's dissolved in water. We combine that with six moles
of molecular oxygen. And then our cells perform
cellular respiration in a whole series of steps. And I'll do more
videos on that. I'll just abbreviate it. And then we end up with six
moles of carbon dioxide. We have to breathe this oxygen
in order to perform cellular respiration and we have to
breathe this carbon dioxide out because it's just a
byproduct of cellular respiration. Six moles of carbon dioxide,
six moles of water. And the whole point of cellular
respiration is, plus some energy is generated
by this reaction. And our bodies store
this energy. Well, some of it is just
turned into heat. But the whole point of cellular
respiration is to store it as 38 ATPs, which we've
learned already is the energy currency of biological
systems. And then our bodies, or
biological systems in general, can use these ATPs to contract
muscles or generate nerve impulses or grow cells or divide
cells or whatever else that a biological system
has to do. In the last video we learned a
little bit about oxidation and reduction, so let's apply
those ideas here. Now we saw in the last video
that a chemist would say-- let me write it this way-- a
chemist would say that oxidation means losing
electrons, or not being able to hog them. While a chemist will
tell you that reduction is gaining electrons. And if you have trouble
remembering, oxidation is losing, that's kind of OIL,
that's the mnemonic. Oxygen is losing electrons. Reduction is gaining. Or, RIG. So OIL RIG. This is what you learned
in chemistry class. Now biologists or biochemists
will say, oh, well, you know I like to define it a little
bit differently. A biologist will say that
oxidation is losing hydrogen atoms. And they'll say reduction
is gaining hydrogen atoms. And we saw in the last
video that this definition is actually hard when you're
applying it to hydrogen because it's not like a hydrogen
atom can lose itself or gain itself. And the reason why we said
that these two ideas are consistent is because if I'm
talking about a carbon and a carbon is losing a hydrogen. So let's say I have some
compound that looks like this. Maybe it's connected to a
bunch of other things someplace else. And then later on the carbon--
let's say I have a carbon that looks like that and I have an
oxygen that's maybe bound to another oxygen. I'm doing a very kind of
hand-wavy explanation here. And maybe that oxygen is bonded
to something else. This is what I start off with. And on the other side of this
equation I end up with something that looks
like this. Where a carbon is bonded
to an oxygen. And maybe that other oxygen is
bonded to this hydrogen. The biologist will say, oh, this
carbon has been oxidized because it lost its hydrogen. The hydrogen went from here--
I'll do it in a different color-- went from this carbon
to this oxygen. And the biologist would
also say that this oxygen has been reduced. It's been reduced because
it gained hydrogens. But the reality, or maybe the
chemists' definition, which I like a little bit more is, over
here because carbon is more electronegative, we
see carbon is much more electronegative than hydrogen. And oxygen is even more
electronegative than carbon. When any of these guys bond with
hydrogen they're going to hog the electron. So here, carbon got to
hog the electron. So here, carbon hogs
electrons. While here, carbon gets its
electrons hogged by oxygen. So here, oxygen hogs. So by losing the hydrogen, the
carbon actually lost its opportunity to hog electrons. And since it ended up bonding
with an oxygen, it not only can't hog hydrogen's electrons
but then it gets its electrons hogged by an even more
electronegative atom. So that's why these two
definitions are consistent. Same thing with the oxygen. Here it's bonding with another
oxygen, not hogging anything. But when it gains the hydrogen
it's able to hog hydrogen's electrons. Because it's so much more
electronegative. Or you could say that it's
gaining electrons. So that's why these two
definitions are somewhat consistent. Although sometimes they fall
apart if we're not dealing with hydrogen. The chemistry definition applies
more consistently to everything. But sometimes the biologists'
definition is easier to kind of glance at. Or you'll actually see it
written in textbooks. So let's go back to cellular
respiration and try to figure out what's being oxidized
and reduced. So if we look over here. Over here we have our glucose. And actually I copied and
pasted from Wikipedia a glucose molecule. And actually there's
one error here. And maybe I should edit
it on Wikipedia. There should be another hydrogen
bonded to that carbon right there. But as you see, all of the
hydrogens, they're either bonded to an oxygen or
a carbon over here. On the left-hand side, they're
either bonded to an oxygen or a carbon. If we were to write its
oxidation state, in every case it's bonded to something that's
more electronegative. So it's going to be giving
up its electrons. So it will have a plus
one oxidation state. And oxygen, in every case,
is either bonded to a carbon or a hydrogen. And so, oxygen, if it's bonded
to a carbon or a hydrogen, is going to hog an electron from
either one of those guys. So in every situation in
glucose, oxygen has a two minus or a minus two
oxidation state. And carbon, since this whole
thing is neutral, one would think that carbon would have
a neutral oxidation state. And if you go through this, you
actually find that most of these carbons do have neutral
oxidation states. Let me circle a few. So for example, this carbon
right here, it's hogging an electron from this hydrogen. But then it gets an electron
hogged by this oxygen. And then of course it does
nothing with the carbon. So that's neutral. This is neutral for
the same reason. This is neutral for
the same reason. This one is also neutral
for the same reason. It's bonded with two carbons. It has an electron
hogged by oxygen. But then it hogs an electron
from hydrogen. So it's neutral. So four of these carbons
are neutral. This carbon right here has two
electrons hogged by oxygens. And then it gets to take one
back from the hydrogen. So it has a plus one
oxidation state. This one is the opposite. It has two hydrogens
that it hogs from. Then it has to give one
away to the oxygen. So this has a minus one. So these two cancel out. On average, you can say that the
carbons in glucose have a neutral oxidation state. And I'm dealing with the
chemist definition. And I'm going to show you
that they're essentially equivalent. Here all of the oxygens have
no oxidation state. Because they're just bonded. Let me do it in a
better color. No oxidation state or neutral
oxidation state because they're double bonded
with oxygen. No one's hogging from anyone. They're obviously equally
electronegative. If we look at the
products, carbon dioxide looks like this. So, in either of these cases,
oxygen is hogging two electrons from this carbon. So it has a minus two
oxygen state. This oxygen is hogging two
electrons from carbon. So it has a minus two
oxidation state. And this carbon is getting all
of its valence electrons, all four, hogged by the oxygen. So it has a plus four
oxidation state. It's lost four electrons,
you can imagine. Because it's getting hogged. So that's carbon. So we could write this as four
plus for the carbon. And then each oxygen
has a two minus. And we can do the math later
on to figure out what the total is. And then, if we look at the
water-- we've looked at this before-- the oxygen is hogging
two electrons, one from each hydrogen. So two minus. And then each of the hydrogens
have a plus one oxidation state. So if you want to do a half
reaction for cellular respiration, and in the
chemists' sense of things, just dealing with electrons,
you can immediately say, I start with 12 hydrogens
on this side. Let me just write it this way. So H12 on this side. They all have a plus one
oxidation state. And then cellular respiration
occurs. And now I have 12 hydrogens. I could write the 12 a little
bit differently here. But they still have
a plus one. Each of them still has a plus
one oxidation state. So nothing from an oxidation
reduction point of view happens to the hydrogen. Now if we do the carbon. On the left-hand side of the
equation, we have six carbons. They have a neutral
oxidation state. But then on the right-hand
side of the equation, what happens? I now have six carbons. Written a little bit
differently. But I have six carbons. And they each have a plus
four oxidation state. Which means that they have
lost four electrons. Or their hypothetical charge,
by losing those four electrons, has gone
up by four. Because they're losing
negatively charged electrons. So the six carbons, after
cellular respiration, end up with six oxidized carbons,
with plus four oxidation states. Plus-- so each of these
lost four electrons. We have six of them. 4 times 6 is 24 electrons. These are the electrons that the
carbon lost. So we see in cellular respiration that
the carbon is oxidized. Oxidation is losing electrons. We see in cellular respiration,
we draw the half reaction, carbon is losing, the
six carbons are losing a collective 24 electrons. And then finally, if I were to
do the oxygen on this side. I've lost my equation up here. So over here I have
two oxygens. And I'm going to draw them
a little bit separate. So I have these six oxygens
here that have a minus two oxidation state. On the left-hand side. So I'll draw it like this. They have a minus two
oxidation state. And then I have these
12 oxygens that are completely neutral. So I won't even write an
oxidation state or oxidation number there. And then after we
perform cellular respiration, what happens? Well now I have, in the carbon
dioxide, I have 12 carbons that have a minus two
oxidation state. Six times O2. So let me write that down
from the carbon dioxide. So I have six O2s that
all have a to minus oxidation state. And then I have another six
oxygens that have a minus two oxidation state. So plus six oxygens that have
a minus two oxidation state. So if you think about it, over
here I had a collective oxidation state on all
of the oxygens. These were neutral. I have 6 times minus 2,
that's a minus 12. You can kind of view it
as collective charge of all six of them. 6 times minus 2. Here I have 6 times minus
2, which is minus 12. And then I have 6 times 2
oxygens per molecule. So that's 12 times minus 2. That's minus 24. So to go from a minus 12 to a
total oxidation or kind of hypothetical charge of
minus 36, I must have gained 24 electrons. And those 24 electrons that
I gained, that the oxygens gained, are the same 24
electrons that the carbons lost. So from the chemistry
point of view, it's very clear. Carbon was oxidized. And oxygen, which gained
electrons-- RIG. Reduction is gaining. Oxygen is reduced. And this is all a
bit of review. But it's nice to see it in
the context of cellular respiration. And this actually kind of
answers one of the questions of where does this
energy come from? In any of these chemical
reactions, when you see energy being produced, it's because
electrons are going from a higher energy state to
a lower energy state. If I have an electron that's up
here in a high energy state and it is able to go to a more
comfortable state, lower orbital or lower
energy orbital. So low energy or more
stable energy state. It'll generate energy in the
form of heat, or maybe this can do some work in some way,
help make ATP molecules. And so when you see these half
reactions, you see these 24 electrons, that are being
lost by carbon, carbon is being oxidized. And they're going to oxygen. They're going in a whole
series of steps. It's not just happening
in one huge explosion. It's happening over a huge
series of steps. And as it does that, it's
entering lower and lower energy states. And as these electrons enter
the lower energy states, essentially by going from the
carbons and being pushed to the oxygens, that's where the
energy is coming from. That's where the energy
to make the 38 ATPs is coming from. So, so far we talked a little
bit about how a chemist views oxidation. I touched at the beginning of
the video of how a biologist views oxidation. And then we saw that cellular
respiration from a chemist's point of view is clearly showing
that the carbon is being oxidized. It's losing electrons. And that the oxygen
is being reduced. It's gaining electrons. It's being reduced. That electrons are going from
this carbon and they're going, essentially, to these
oxygens right here. Now how does the biology
definition of our position hold up? Well here it holds
up pretty well. Because you can imagine, over
here, all of the hydrogens in the equation are associated
with glucose. And so they're either bonded,
if you look at the structure of glucose. The hydrogens are either bonded
to carbons or oxygens. So these are bonded to
carbons and oxygens. And when you go on the
right-hand side of the equation, all of the hydrogens
are only bonding with oxygen. So net-net, carbon definitely
lost hydrogens. And hydrogens and oxygen
definitely gained hydrogens. Let me write that down. We see in respiration, carbon
lost hydrogens. And oxygen gained hydrogens. And that's consistent. Because we see that by losing
hydrogens we are being oxidized from a biologist
point of view. And by gaining hydrogens,
oxygen is being reduced. And just so you can kind of
makes sense of this when you see this-- and when I start
drawing out the mechanisms, which I will hopefully not make
too hairy-- this process of transferring these hydrogens
is facilitated by molecules like NAD
plus and FAD. And we'll see that. But really, if we just want to
reconcile the two notions, as the hydrogens are being
transferred from one electronegative atom to another
electronegative atom, what's really being transferred
is the opportunity to hog electrons. If carbon has the hydrogen, it
gets to hog the electrons. But if that hydrogen goes from
the carbon-- and the whole atom; not just the nucleus, but
the whole atom goes to the oxygen-- now the oxygen
has gained that electron that it can hog. And carbon has lost
the electron. So carbon has oxidized and
oxygen has been reduced. And I mentioned this
in previous videos. But probably the most confusing
thing about oxidation is that you always
want to say, all right, that must have something
to do with oxygen. And it does. The word really comes
from, what would oxygen do to something? So oxygen, when it bonds with
things, it loses, it takes away their electrons. Or, in a reaction, it'll often
take away the hydrogens. It took away the hydrogens
from the carbon in this situation. So that's where the term
oxidation comes from. But you don't have to have
oxygen anywhere in your reaction for oxidation or
reduction to occur. Anyway, hopefully you found
that reasonably useful. This was actually a huge pain
point for me when I learned, I got comfortable with the
chemistry definition of oxidation reduction. And then all of a sudden you
open up your biology book and they start talking about losing
and gaining hydrogens, as opposed to electrons, and it
took me a while to really reconcile these two notions.