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Course: Biology library > Unit 12
Lesson 2: Steps of cellular respirationOverview of cellular respiration
Cellular respiration is how cells get energy from glucose. The process has three main parts: Glycolysis happens in the cytosol and breaks glucose into two pyruvate, producing 2 ATPs and 2 NADHs. The Krebs cycle occurs in the mitochondrial matrix, where pyruvate is turned into acetyl-CoA, which then goes through a series of reactions, producing ATP, NADH, and FADH2. Finally, oxidative phosphorylation uses NADH and FADH2 to create a proton gradient, which helps make more ATP.
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- What is the QH2 that Sal was talking about? In my classes, we only learned about FADH2, not about QH2. Is it important that we know this?(32 votes)
- Q is the coenzyme Q, an antioxidant called ubiquinone with vitamin-like properties, and QH2 is the reduced form of the coenzyme. I have to acknowledge that Sal is right in using QH2 as the electron carrier. Although textbooks teach that FADH2 does this, it is not correct. FADH2 remains bound to the Succinate dehydrogenase (complex II) and as such it does not carry the electrons to the next complex III (cytochrome bc1); QH2 does. Therefore, QH2 is the "true" electron carrier. Regarding what you have to learn? Keep in mind that you need to pass the exam, thus, it would be better to use FADH2 as your answer, but remember that it is wrong until the AP college board changes its thinking. There is an excellent discussion on using QH2 instead of FADH2: http://sandwalk.blogspot.com/2007/06/cellular-respiration-ninja-enzymes.html(59 votes)
- So a muscle cell in an environment with a large amount of oxygen available would produce a number of ATP closer to the maximum 38 ATP versus a skin cell in an environment with less oxygen available?(9 votes)
- Yes. More oxygen (and the mere presence of it) defiantly increase ATP production, although there is a threshold above which more oxygen will not increase ATP production. :)(7 votes)
- If cellular respiration produces 10 x NADH molecules then where do the NAD+ come from to reduce into NADH?(6 votes)
- Every time NADH drops off the electrons in the ETC, it becomes NAD+ again and can be recycled.(11 votes)
- how can you have 1.5 atp's?(9 votes)
- maybe because the number of H ions that specific electron carrier(QH2) can pump via the electron transport chain into the reserve are more than required to make one complete ATP and less than the number of protons required to make 2 ATP.(5 votes)
- At, NAD+ --> NADH. Where does the proton come from? Is it just floating around in the cytosol? 2:09(6 votes)
- The hydrogen comes from the glucose that is broken down to produce ATP. This is the source of the hydrogen ions that are floating around. Energy is released when bonds are broken, so when hydrogen is removed from the glucose, this releases energy (in the form of electrons) that is incorporated into the reduced NADH molecule and subsequently ATP.
If you add only a proton to NAD+ you won't get NADH. A proton is positive charge. NADH is neutral (no charge). You cannot add a hydrogen ion to NAD+ to get NADH.
The actual reaction is NAD+ and 2H ----> NADH and H+
So a hydrogen molecule (proton + electron) and an additional electron are added to NAD+ to form NADH. The leftover hydrogen ion (H+) is now free to float around and participate in the electron transport chain.(3 votes)
- I know sal already taught about NAD+ and NADH but what is the difference? could someone help me understand, please(4 votes)
- Great Question! I was wondering the same thing. This is what I found:
The molecule NAD (nicotinamide adenine dinucleotide) can exist in two forms: NAD+ and NADH. One form is the reduced version of the molecule, and the other is the oxidized version (reduced is the gaining of electrons, oxidized is the loss of electrons). This makes a redox couple. (red- from reduced, -ox from oxidized) NAD+ turns into NADH through a redox reaction. NAD+ gains a hydrogen ion (H+) and 2 electrons, reducing it to NADH. The 'H' at the end of NADH is because of the hydrogen ion it gained.
I really hope this helps! Here are some links for further explanation:
https://www.elysiumhealth.com/en-us/science-101/whats-the-difference-between-nad-and-nadh
phschool.com/science/biology_place/biocoach/cellresp/closer2.html
https://biology.stackexchange.com/questions/81770/where-does-the-proton-come-in-the-reduction-of-nad(4 votes)
- What is the difference between NADH and NADPH?(3 votes)
- The difference between NADH and NADPH is that NADPH has an extra phosphate group. While NADH is used to power cellular reactions such as glycolysis, during which molecules are broken down, NADPH is used to power photosynthesis.(6 votes)
- Is there such a thing as a phosphorylated nucleotide with Thymine or Cytosine as a nitrogenous base? If only Adenine and Guanine can function as nitrogenous bases in energy baring molecules (i.e. ATP and GTP), then why is that the case?(4 votes)
- You appear to be asking about the energetic roles for ribonucleotides other than ATP and GTP — all four standard ribonucleotides function as energy bearing molecules (though ATP is by far the most commonly used), they are also used in the synthesis of ribonucleic acids (RNA molecules).
The standard ribonucleotides are:
• Adenosine triphosphate — adenine bonded to ribose bonded to 3 phosphoryl groups.
• Guanosine triphosphate — guanine bonded to ribose bonded to 3 phosphoryl groups.
• Cytidine triphosphate — cytosine bonded to ribose bonded to 3 phosphoryl groups.
Finally, there is a fourth ribonucleotide, but it isn't based on thymine — remember these are ribonucleotides not deoxyribonucleotides!
• Uridine triphosphate — uracil bonded to ribose bonded to 3 phosphoryl groups.
The four corresponding deoxyribonucleotides are almost exclusively used for the synthesis of DNA.
They are:
deoxyadenosine triphosphate,
deoxyguanosine triphosphate,
deoxycytidine triphosphate, and
deoxythymidine triphosphate — confusingly this is often written as thymidine triphosphate.
There are reasonable wikipedia articles on each of these compounds if you wish to learn more.(3 votes)
- what exactly does net mean?(3 votes)
- Net is the resulting amount of something including the deductions. For example, if I spend $10 on supplies to start a lemonade stand, and my stand makes $30 dollars, my net gain is $20 (30-10). In the case of glycolysis, we need 2 ATP to get the reaction started, and the process makes 4 ATP, making a net of 2 ATP (4-2).
Hope this helps!(4 votes)
- Do every cell in our body produce the exact same amount of ATP by completely oxidizing glucose?(5 votes)
- No, as is said in the video the amount of ATP made varies from cell to cell and with conditions.
Some extreme examples:
• Mammalian red blood cells do not have mitochondria and so only perform glycolysis!
• Similarly, when exercising very intensely muscles will switch to anaerobic respiration (lactic acid fermentation to be exact) because they cannot get oxygen fast enough from the circulatory system — this also bypasses oxidative respiration.(2 votes)
Video transcript
- [Voiceover] So what I
wanna do in this video is give ourselves an overview
of cellular respiration. It can be a pretty involved
process, and even the way I'm gonna do it, as messy as it looks, is going to be cleaner
than actually what goes on inside of your cells,
and other organs themselves, because I'm going to show clearly from going from glucose,
and then see how we can produce ATP through glycolysis,
and the Krebs cycle, and oxidative phosphorylation, but in reality, all sorts
of molecules can jump in at different parts of the
chain, and then jump out at different parts of the chain,
to go along other pathways. But I'll show, kind of
the traditional narrative. So we're gonna start
off, for this narrative, we're gonna start off with glucose. We have a six-carbon-chain
right over here. And we have the process of
glycosis, which is occurring in the cytosol, the cytosol of our cells. So if this is a cell right over here, you can imagine, well the glycolysis, the glycolysis could be
occurring right over there. And that process of glycolysis
is essentially splitting up this six-carbon glucose molecule into two three-carbon molecules, and these three-carbon molecules, we go into detail in another
video, we call these pyruvate. Pyruvate. And in the process of
doing so, and this is, I guess you could say,
the point of glycolysis, we're able to, on a net
basis, produce two ATP's. We actually produce four,
but we have to use two, so on a net basis, we produce two ATP's. I'm gonna keep a little
table here, to keep track. So we produce two ATP's, and we are also, we're also, in the process of that, we reduce two NAD molecules to NADH. Remember, reduction is
gaining of electrons. And you see over here,
this is positively charged, this is neutrally charged, it
essentially gains a hydride. So this is reduction. Reduction. And if we go all the
way through the pathway, all the way to oxidative phosphorylation, the electronic transport chain,
these NADH's, the reduced form of NAD, they can be, then, oxidized, and in doing so, more energy
is provided to produce even more ATP's, but we'll get to that. So you're also gonna get two NADH's. Two NADH's get produced. Now at that point, you
could kind of think of it as a little bit of a decision point. If there's no oxygen around, or if you're the type of organism that doesn't want to continue, for some reason, with cellular respiration,
or doesn't know how, this pyruvate can be
used for fermentation. We have videos on fermentation, lactic acid fermentation,
alcohol fermentation, and fermentation is all
about using the pyruvates to oxidize your NADH back into NAD, so it could be re-used
again, for glycolysis. So even though the NADH has energy that could eventually
be converted into ATP, and even though pyruvates have energy that could eventually
be converted into ATP, when you do fermentation,
you kinda give up on that, and you just view them as waste products, and you use the pyruvate to
convert the NADH back into NAD, And then, glycolysis can occur again. But let's assume we're not gonna go down the fermentation pathway, and we're gonna continue with traditional aerobic cellular
respiration, using oxygen. Well, the next thing
that's going to happen, is that the carboxyl group, and and everything I'm going to show now, it's going to happen for
each of these pyruvates. So, you can imagine these
things all happening twice. So I'm gonna multiply a
bunch of things, times two. But what happens in the next step, is this carboxyl group, this
carboxyl group is stripped off of the pyruvate, and it, essentially, is going to be released as carbon dioxide. So this is our carbon
dioxide being released here, and then the rest of our
pyruvate, which is, essentially, an acetyl group, that
latches onto coenzyme A. And you'll hear a lot about coenzyme A. Sometimes I'll write just CoA, like this. Sometimes I'll do CoA, and then the sulfur, bonded to the hyrdrogen. And the reason why they'll
draw the sulfur part, is because the sulfur is what bonds with the acetyl group, right over here. So, you have the carbon
dioxide being released, and then the acetyl group,
bonding with that sulfur, and by doing that, you form acetyl-CoA. And acetyl-CoA, just so
you know, you only see three letters here, but this is actually a fairly involved molecule. This is actually a picture of acetyl-CoA, I know it's really small, but hopefully you'll appreciate that it's
a more involved molecule. That, the acetyl group
that we're talking about is just this part, right over
here, and it's a coenzyme. It's really acting to
transfer that acetyl group, and we'll see that in a second. But it's also fun to
look at these molecules, because once again, we see these patterns over and over again in
biology or biochemistry. Acetyl-CoA, you have an
adenine right over here. It's hard to see, but you have a ribose, and you also have two phosphate groups. So this end of the
acetyl-CoA is essentially, is essentially an ADP. But it's used as a coenzyme. Everything that I'm talking about, this is all going to be facilitated by enzymes, and the enzymes will have cofactors, coenzymes, if
we're talking about organic cofactors, that are gonna
help facilitate things along. And as we see, the acetyl group joins on to the coenzyme A, forming acetyl-CoA, but that's just a temporary attachment. The acetyl-CoA is,
essentially, gonna transfer the acetyl group over to,
and now we're going to enter into the citric acid cycle. It's gonna transfer these two carbons over to oxaloacetic acid,
to form citric acid. So it's gonna transfer these two carbons to this one, two, three,
four carbon molecule, to form a one, two, three,
four, five, six carbon molecule. But before we go into the depths of the citric acid cycle,
I wanna make sure that I don't lose track of my accounting, because, even that step right over here, where we decarboxylated the pyruvate, we went from pyruvate to acetyl-CoA, that also reduced some NAD to NADH. Now, this is gonna happen
once for each pyruvate, but we're gonna- all the accounting we're gonna say, is for one glucose molecule. So for one glucose molecule, it's gonna happen for each of the pyruvates. So this is going to be times- This is going to be times two. So we're gonna produce two, two NADH's in this step, going from pyruvate to acetyl-CoA. Now, the bulk of, I guess you could say, the catabolism, of the
carbons, or the things that are eventually going to produce our ATP's, are going to happen in what we call the citric acid, or the Krebs cycle. It's called the citric
acid cycle because, when we transferred the acetyl
group from the coenzyme A to the oxaloacetic acid,
we formed citric acid. And citric acid, this is the thing that you have in lemons, or orange juice. It is this molecule right over here. And the citric acid cycle, it's
also called the Krebs cycle, when you first learn it,
seems very, very complex, and some could argue
that it is quite complex. But I'm just gonna give you an
overview of what's going on. The citric acid, once again, six-carboned, it keeps getting broken
down, through multiple steps, and I'm really not showing
all of the detail here, all the way back to oxaloacetic acid, where, then, it can accept
the two carbons again. And just to be clear, once the two carbons are released by the coenzyme A, then that coenzyme A can be used again, to decarboxylate some pyruvates. So there's a bunch of cycles going on. But the important take-away,
is as we go through the citric acid cycle, as we go from
one intermediary to the next, we keep reducing NAD to NADH, in fact, we do this three
times for each cycle of the citric acid cycle, but remember, we're gonna do this for each acetyl-CoA. For each pyruvate. So all of this stuff is
going to happen twice. So we're going to go through it twice for each original glucose molecule. So, here we have one, two,
three NADH's being produced, but since we're going
to go through it twice, and we're gonna be accounting for the original glucose
molecule, we could say that we have six six NADH's, or you could say, six
NAD's get reduced to NADH. Now, you also, in the process, as you're breaking down, going from the six-carbon molecule
to four-carbon molecule, you're releasing carbon,
as carbon dioxide, and you also have, traditionally
GDP being converted into GTP, or sometimes
ADP converted into ATP, but functionally, it's
equivalent to ATP, either way. So, we could also say
that we're gonna directly- Remember, we're gonna do
all of this stuff twice. So, we could say that two, I'll just say two ATP's,
to make it simple. We could say GTP, but I'll say two ATP's. Because once again, this
happens once in each cycle, but we're gonna do two
cycles, for each glucose. And then, we have this other
coenzyme right over here, FAD, that gets reduced to FADH2, but that stays covalently
attached to the enzymes that are facilitating it, so eventually, that's being used to reduce . coenzyme Q to QH2. So I'm just gonna write the QH2 here, but once again, you're gonna get two of these. So two QH2's. Now let's think about what the net product, over here, is going to be. And to think about it, we
should just, we'll just- I'll do a little bit of a shorthand. We'll go into more
detail in future videos. These coenzymes, the NADH, the QH2, these are going to be oxidized, during oxidative phosphorylation, and the electron transport chain, to create a proton gradient across the inner membrane of mitochondria. We're gonna go into much
more detail in the future, but that proton gradient is going to be used to produce more ATP. And one way to think about it, is each NADH is going to produce, and I've seen accounts, it
depends on the efficiency, and where the NADH is actually going to be produced, but it's going to produce anywhere between two and three ATP's. Each of the reduced coenzyme Q's, so QH2, that's going to each produce
about one and a half ATP's. And people are still getting a good handle on exactly how this is happening. It depends on the efficiency of the cell, and what the cell is
actually trying to do. So, using these ranges, actually I'll say one and a half to two ATP's. And these are approximate numbers. So let's think about what
our total accounting is. So if we just count up
the ATP or the GTP's, we're gonna get two there, two there. So we're gonna have four direct, or very close to direct,
ATP's net, being created. And then how many NADH's? We have two, four, and then we add six. We have ten NADH's. Ten NADH's. And then, we have two of the coenzyme Q's. Two QH2's. So that's gonna be four ATP's, this is going to be between- this is going to be
between 20 and 30 NADH's. Sorry, 20 and 30 ATP's. 20 to 30 ATP's. And then, this is going
to be three to four. Three to four ATP's. So if you add them all together, if you add the low ends
of the range, you get, let's see, 20 plus three, plus four. That's 27 ATP's. 27 ATP's. And the high end of the range, let's see. You have four plus 30 plus four. You have 38. 38 ATP's. And 38 ATP's is currently
considered to become the theoretical maximum,
but when we actually observe things in cells, it looks like it comes right at around 29 to 30 ATP's. And once again, it depends what the cell's trying to do, the type of cells,
and the type of efficiency. But all of this is happening
through cellular respiration. And just to get a better sense of where all of this is occurring. Where all of this is occuring, we said glycolosis is
occurring in the cytosol. The citric acid cycle,
this is occurring in the matrix of the mytochondria. So this space right over here, that is the citric acid cycle, in that little magenta
space that I've drawn. So that's the matrix. In the video on mitochondria, we go into much more detail on that. And then, the actual conversion of the coenzymes, of the
electron transport chain, that's occurring across
the membrane of the crista. And the crista are these
folds, these kind of, inner membrane folds, of our mitochondria. So, it's occurring across the membranes of those, actually the plural is cristae. Crista is the singular of the cristae. And we'll go into more detail
into that, in other videos.