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Calvin Cycle Review · Acedra
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Calvin Cycle Review
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0:00
Let's review the Calvin cycle a little bit. Maybe I'm going to go into a little bit more detail than the last video. And by doing that, we'll be able to appreciate an inefficiency, or what we think is an inefficiency that occurs in many photosynthetic plants. So just to review the Calvin cycle, you start off with some CO2. In the last video, I had six molecules of CO2 and six molecules of ribulose bisphosphate. But in this one, just to show you that I could have
0:28
literally just divided that last cycle by two, I'm going to start with three and three. Three molecules of carbon dioxide. Three molecules of ribulose bisphosphate. If you don't remember what ribulose bisphosphate was, it was just a 5-carbon molecule. It had two phosphates on it. I can draw the phosphates. So you have a phosphate there. And you have a phosphate there. And we saw on the last video-- and here I'm going to do a little bit more detail.
1:01
Of course carbon dioxide, there's only one carbon. I can just draw a carbon like that. I mean, I don't want to draw the oxygens right now. I just want to focus on just the carbons and maybe the phosphates. But in the last video we saw that these two merged. Or these two reacted. They reacted in the presence of ribulose bisphosphate carbocylase, which is this enzyme. It was that big protein that I showed you in the last video. And it's called RuBisCo for short.
1:29
So I'm going to write RuBisCo in the middle of this whole cycle. Because all of this is going on with the RuBisCo enzyme. These molecules are joining on to the RuBisCo enzyme and then the ATP and the NADHs are reacting on different parts. And that's essentially what's driving the whole reaction. So you have all of this is happening with that big RuBisCo protein, or enzyme, or whatever you want to call it. And this just stands for ribulose bisphosphate carboxylate.
1:58
So it essentially telling you that it merges a carbon onto ribulose bisphosphate. And the last video, I just did a very hand-wavey, I didn't show you all of the intermediates. But I showed you, look, if in this situation you end up using six ATPs and six NADHs, you'll end up with glyceraldehyde 3-phosphate. Or G3P. Or that's also called phosphoglyceraldehyde so you can also write it as PGAL. But a very easy way to visualize it as just a 3-carbon chain with a phosphate. With a phosphate group.
2:34
And the last video when I started with six carbon dioxides and six ribulose bisphosphates. And I ended up with 12 of these. Now I have three and three. Now I'm going to end up with six of these. So I'm going to end up with six G3Ps. And the math works out, right? I have three carbons plus three times five carbons. That's a total of 18 carbons. And here I have six times three carbons. Or 18 carbons. And that's what I showed you in the last video, but there are actually some intermediary steps.
3:08
So the very first thing that's actually formed is 3-phosphoglycerate.
3:20
And the whole reason why I'm showing you this detail, and maybe you're taking a biology class where they actually go into this level of details, is I want to show that this compound also gets produced in that inefficient cycle that we're going to explore in this video. So you get 3-phosphoglycerate, which is also a 3-carbon chain. But it's a different one than this. And this isn't ready to produce glucose just yet. But you could also imagine that as a 3-carbon chain with a phosphate group.
3:46
And then, and course you're going to produce six of these. Just to make sure all the carbon gets accounted for. You have 18 carbons here, you have 18 carbons here. So you're going to have six of these 3-phosphoglycerates get produced. And then each of those gets a phosphate from ATP. So you're going to have six ATPs come in. Remember these ATPs got produced in the light dependent reactions that occur in the membrane of our thylakoids. Anyway, we have these ATPs. You have six ATPs.
4:22
And then they become six ADPs. So essentially they have given away their phosphate groups. So they're going to give one phosphate group to each of these six 3-phosphoglycerates. And then you're going to end up with this, right here. I'm running out of space. Maybe I didn't do it as neatly as I should have. But you're going to end up with this three chain carbon, or 3-carbon compound. It's going to have two phosphates then. And this, you could know the name. This is 1 3-bisphosphoglycerate.
4:53
This was just 3-phosphoglycerate, which means that the phosphate is on the third carbon. Now we have 1 3-bisphophoglycerate. We have a phosphate on the 1-carbon and the 3-carbon. We have two of them. So it's bisphosphoglycerate.
5:09
So that's to get us right there. And then to get from the 1 3-biposphoglycerate, and of course we have six of these as well. Six of these turn into six of these. And to turn these six 1 3-phosphoglycerate into our three glyceraldehyde 3 phosphates, or phosphoglyceraldehyde the names are very daunting. This is where we're actually going to oxidize our NADPH. Remember oxidizing is losing electrons or losing hydrogens with their electrons, either way.
5:41
So here, on this step right here, two things are going to happen actually. You're going to have six NADPHs become six NAD pluses. So they're losing that hydrogen and the electrons. And also one phosphate is going to be lost from each of these molecules. And then you're going to also have plus six of the phosphate groups. The phosphate groups don't get lost here. They get added onto these molecules right there. And then we end up with our sixth glyceraldehyde
6:16
3-phosphates, which we learned in the last video, we can then use to produce fuel or glucose or other carbohydrates in the cell. But we also learned that this is called the Calvin cycle. So all of it doesn't get used to produce actual glucose or other carbohydrates. Most of it, out of the six, five of them are going to be used to produce our three ribulose bisphosphates. So let me do it like this. So you're going to have five G3Ps go in that direction.
6:55
So for every six G3Ps you produce, five go back into the cycle and then one exits the cycle. We saw this in the last video. In the last video I had 12 here, so 10 went in this direction and two in this direction. I'm just dividing everything by two in this video. And the whole reason why I multiplied by two in the last video is so that we end up with two going into this direction because two is enough to produce at least one glucose. But we don't have to have two.
7:17
We could just say we do this whole cycle twice to get two. But anyway, I have one glyceraldehyde 3-phosphate coming out of this end. And then these five glyceraldehyde 3-phosphates that stay in the cycle. Remember those are just 3-carbons with a phosphate group. I drew it over here already. They will then use another three ATP.
7:51
So we end up with three ADPs. And the phosphate groups actually end up on the ribulose bisphosphate and they get essentially recycled into ribulose bisphosphate for the whole cycle to happen again. Remember when all this is happening you can kind of imagine in or on the surface of this big enzyme. This ribulose bisphosphate carboxylase.
8:12
And actually, just so you know a piece of terminology, this type of photosynthesis is called C3 photosynthesis.
8:26
And the whole reason why it's called C3 photosynthesis is because the very first product that you get when you react with carbon dioxide, when the first time you fix your carbon dioxide-- remember carbon dioxide fixation is when you take it in its gaseous form and you actually put it into a molecule. The very first carbon molecule you get once the carbon dioxide gets fixed is a 3-carbon molecule. It's phosphoglycerate. That's why I showed it to you there. Because that's where the three in the C3
8:57
photosynthesis comes from. Now, everything I've just done is a review of what we did in the last video. Maybe with a little bit more detail. And I showed you that you don't have to start with six and six. You could start with three and three and just have five go down here and one go down here instead of doubling everything. But what I want to show you now is an inefficiency that goes on in plants. This RuBisCo doesn't necessarily just fix carbon dioxide. It can also react with oxygen.
9:29
And actually, its name is ribulose bisphosphate carboxylase oxygenase, which means it can react with carbon or oxygen. And this mechanism-- so let's say you have your ribulose bisphosphate hanging around. You know we're in the stroma of our chloroplast. So let's say we have our five ribulose bisphosphates.
10:03
And instead of reacting with carbon it can actually react with oxygen. So instead of having carbon dioxide here, I can have O2 coming in here. We have oxygen coming in here. And all of this, once again, is occurring on the surface or with the assistance of the RuBisCo. The exact same, the RuBisCo enzyme. Ribulose bisphosphate carboxylase oxygenase.
10:31
That's why. Because it can also fix oxygen. And when these two things react, you don't get useful things that can be used for fuel and all of that. You end up with-- and my drawing is a little messy-- you end up with-- well when these two guys react-- you'll end up with five molecules of this guy right here. Five molecules of this 3-phosphoglycerate right there. Remember here we ended up with six of them. But now we're only going to end up with five phosphoglycerates.
11:17
I know the names are all very confusing. But the basic idea is to remember that what's happening here is if carbon dioxide isn't getting fixed, oxygen can get fixed. And you end up with five phosphoglycerates. And you actually also end up with five-- this is called phosphoglycolate. I know these are very daunting names. Phosphoglycolate, which is a 2-carbon molecule. Which makes sense because we only have five carbons to start with. This thing right here is a 3-carbon molecule.
11:51
That's a 3-carbon molecule and this right here is a 2-carbon molecule. So this right here is going to be a 2-carbon molecule and it has a phosphate group. So as you can imagine, in this situation we're not going to be able to keep going forward and produce our glyceraldehyde 3-phosphate, which we can then use to make up carbohydrates. We're stuck. We just have these five phosphoglycerates.
12:16
These can go on and some percentage of them, but the ratios are all getting messed up. But everything doesn't necessarily happen this cleanly in the cell. These things can go on, but we have one less being produced. And this thing right here, it's actually using up some of our carbon from our ribulose bisphosphate. And if this thing kept going up, all of our ribulose bisphosphate is going to get eaten up. We're not going to be able to continue on into this cycle over and over again.
12:44
And actually this is kind of a waste product. Right here this is a waste product. Or we think it's a waste product. And this actually has to exit your chloroplast and actually get processed by other organelles in the plant cells. And these are called, these waste processing organelles in your cells, these are peroxisomes I know there's a lot of complicated terminology here. But the important thing to remember, when RuBisCo fixes oxygen, this is actually called photorespiration.
13:19
That's an important word to know. Photorespiration.
13:24
All of a sudden, instead of being able to carry forward with your Calvin cycle and produce a lot of sugar, instead you are depleting your RuBisCo. So this is a very bad process. This is going to get in the way of your Calvin cycle. And remember how important each of these G3Ps are. Because for every turn of the Calvin cycle, or at least the way I did it, we only produce one G3P that actually gets used for something useful. The other five G3Ps have to go back to produce ribulose bisphosphate.
14:00
RuBisCo of course is the enzyme. So in this situation we only have five phosphoglycerates to begin with. Maybe if we have our ATP or an NADH, we can convert these five-- and it doesn't have to go in this direction-- but maybe we can convert these five phosphoglycerates into five glyceraldehyde 3-phosphates. G3Ps. Then all of these are going to be used back to produce our ribulose bisphosphate. So to go from here to here, we actually had to use up ATP and NADH.
14:33
And then to go from here to here, we have to use more ATP. But we went through this whole cycle and we didn't produce anything, in terms of useful things that can be used to-- essentially sugars or carbohydrates that can be used to fuel or provide structure for the plant in any way. So this is a completely-- or we think-- this is a wasteful process.
14:55
So people wonder, when you look a lot of biological systems you don't see wasteful processes all the time. You would say, well wouldn't natural selection have selected against this? And some people believe that this is just a remnant from our evolutionary past, or our plants' evolutionary past, where there wasn't a lot of oxygen in the atmosphere. And if there wasn't a lot of oxygen in the atmosphere, this was not that likely of an occurrence.
15:23
But there's some people who actually believe that no, this might actually have been selected for in natural selection. Because if there is a lot of oxygen hanging around in the cell, more than you need, that oxygen might actually react with your ATP and create other harmful compounds in the cell. And this might be a way of sopping up the harmful oxygen that's actually hanging around the cell. So who knows? But it's interesting idea. That you have this one enzyme that can react with ribulose
15:53
bisphosphate and carbon dioxide. And if that happens, we just get our regular Calvin cycle. Or it can react with ribulose bisphosphate and oxygen and actually fix oxygen into these two molecules. Especially the phosphoglycolate which we think is a waste compound. And if you just went on and on in the Calvin cycle in this way, you will not produce any useful sugars. So in the next video, what I want to do is study some plants that have been able to get by this photorespiration problem.
16:26
And you could imagine, photorespiration could be a really big deal or it could be very harmful in situations where, one, it's very important for a plant to be very productive of sugars. And it can also, we'll see, it can be a problem-- it's definitely a problem if there's a huge amount of oxygen content. But in the next video I'm going to show you plants that have gotten around this problem by, instead of performing C-3 photosynthesis, which is the classic Calvin
16:55
cycle that I just showed you, they perform C-4 photosynthesis. And I'll show you what that means in the next video.
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