3 Phases of the Calvin Cycle (C3 Pathway)

by Jason Amores Sumpter
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in this video, we're going to introduce the three phases of the Calvin Cycle, which we know. The Calvin Cycle is the second stage of photosynthesis following the light reactions, and we're gonna talk about the three phases of the Calvin cycle and what's known as the C three pathway, which we'll get to define what that means here very shortly. Now, notice that the three phases of the Calvin Cycle we have number down below as number one number two and number three, and notice that we have each of these phases color coordinated so that the color coordinate to the image that we have down below for each of these three phases of the Calvin cycle. And so that's important to keep in mind. Now. The first phase of the Calvin cycle is what's known as carbon fixation, and the fixation part here actually has nothing to do with fixing something that's broken. Instead, this term fixation is derived from the term to a fix something, and the term of fix is really just a term that means to fasten something to something else, to attach something to something else. And really, that's what's happening here in carbon fixation. And so carbon fixation is going to utilize the enzyme that's known as Rube Isco and rub isco is a very important inside because Rube Isco has this amazing ability to a fix or to add carbon dioxide gas from the atmosphere to, ah, five carbon sugar molecule called Regulus Biss phosphate. Or are you BP? And so Rube isco is the enzyme that takes carbon dioxide and adds it toe, Are you BP? And so because, uh, this is adding carbon dioxide, it's called carbon. And because it's being a fixed, uh, to our UBP, it's called carbon Fixation. Now it turns out that the very first stable molecule that's produced in this carbon fixation phase is a three carbon molecule. And because the very first stable molecule that's produced is a three carbon molecule or a C three molecule, this pathway is called the C three pathway. Now, later, in our course, when we're talking about different types of photosynthesis, we'll talk about other pathways that do not form a stable three carbon molecule. Instead, they end up forming a four carbon molecule, But we'll talk about those other types of photosynthesis, including C four and Cam pathways later in our course. For now, this pathway the C three pathway is the standard pathway, and it forms a stable three carbon molecule. And that stable three carbon molecule is called Foss Foege, Lister Aldo hide. And so far, so blister alga hide is also known as P G a. Now, if we take a look at our image down below at the first phase of the Calvin cycle, just focusing on this left hand side notice that the first phase of the Calvin cycle is right here in green called carbon fixation. And what it does is it uses this enzyme rube isco and the ends. I'm Rob ISCO has this amazing ability to take carbon dioxide, which we are showing you up here. And it could take the carbon dioxide and a fix it to the R. U B P molecule that we're showing you here and ultimately through a Siris of reactions, the very first stable molecule that's produced is a three carbon molecule called P g A. And again, each of these black circles that you see throughout here represent carbon Adams. And so that's really it for the carbon fixation stage. Uh, rube ISCO is going to take carbon dioxide and attach it to our UBP and ultimately through a series of reactions, is going to generate PG A three carbon molecule now moving on to the second phase of the Calvin Cycle. What we have is G three p synthesis and in this G three piece synthesis face, what's going to happen is the P G A is going to be used to synthesize glycerol. The high three phosphate, or G three p and G three p is a precursor molecule that's needed to build glucose. And so the cell is going to use specifically to G three p molecules to synthesize a single glucose molecule. And so, if we take a look at our image down below at the G three p synthesis phase, notice that it starts with p g A. And it converts this PGA into another three carbon molecule called G three p and G three p is going to be the precursor that's needed to build glucose. And so ultimately, there are going to need to be to G three p molecules needed to build one glucose, and also in this G three p synthesis phase, it's actually going to require the use of energy, and this energy is going to come in the form of any DPH and p and this n a. D pH and 80 p that's needed as energy for G three p synthesis. This ntd Ph and 80 p is coming from the light reactions, which is the first phase of photo synthesis. And so, uh, the light reactions produces the energy in the form of any DPH and 80 p that's needed to power the Calvin Cycle specifically needed to power the second phase of the Calvin Cycle and G three piece synthesis. And so, ultimately, what we have here are some G three piece, and it takes two G three p molecules again to make just one single glucose. But ultimately there's gonna be some left over G three p molecules that are not gonna be utilized to build glucose. Instead, thes leftover G three p molecules are gonna be used in the third and final phase of the Calvin cycle. And so, in the third and final phase of the Calvin Cycle, what we have is our UBP regeneration and recall that are UBP is rebelo sbis phosphate It's the very first molecule that was generated, the very first molecule that was reacting here in the Calvin cycle. And so, in order for our UBP, in order for the Calvin Cycle to be a cycle in order for it to start and end in the same place, it needs toe have a phase that's dedicated to regeneration, essentially getting our UBP back to its original state. And so here in the third phase, what we have is our UBP regeneration, which is gonna use the remaining leftover G three p that's not utilized to build glucose. And it's gonna rearrange this g three p in a series of enzymatic reactions that's driven by a teepee to regenerate our UBP and then whence are you? BP has been regenerated. It's ready to go for a second round of the Calvin Cycle here. And so when we take a look at our UBP regeneration phase down below is over here and notice that it takes the remainder of the G three p and uses it to rebuild to regenerate our UBP here, and it's going to take energy in the form of a teepee in order for this regeneration to occur. And so, ultimately, that is the end of the Calvin cycle. Those three phases and at the end of the Calvin cycle again noticed that carbon dioxide is ultimately being used as a reactant, and also n a. D P. H and a teepee are being used as reactant as well. And in terms of the products, what we get is a glucose molecule. And so, in order to remember the reactant and the products of the Calvin cycle over here on the right, what we have is a memory tool to help you guys remember the reactant and the products of the Calvin cycle. And so when you think about the Calvin cycle, you really want to think about Calvin's can of sugar, which is really just this soda can right here. And so here we have Calvin, and you could see he's got even a little name tag that says, Hi, my name is Calvin, and so Calvin has his can of sugar here. This soda can hear, And so when you're thinking about the Calvin cycle, just think about Calvin's can of sugar. And when you think about that again, that will help you remember the reactant of the Calvin Cycle and the products of the Calvin cycle. And so the reactant of the Calvin cycle are going to come from this can over here, where the sea and the can represents the C and C carbon dioxide. The A in the can represents the A, n A, T P and the N in the Can represents the n N N a. D pH and so Co. 2 80 p and N. A d. PH are the reactions needed for the Calvin cycle and, of course, Calvin's can of sugar. The sugar over here is going to represent the product, which is glucose. And so, by remembering Calvin's can of sugar once again, you'll be able to remember the reactions and the products of the Calvin cycle. Now, in terms of this image over here and what you're supposed to memorize, that is going to be up to your professor. Your professors are going to vary in exactly the amount of detail that they want you to know about the Calvin cycle. But one thing that is pretty consistent about what you should know is that it's going to take a total of six carbon dioxide molecules entering the Calvin cycle to create glucose. And one way to help you remember that is that within six carbon dioxides there are six carbon atoms, and glucose also has six carbon atoms. And so ultimately, these six carbon atoms of the carbon dioxide end up as the six carbon atoms that air in a glucose molecule. And also you should know that it takes to G three p molecules to make a glucose. So basically the interactive portions here are the portions that you really want to focus on the most. And so this year concludes our introduction to the three phases of the Calvin Cycle in the C three pathway. And we'll be able to get some practice applying these concepts as we move forward in our course. So I'll see you all in our next video.