M.Ed., Stanford University
Winner of multiple teaching awards
Patrick has been teaching AP Biology for 14 years and is the winner of multiple teaching awards.
The Calvin cycle is the second step of photosynthesis which converts ATP into glucose for storage. It is also called the "light independent reaction" or the "dark reaction" because unlike the light dependent reaction, light plays no role in the reaction. The Calvin cycle takes place in the stroma, the semi-liquid fluid inside chloroplasts.
Photosynthesis is a 2 step process, the first step is the light dependent reactions the second step is the called the light independent reactions. I often will just simply call the Calvin cycle makes it easier for my students to remember and hear the difference between the 2 steps. But they call the light independent reactions because it does not depend on light. Now if you turn off the lights this process will grind to a halt because this is the step that use the energy from earlier to build glucose. Now the building of glucose from carbon dioxide in the air, this is something that is often called carbon fixation. To fix any element or atom in scientific terms means to put it into a useable form. So carbon fixation is the building of glucose or other sugars using carbon dioxide in the air.
Now this happens when the stroma of the chloroplast the organelle of photosynthesis and it's using the energy that's in ATP molecules and the energy of the electrons being carried by NADPH that were being provided to the stoma by the light dependent reactions. Let's take a closer look again I always like to remind students that photosynthesis is happening in the chloroplast that are inside a plant cell and every plant is made out of gazillions of these plant cells. So if we zoom in on one of these chloroplasts we'll see there's the inner and outer membrane and then there's these stacks of membranes each of these little sacks is called a thylakoid and they're arranged in the stacks called grana.
They're floating in the liquidy stroma. Now the light dependent reactions occurred on the thylakoid membrane and they send their materials the ATP and NADPH to the stroma. And that's what we see going on here, so the light reactions are providing ATP and NADPH and those are being consumed by the Calvin cycle as it takes in carbon dioxide from the air does that carbon fixation I mentioned and spits out some sugars. Now you'll notice it's a cycle so it's starts off with some building materials add the CO2 to them build some glucose but it has to recycle a lot of those materials in order to have your starting materials from the beginning again. It sends the used up adenosine diphosphate and NADP positive back to the thylakoid membrane where it can be recharged with energy so you can continue doing the Calvin cycle.
And now let's take a closer look, so there's a lot of names of molecules in here and depending on your teacher you may need to memorize the names or not. A lot of times I don't bother, but I'll give you those names just incase. So it begins with a 5 carbon molecule here called ribulose bisphosphate the bis just means it has 2 phosphates on the end of this 5 carbon ribulose molecule. I've used the blue marbles here to represent the carbons and the yellowy white ones to represent the phosphates. Now are there other atoms? Yeah there's a bunch of oxygen's and hydrogen's but I'm just going to focus in on the carbons. Just know that sometimes when I'm going from a say a 3 carbon molecule or to a 3 carbon molecule there, there are significant differences. I may be removing hydrogen from one position putting it on another breaking off an OH group putting on a double bound oxygen.
But let's just follow the carbons, so here we have our 5 carbon ribulose bisphosphate and we ram it together with this carbon dioxide up there which has 1 carbon in it. That makes a 6 carbon molecule now scientists love to name things. Why don't they name that 6 carbon molecule because it falls apart immediately, it's unstable so luckily we don't have to remember its name it breaks apart into a pair of these phosphoglycerate molecules here I'm used to calling it phosphoglyciric acid, now for every 6 ribulose bisphosphates that I add 6 carbon dioxides to that will give me 12 phosphoglycerates now I need to energize this molecule in order to make some if the chemical changes that I want to make to it. So I add a phosphate onto the opposite end it already had 1 phosphate now it has 2 so we call it bisphosphoglycerate.
Sometimes other books I've seen they call it di, di and bis they both mean 2 I don't know why the difference maybe it's a East Coast, West Coast kind of a thing. So anyway now we have our bisphosphoglycerate molecule here now we can make some major changes we add some additional electrons from NADPH and these are special electrons they're electrons that have lots of energy. Just like a baseball that's flying through the air has lot of kinetic energy or a bowling ball on top of a shelf has energy as opposed to a bowling ball on the bottom of the floor it has very little energy in it. These high energy electrons allow us to make some major alterations and turn this, this phosphoglycerate into a different molecule that's a kind of glyceraldehyde now this glyceraldehyde is a 3 carbon molecule now that's an alderhyde and on it's third carbon it has a phosphate ion. So we call it glyceraldehyde that has a phosphate on its third ion and glyceraldehyde-3-phosphate. Now these names are really big so I'm going to give you their abbreviations ribulose bisphosphate is called RUBP, phosphoglycerate is often just called PG, bisphosphoglycerate would be called BPG or in those di text books DPG, and glyceraldehyde-3- phosphates is commonly called G3P alright so you get the pattern here.
Now from these 12 here, there's 12 these 3 carbon, molecules. If I count up the carbons I get 36. So that's 36 carbons I need to recycle back to my beginning and I had 6 of these 5 carbon molecules let's see 6 times 5 is 30 so I need to keep 30 of my carbons in order to keep doing the cycle but that does mean that I can scheme of 6 of my 36 carbons down here. And how can I do that, hey if I take 2 of these glyceraldehyde-3- phosphates or G3Ps and ram them together I can actually make glucose or any other sugar or I can use it to build most of the other sugars with some modification.
That leaves me with 10 of them left over, now every step here has been pretty straight forward you have 5+1 divide in half that gives you 3, 3, 3 everything has been pretty simple and down here the pair of G3Ps that I've schemed off to make glucose, glucose is a 6 carbon molecule. Here I have 3 there I have 5, there's no easy Math here and in fact most text books don't show all of the steps here and nobody really cares except some BioChemists so just let's pretend some magic happens. So the magical ATP fairy comes along add some energy and a bunch of enzymes help this process happen and we've recycled our way back to our starting material. So this is it, this is the Calvin cycle or the light independent reactions, it takes carbon dioxide adds it to REBP turns it into a series of different chemicals and ultimately allows you to pull out a few of those in order to make the glucose and then you spend a little bit more energy to recycle back to the beginning.
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