The Calvin Cycle, Light Independent Reactions in Photosynthesis


Hi, everybody! This is going to be about light independent reactions; the “synthesis” part of photosynthesis. Just to make sure we’re all on the same page, we’re going to look at this diagram one more time. So, here we have a chloroplast which has a space inside called the thylakoid membrane and that’s where the light-dependent reactions are going to occur. In the light-dependent reactions, we add together photons of light and water and then we create ATP and NADPH and then oxygen as a by-product. Then for the light independent reactions, which occur out in the stroma, we’re going to add together carbon dioxide, ATP, and NADPH along with more water in order to get PGAL, the precursor of glucose. So this is our framework for drawing out how the light independent reactions work. We’re going to include some aspects of the light-dependent reactions so we can see how they all work together. This looks pretty complicated, but you have this exact template in your dropbox folder. The first thing we’re going to do is we’re going to label all the different reagents that are necessary for photosynthesis. Since every one of these reagents are going to be added into the light-dependent reactions, the first part of photosynthesis, we’re going to start by labeling the box in the middle as the light-dependent reactions. So, here are three major reagents of photosynthesis; water, carbon dioxide, and photons of light, and those are all going to enter in different ways. Photons of light will be absorbed by the two photosystems; photosystem 2 and photosystem 1, and then will also be involved in the photolysis of water in the light-dependent reactions. However, they don’t directly play a role in the light independent reactions. The next reagent, water, will be crucial in both the light-dependent and the light independent reactions. It is pulled from the soil and enters the plant tubes called the xylem. Carbon dioxide is not involved in the light-dependent reactions, but it does become important in the light independent reactions. It enters through the openings called stomata. Each molecule of carbon dioxide has only one carbon atom. In order to create one molecule of PGAL and combine them to make glucose, we need to use six carbon dioxides. The light independent reactions happened in the thylakoids, however the light independent reactions are going to happen in the stroma. Since the light independent reactions are a cycle, it’s hard to know where to start but the easiest place for us is probably going to be to start up here at the top. This process starts with molecule called RUBP. Now, RUBP stands for Ribulose 1-5 biphosphate, so we’re just going to stick with calling it RUBP. One molecule of RUBP contains five carbons and it’s shaped like a Pentagon, and in this process we’re actually going to use six molecules of RUBP for a total of thirty carbons. Now, if you take the 5 carbon RUBP and add together the single carbon from carbon dioxide, what you get is a six-carbon intermediate molecule. This long chain of carbon is highly unstable, and so it doesn’t stay together for very long. Because these 6 carbon intermediates are so unstable, they almost immediately break apart. When the intermediates break apart, they rearrange themselves into shorter molecules which are called PGA. The real name for PGA is phosphoglyceric acid, so we’re going to call it PGA. Just to make sure we’re on the same page in terms of the number of carbons, let’s do some quick math. During the intermediate step of the calvin cycle, we had six six carbon molecules for a total of thirty six carbons. When they rearrange themselves after breaking apart from their more unstable form, they form twelve smaller molecules each with three carbons. So, 6 times 6 equals 36, and 3 times 12 is also equal to 36. In our next step of the process, each molecule of PGA will combine with inorganic phosphate, hydrogen an electron and energy. So, now we need to ask ourselves “where do we get the inorganic phosphate, H+, the electron and the energy?” and this is where the product of the light-dependent reactions are going to come into play. One molecule of NADPH will carry one hydrogen ion one and one electron. This accounts for our first two ingredients. Our final ingredient, inorganic phosphate, will come from our good friend ATP from the light-dependent reactions. Once the ATP and NADPH have dropped off the important components to this chemical reaction, they are in their lower energy form. After ATP has lost its phosphate molecule, it becomes ADP the lower energy form and must return to the light-dependent reactions to be “recharged”. Once NADPH has dropped off at hydrogen and its electron, it becomes something called NADP+ and must also return to the light-dependent reactions to be “recharged”. In addition to providing an inorganic phosphate molecule to combine with the PGA, the hydrogen, and the electron, the ATP molecule will also provide a boost of energy for this reaction to occur. When we have combined all these ingredients together, we wind up with a total of twelve PGAL. Each PGAL has three carbons on it. Of the 12 PGAL we’ve created, we’re going to borrow two of them to convert into a glucose molecule. 1PGAL plus 1 PGAL equals 1 glucose. However we now have to ask ourselves “what do we do with the 10 PGAL that are then left over after we steal two of them to create a glucose?”. The answer is that we re-invest the remaining PGAL order to keep the cycle turning. So here my 10 PGAL and I’m going to have to rearrange them in order to reinvest them in the cycle and somehow transform them back into RUBP. In the next step of the process, 10 gal re-arrange themselves in a process that we will simply refer to as “10 PGAL get crazy” and the reason that we’re keeping it simple because people have spent decades studying how all these crazy reactions happen. But for our intents and purposes, they’re simply getting a little help from ATP and they’re rearranging themselves again. So, a little through ATP and its consequential boost of energy, the molecules will rearrange themselves. The 10 PGAL start off as 10 groups of 3 carbons each for a total of 30 carbon. After the “get crazy” step, we still have 30 carbons, but they’ve rearranged themselves to become 6 groups of 5 carbons each, also known as 6 RUBP and the cycle can begin again. So, like this. This process also releases water as a byproduct in the form of water vapor. The water vapor will then diffuse out through the stomata just like we saw with the other gasses. The only thing we haven’t addressed is what happens to the oxygen from the light-dependent reactions. Since oxygen is a gas, it can also diffuse out through the stomata. So, there you have it! That is the Calvin Cycle, also known as the light independent reactions. I know it’s a lot so if you need to go back and watch it again, please feel free to do so. Please make sure to bring questions to class and I’ll see you all soon. Bye!

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