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Photosynthesis: Understand The Second Step of Calvin Cycle - The Reduction Phase

Writer's picture: PlantHouse EnterprisePlantHouse Enterprise

Updated: Nov 24, 2024

Delivered on: 23 OCT 2023


Reduction. In the second stage, ATP and NADPH are used to convert the 3-PGA molecules into molecules of a three-carbon sugar, glyceraldehyde-3-phosphate (G3P). This stage gets its name because NADPH donates electrons to, or reduces, a three-carbon intermediate to make G3P. In this session, we will explore more in details on the reduction phase in Calvin cycle.


Video Transcript:

Now this product phosphoglycerate will enter the second phase of the Calvin cycle which is the reduction phase. Reduction has nothing to do with something get trimmed off something get pruned off we are talking about biochemistry meaning that it receives hydrogen. As simple as that reduction in chemistry means some somebody is receiving hydrogen. Right so what happens here so I have highlighted here this 3-phosphoglycerate will react with one molecule of ATP to become,1, 3 bisphosphoglycerate and this process is what we call as phosphor relation. So, the phosphor relation of 3-phosphoglycerate this phosphoglycerate sometime people put it PGA, phosphoglycerate it depends on the that's why I'm not so big on this abbreviation. You read 10 books will have different acronym abbreviation for it. So, for now we use the full name right ATP is added to phosphoglycerate now carbon number one also has phosphate. 1 three carbon one got phosphate carbon three got phosphate. But it is still glycerate it's not changing to anything else okay right. This 1,3 bisphosphoglycerate will then receive hydrogen will get reduced by the action of NAPDH now it becomes a new molecule you see it adds the name here glyceraldehyde 3 phosphate. In this process you can see that there is no more 1, 3. Carbon number one get dephosphorylated. That's why at the end of here you get NADP+ plus inorganic phosphate. A phosphate group is released from the 1,3 bisphosphoglycerate.


Why it has to be removed? If it is not removed, how can it receive extra hydrogen. Right, that gets removed now it becomes the new G3P some books say it. It's good to know a bit G3P, glyceraldehyde 3 phosphate and guess what this is the sugar precursor. Two times glyceraldehyde 3 phosphate, you will get a glucose but there's no glucose happening right now. Remember who decides to get glucose sucrose or starch? The plants will decide depending on the energy demand depending on whether it has babies or not maybe some plants. For example, like the perennial plants it will decide during the fruit season to shuttle all the sugar to the fruits but during non-fruit season it only has what less organs only leaves no fruit right. So it will decide much of it go to starch only some leave as sucrose to the leaf. Because leaf is not really a sink sugar it is a sink organ but if you compare leaf to fruit which is the strong sink organ? Fruit right is that but some leaves are sweet right? What example sweet leaf cannot think of one? Cekur Manis? You know that plant? Soropus androgenous that leaf is sweet. Okay right so up to this point you can see that ADP and ATP has been used. So whatever that I've said just now that is the summary of it.


To get this remember at this point the Calvin cycle not only fix one carbon with one RuBP at a time. No that's not what's happening okay actually in reality is, in order because one need to exit if you start as one carbon and RuBP when it reaches at this point everything needs to exit who's going to continue right you cannot continue so the reality is actually, Three * RuBP react with 3 * CO2. So, the total carbon here is 18. When it reaches here, it reaches in the form of six molecule of glyceraldehyde 3 phosphate this is three carbon right if you time again the carbon is still 18. 18 carbons remember one need to exit here so when it exits now it becomes. Who exit who continue? Exit is one * G3P which is three carbons, who's continue regenerate five molecules of G3P which is 15 covered. Okay this is how it works. And these five molecules of G3P which you will I think we will just talk up to this point, it will continue to become only three molecules of RuBP it becomes this way. Start with three again can you see the magic here? It starts as three RuBP react with three carbon the total is 18 carbons at the end of the reduction it is left with six molecules of G3P with the total of 18 carbon right. However not all six molecules of G3P will exit and do sugar do starch do whatever only one precursor hence the word precursor. That's why it's called C3 photosynthesis this is why because the first product the first product is G3P three carbon glyceraldehyde 3 phosphate. One carbon exit so what about the balance? The balance is five units of G3P proceeds to regenerate. During regeneration a shuffling is involved like you are playing a Lego and cut reshuffling is involved originally you start as five molecules then it reshuffles… you come with three molecules of RuBP. This is five carbons, right? What about three molecule RuBP how many carbons still 15 and you get exact the same you end with five carbon you start with five carbons as well oh sorry 15 Carbon. This is 15 carbons so that is the reason why this has to be in the form of uh cycle it has to be sustainable in in any way RuBP level. Because plants do not have time to make new RuBP it needs to find a way it's like a riddle how can we use this sugar without losing the sugar at the end that's the riddle. So, evolution, all the magic happens millions of years they come up with this strategy. Very smart who tells the plan to do this that's a mighty good question right who tells the plan to do this?


Reference book: Plant Physiology and Development 7th Edition

by Lincoln Taiz, Ian Max Møller, Angus Murphy, Eduardo Zeiger

Attribution 4.0 International — CC BY 4.0 - Creative Commons

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