Delivered on: 23 OCT 2023
The carbon fixation phase is the initial step in the Calvin cycle, where inorganic carbon dioxide (CO₂) is incorporated into organic molecules. In this session, we will explore more in details on the carbon fixation phase in Calvin cycle.
Video Transcript:
These 3 phases of Calvin cycle you have 2 ways to look at it. If you want to look at it in the form of chemical reactions, what chemical components reacting with what, you go with the left one. The carboxylation, reduction phase and regeneration phase. You can see the complexity of it. Of this molecule the good thing is you can already see, this is the stage that uses ATP and NADPH and then this thing is actually they kind of split before they made up at the end. Right so you get the appreciation of it. Another way to look at it is just look at the summary. This when you are interested with the reactors the products and also the enzymes responsible for it. The reason we need to have this not only this even though biochemistry student kind of you know very fond of this diagram. The reason we are dealing this because you learn physiology when you learn physiology you understand that in any biochemical reaction there is always a limiting factor. You need to id the limiting factor so that you understand how the plants speed up the reaction or slow down the reaction. Because at the end of the day it is the very process that is manufacturing your harvest.
Look at this the number one, the one that I highlighted here is actually belonging to the carboxylation phase. So, what is the reaction here are you RuBP + CO2 + water. Now you learn something new maybe before this you have learned Calvin cycle, but you didn't know Clavin cycle need water as well. Yes, Calvin cycle just like the light reaction requires water to run. In the light reaction at the thylakoid membrane the photolysis you use the water split the water so that you get what out of the water? Electron yeah you get the electron. So, in this process you also need water but not so really for the electron transport you need the water because to hydrate the molecule. The product of this is you get 2 molecules of phosphoglycerate actually it was originally one molecule but this molecule scientists omit this because it's very unstable scientists go straight to three carbon phosphoglycerate. How come? Why 2? RuBP, 5 carbon CO2 1 carbon you bring together you will get six carbon molecules. This six carbon molecules super unstable maybe like less than a second it will quickly broken down. It breaks down it becomes 3-phosphoglycerate two molecules of it right. So, what actually happens here in this first phase of Calvin cycle will greatly determine the activation of the Calvin cycle will determine whether the Calvin cycle starts or not will determine the rest of the step here. You see there so many statement here. 12. Actually, there's small if I use other book. Then you'll cry even harder.
Alright so let's look at what happens a bit detail in number one in here. This is what happens to the RuBP the five carbon sugar and the Rubisco. So, this RuBP it will go some kind of chemical transformation to itself. Right isomerization and condensation because this RuBP depending on rubisco the enzyme that act upon it will either bind to O2 or CO2 and that is the problem with Rubisco. You cannot blame RuBP okay, blame Rubisco. So, the problem with rubisco is it has carboxylate reaction and also it has oxygenase reaction. If the rubisco takes CO2 to and then fuse with RuBP you go up here. you will get the one that you saw earlier the 3-phosphoglycerate and then this will proceed to photosynthesis. 2 molecules of it. However, if this rubisco decides to do the oxygenase it gets the liquid oxygen fills with RuBP then you go down here.
It will produce two molecules as well but only one is phosphoglycerate the other one is phosphoglycolate. Yeah, these two phosphoglycolate has got the synonym we call it the C2 photosynthesis. All this why you learn about the C3 C4 and CAM photosynthesis right? That's another one C2 photosynthesis. Who is the C2 photosynthesis? This thing photorespiration so photorespiration is C2 photosynthesis is it really photosynthesis no it's carbon Salvage pathway. What seems to be the issue here if it proceeds with this you get your sugar but if it proceeds with this you only get half of the sugar precursor not only that. The moment phosphoglycolate plus another one glycolate, the moment this thing is manufactured it's a bit toxic. The plants need to deal with it when the plants need to deal with it not only that it will use energy it will cause another organelle to intervene as well. This glycolate needs to be detoxified to detox glycolate you need 3 organelle you need the chloroplast, peroxisomes and mitochondria.
We will learn I think next week about this photorespiration in detail but for now it's good to know this thing can happen. When does this thing happen? When CO2 concentration surrounding Rubisco decrease when it decreases? When the temperature is high. when it gets too acidic. This is why plants not doing well in in the field when it's too acidic this is another one reason usually you learn because of the nutrient availability right? When it's too acidic some nutrient is not well. That is one thing that's another thing you create acidic environment in the chloroplast as well. When it's too acidic, CO2 cannot attach to Rubisco properly because it's competing. That's the keyword CO2 competing with O2 for the catalytic site of Rubisco as simple as that.
Reference book: Plant Physiology and Development 7th Edition
by Lincoln Taiz, Ian Max Møller, Angus Murphy, Eduardo Zeiger
Full video: https://youtu.be/t1USGntKCfM?si=Me3PptSa5Gs_tuM_
Attribution 4.0 International — CC BY 4.0 - Creative Commons
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