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Photorespiration: In Peroxisome & Chloroplast - The Final Path from Serine to Phosphoglycerate

Writer's picture: PlantHouse EnterprisePlantHouse Enterprise

Delivered on: 27 NOVEMBER 2023


Photorespiration is a complex process that occurs in plants when the enzyme rubisco, responsible for carbon fixation in photosynthesis, binds with oxygen instead of carbon dioxide. This creates a toxic byproduct that the plant must recycle through a series of reactions involving three organelles: the chloroplast, peroxisome, and mitochondrion. This process, while metabolically expensive, is essential for plants to recover carbon, maintain metabolic stability, and protect themselves from damage.


Video Transcript

From Mitochondrion back to the Peroxisome

This serine here needs to exit the mitochondria now. Go where? Go back to the peroxisome. Now we are in the peroxisome, okay? What happens now? So, in the peroxisome, serine can again react with what? Not glutamate, sorry, not glutamate. It wants to produce glutamate. It reacts with this thing because it needs to change itself again. Remember, amino acid wants to change itself again. You always need this 2-oxoglutarate, okay?


And the name of the enzyme here should be glutamate-oxoglutarate aminotransferase to get what? To get glutamate plus, this is critical, hydroxypyruvate, okay? I need to make sure it's all the way up, okay? So, what happens to hydroxypyruvate? Don't worry about the glutamate because this glutamate will be recycled to be used again here, react with glyoxylate because this is peroxisome, remember?


We saw earlier, glutamate is used in a reaction, a reaction where with glyoxylate. Glyoxylate reacts with glutamine, oh sorry, glutamate in order to get glycine, right? So, we are not going to talk about the glutamate. We're going to proceed straight to the hydroxypyruvate. This hydroxypyruvate, it can be reduced with the help of hydroxypyruvate reductase plus NADH plus H⁺ to get what?


You will start to get back your glycerate. You see, you see the magic there? So, this thing has become plus NAD⁺. So, here's the, that the magic happened. You start to get your glycerate, remember? Isn't it the very thing that will be the basic of the Calvin cycle? However, for the Calvin cycle to happen, the first molecule is phosphoglycerate.


Glycerate moves back to chloroplasts

This newly created glycerate will be phosphorylated. So, glycerate reacts with glycerate kinase plus ATP. Now you will get your phosphoglycerate. And this phosphoglycerate is actually this thing that proceeds with the regular Calvin cycle. And oh, I forgot to tell you, this actually, this glycerate now has come back to the chloroplast. This is actually in the chloroplast.


Hydroxypyruvate that has been reduced to form glycerate and NAD⁺, the glycerate will move back to the chloroplast, and then it will be phosphorylated by a kinase, glycerate kinase to become 3-phosphoglycerate, right? And you just saved the carbon from missing using this one very long process, okay?


However, this only talks about the carbon cycle of it, right? So, this is why photorespiration, the other name of it is the C2 cycle. C2 cycle, C2 or C2 metabolism.


Keywords: Photorespiration, C2 cycle, Calvin cycle, Plant metabolism, Carbon fixation, Oxygenation, Chloroplast, Peroxisome, Mitochondrion


Watch the Introduction and the Brief Operation Mechanism photorespiration: https://youtu.be/UkPosfJ8IwA

Watch the Chloroplast Reactions - Rubisco's Oxygenase Activity & Phosphoglycolate Production: https://youtu.be/ISE3Ln_5aHY

Watch the reactions in the Peroxisome - Glycolate Oxidation & Glycine Formation: https://youtu.be/pkqkajc-2Tw

Watch the reactions in Mitochondrion - Converting Glycine to Serine & Releasing CO2: https://youtu.be/n8siyvawuiE


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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