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Photorespiration: Chloroplast Reactions - Rubisco's Oxygenase Activity & Phosphoglycolate Production

Writer's picture: PlantHouse EnterprisePlantHouse Enterprise

Updated: Jan 23


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:

Okay, so we start in chloroplast. Chloroplast, yep. This is good, this is good. So, from last week, you learned that RuBP, which is the five-carbon sugar, will react with Rubisco that has been activated. Remember, this needs to be activated from your last week's lesson. So, in the activated form, it's called ECM. You need to recall back, Rubisco has been carbonated, okay? It has been carbonated with CO2.


Remember, the CO2 needs to activate Rubisco. It is not the same as Rubisco reacting with CO2. This is just a free CO2 attached to Rubisco and then stabilized with magnesium to form the activated form of Rubisco, okay? Then this will react with oxygen. We are talking about photorespiration, right?


This will produce, okay, we are in chloroplast, this will produce one molecule of phosphoglycerate. This is fine, no problem with it, plus phosphoglycolate. Three, with this, with a "C." Now the issue is glycolate, right? Now the problem.


So, this phosphoglycolate, I think I'll just put it as 2-PG, okay? To simplify it. It needs to become phosphoglycerate. This is the idea, however, there is no direct conversion. So, it needs to begin the journey now to convert itself. How does it happen? It needs to be dephosphorylated first. So, it will undergo dephosphorylation. It will become glycolate plus ADP plus Pi.


The name of the enzyme is actually on this page. So, what's the name of the enzyme? It should be phosphatase, as it removes phosphate. Dephosphorylation. The enzyme here is phosphatase, glycolate phosphatase. Is it glycolate phosphatase? Phosphoglycolate phosphatase. Look at number two, okay? Meaning that it now becomes glycolate after phosphate has been removed.


Now, this glycolate can now exit the symporter transporter. You got glycolate, right? This glycolate does not stay forever in the chloroplast. It will now leave the chloroplast and then enter through this transporter, a symporter. You got two here, two doors here. It must be very toxic.


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


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

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

Watch the reactions in Peroxisome & Chloroplast - The Final Path from Serine to Phosphoglycerate: https://youtu.be/43m8xoweG48


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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