Original video: https://youtu.be/UkPosfJ8IwA
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:
Now we proceed to the second part of it which is photorespiration. Photorespiration actually happens because, by the way, the Calvin cycle—the other name for it is C3 cycle, pentose phosphate cycle, Calvin cycle, dark cycle, many other names—however, the star enzyme involved in the Calvin cycle, Rubisco, Ribulose bisphosphate carboxylase/oxygenase, is an ancient enzyme. Therefore, it has the issue of not being specific enough to catalyze a single reaction, which is the carbon reaction up here.
If it proceeds with this carboxy reaction, then that's fine. You get your phosphoglycerate that can be reduced to become your sugar precursor. However, the issue with Rubisco is it can add oxygen to the RuBP and proceed with, many like to call it a wasteful process. I would like to call it a metabolically expensive process called photorespiration.
When it proceeds this way, it will create two molecules. The first molecule is the regular phosphoglycerate to proceed with the regular Calvin cycle. The problem is here, it creates the two-carbon product, not a three-carbon, two-carbon product called two-phosphoglycolate. This thing actually is a bit toxic to the plant.
So, what the plant does? The plant needs to eliminate this or needs to turn this back so that I can reuse this carbon for the sake of making sugar precursor again because this thing—another problem is it cannot be converted directly from here, jump to here. Plants do not have the enzyme. Unless some kind of mutation happens, then we do not have this photorespiratory issue, right? So, photorespiration is a type of respiration, meaning that it consumes ATP. It breaks down something to get something, but the trouble with photorespiration is it's a lengthy and expensive process.
To start with, photorespiration involves three organelles: chloroplast, from where it started originally, then peroxisome, then the third one is mitochondria. Okay, mitochondria, that's the singular, mitochondria, that's the plural of it. So, these three organelles. Without the oxygenation reaction done by the Rubisco, these two are really out of the question. Chloroplast alone is enough to make the sugar. Now, since the plants have produced this toxic two-carbon product, phosphoglycolate, this needs to be turned into something useful again.
The whole idea is, from two-phosphoglycolate, how to get back to three-phosphoglycerate? You see, it's just from here to here, but there is no enzyme to do this. So, in order to do that, the plant has to go all this path, go back, go down to the peroxisome, mitochondria, coming back to peroxisome, coming back to chloroplast. It's such a long process. So, you are equipped now with the enlarged version of this so that you can study it later.
For the second page of this, this is the summary of the reaction that's going to happen. But let's be realistic. Yeah, why is there a question mark? Oh, I'll come to that in a bit. You see these light purple boxes here? These are called transporters, meaning that it's like a portal or doors from which molecules can exit or enter. Okay, two years ago when I taught this photorespiration, people had not confirmed the name of this transporter.
So, this transporter actually has a name, so it's PLGG1, BASS6. So, P stands for plastidial glycolate glycerate number one. And BAS, what is that? This is, by the way, this is a transporter. Okay, this is called a transporter. BASS6 acid, is called a symporter, meaning that one door is used by many people. See, BAS stands for bile acid sodium symporter 6. Whenever you see this "?," it means that people have not found the name or the protein involved to make this.
So, this is a research opportunity, actually, to study this. I tell you, it's not easy. In my thesis, there is a technique that can be used to detect this. If you ever read my thesis in chapter 5, I describe there is a technique to detect this, right? Okay, coming back to this cycle, I just want to be realistic about this.
The realistic bit that I'm talking about is you need to cover this in one lecture. You need to find a happy, peaceful place to read it and to absorb it. But what I can assist you with here is to create the story so that you can see that it starts with something, then it comes back to something.
Keywords: Photorespiration, C2 cycle, Calvin cycle, Plant metabolism, Carbon fixation, Oxygenation
Watch Full video: https://youtu.be/X3c81GRbN2M
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
Watch the reactions in Peroxisome & Chloroplast - The Final Path from Serine to Phosphoglycerate: https://youtu.be/43m8xoweG48
Reference book:Â 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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