Delivered on: 23 OCT 2023
In this lecture, we will explore the intricacies of the Calvin cycle, a vital process in photosynthesis. We'll delve into the three main phases, the key enzymes involved, and the significance of this cycle in producing organic molecules essential for life.
Video Transcript:
Okay right let's look at what happens actually in plant even though it has doesn’t have the CCM. How does it achieve the carbon cycle. First thing first the carbon cycle or the Calvin cycle has 3 phases. Bear that in mind these 3 phases are carboxylation, or the other name is carbon fixation followed by reduction, meaning that you change the molecule of the cycle by allowing it to receive extra electron and then regenerate again. The whole thing. Right so if you look at in the terms of the cycle it looks something like this okay very easy 3 phases.
However, the detailed story for each of these, that's what going to make people cry. So, let's read this, carboxylation it's actually the action of the existing sugar molecule. you see in the chloroplast that's already a five-carbon sugar molecule which is called ribulose 1,5 bisphosphate. 1,5 meaning that in the carbon change, carbon number 1 carbon number 5 has phosphate group attached to it. Ok just to refresh your biochemistry right and these sugar molecules come together with CO2 not in the form of gas, but in the form of liquid. Liquid CO2 fuse together by the power vested by a catalytic enzyme called Rubisco. So that is the carboxylation. And then when it enters this will give rise to a molecule, 3-phosphoglycerate. I should warn you because you're going to see a lot of this a lot later. Due to this biochemical reaction that can act upon one molecule esterification, carbonylation, isomerism. You're going to see so many things that is actually originally the same thing. So, it starts with glycerol, glycerate glyoxylate I think and so on. There are so many, so these are all start as glycerol there's another one glycolic acid. you should know glycolic acid if you go to Watson all the time right.
That's a lot of that on the cosmetic shelf. What's that for glycolic acid? Or you don't go to Watson. So, these things uh due to this climate correction we're going to see later, it will change the form from glycerol to glycerate, glyoxylate and also the glycolic acid. But there is actually basically glycol thing. Right so in here it is phosphoglycerate it is actually glycerate with a phosphate attached to it. Then these products enter the reduction with the help of ATP and NADPH this is further changed into another new molecule called glyceraldehyde-3-phosphate. You see the three phosphate is not changing it's just it becomes a new thing due to the reduction action acted upon it. Right, this is very critical here you see there's a opening here meaning that the product at the end of reduction phase some exits the rest continue very important not everybody exit it doesn't work like that because this is cycle. The one that exit becomes the sugar precursor that I was talking about why do I say sugar precursor? You learned so far, it's all about glucose, right? Photosynthesis produce glucose that is actually incorrect. Whether it gets turned into glucose or not it's plant's decision but what's for sure is at the cytosol level, cytoplasm level two things get turn turned into first it gets turned into starch and also sucrose. So, this sugar precursor in the chloroplast, correct? Then it will exit chloroplast where is it now in the cytosol. So, in the cytosol it will be changed into starch because that is the storage form of it.
However not all of these can be turned into a storage form. Some needs to be transported into the baby leaves so if the plants decide some of the sugar needs to be transported away, it is not going to be changed into starch but changed into sucrose. So, sucrose is the transport molecule. Why sucrose? You see did I mention anything about glucose here actually plant decide to do the glucose if it needs the further ATP because glucose never leaves the cell. Remember glucose that has been formed in the cell never leaves the cell itself. The only thing that leaves the cell is actually sucrose. Why is that? Glucose is very reactive. It's a reactive sugar meaning that it will cause havoc with whatever it touches. That's why you cannot have so much sugar in your in your blood. It's detrimental because it's very reactive. So the sugar if it turns into glucose it will be quickly transported into mitochondria for cellular respiration. You see very brief actually the lifespan of a glucose in the cell plant cell. It gets manufactured the glucose then straight go to the mitochondria to be broken down so that more ATP is produced.
Do you see anywhere in here glucose gets stored? No so that's why it is a struggle with us human because we always have some kind of glucose in our blood, we need that but not too much okay right. That's the essence of this and what about the remaining of it? The remaining of it uh of this end product G3P usually people call it G3P will proceed to the beginning in order to produce this again ribulose bisphosphate or RuBP. So that the cycle does not stop it will continue on going on and on. Don't think there's only one cycle in the chloroplast in the chloroplast, in the stroma of chloroplast there are millions of this going on at the same time. So, they kind they kind of complement each other at different speed so if you look at let's say that at 10:00 a.m. at any given time some Calvin cycle at this stage some at this stage some at this stage some at this stage some at this stage some at this stage. So, all the stages are present at any time because they cannot stop to happen. If it stops to happen it will cause backlog it will stop the sugar production right talking about probability now right.
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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