Original video: https://youtu.be/kzoB9p-aUuE
C4 plants like corn and sugarcane are known for their high productivity. But what makes them so efficient? This video reveals the biochemical secrets of C4 photosynthesis, explaining how these plants overcome the limitations of Rubisco and the wasteful process of photorespiration. Learn about the unique roles of mesophyll and bundle sheath cells, the importance of four-carbon compounds, and the key enzymes that drive this remarkable pathway.
Video Transcripts:
For the C4 plant, the reason it is called C4 is because the first carbon fixation event, the first product is not phosphoglycerate like you learned in the Calvin cycle. Rather, the first product is a four-carbon compound such as malate.
The image you're looking at is a typical representation of a C4 photosynthesis organism, such as corn. You can see the first cell here, the mesophyll cell, and the bundle sheath cell. This is rather different because previously you only learned about mesophyll cells, and suddenly there is another cell now. They have a function.
CO2 in C4 plants gets into the mesophyll cell. Mesophyll cells are filled with water, just like any other cell, and CO2 can dissolve in water. The moment CO2 dissolves in water, you get carbonic acid. This carbonic acid can further be dissociated into bicarbonate ions. This bicarbonate ion is the one that is going to react with a special three-carbon compound called phosphoenolpyruvate.
This phosphoenolpyruvate is not a sugar precursor, it's just a three-carbon compound that will capture the CO2 in the form of bicarbonate ion to form malate. Three carbons here, one carbon here, you got four carbons. This is why it is called the C4 metabolism. This is all achieved with the presence of carbonic anhydrase and PEP carboxylase. Carboxylase means the incorporation of carbon into a compound. Decarboxylation means the removal of CO2 out of the compound. The terminologies are very important here.
One thing about carbonic anhydrase is it can do this thing. You got your CO2, this is all liquid, not gas. CO2 + H2O, it means reversible reaction. Carbonic anhydrase. You will get H2CO3, carbonic acid. This thing can be dissociated into bicarbonate ion, HCO3⁻ plus hydrogen proton. Carbonic anhydrase can make CO2 that has dissolved in water to become carbonic acid, and then from carbonic acid, it can further change itself into bicarbonate ion. This is the bicarbonate ion. Bicarbonate plus hydrogen. And from this, it can revert back to CO2 and water. It's kind of like this triangular reaction with carbonic anhydrase. The star that we are talking about here is this guy here, the important thing, bicarbonate ion.
This bicarbonate ion now reacts with phosphoenolpyruvate, or short is PEP. This PEP now becomes oxaloacetate after it has reacted with bicarbonate ion. It becomes oxaloacetate plus Pi, inorganic phosphate. Sometimes it doesn't say Pi, it says orthophosphate, kind of the same thing. You are still in the mesophyll cell. We are not in the bundle sheath, we're still in the mesophyll cell.
This oxaloacetate, two things can happen, depending on the subspecies of the C4. Some species, from the oxaloacetate, it will be reduced, you get malate, or the oxaloacetate reacts with an amino acid, undergoing transamination to get another kind of amino acid called aspartate, plus 2-oxoglutarate. If you recall from the photorespiration lesson, you found this guy a lot, 2-oxoglutarate. The moment an enzyme, aminotransferase is present, when you find something using the enzyme aminotransferase, this guy, 2-oxoglutarate will be present.
This is the reduction process. Reaction number three is the reduction of oxaloacetate. Number four is the transamination reaction. In any case, you get a four-carbon compound, either malate if the plant is using reaction number three, or you get aspartate if the plant is using reaction number four. Some plants kind of overlap, because the evolution is actually still not finished. There is overlap and some plants kind of do halfway, follow the regular C3.
You have finished the first part of the C4 step, which is carbon fixation, but this is in the mesophyll cell. From the mesophyll cell, the four-carbon compound, either malate or aspartate, will move to the second cell, which is called the bundle sheath cell. Sheath means the cells are actually in the concentric arrangement surrounding the vein. So, if I may draw the vascular system like this, in the middle here, you got your vascular system, xylem and phloem. Surrounding here, you got your bundle sheath. Outside the bundle sheath, you got your mesophyll.
The key point here is the bundle sheath is right next to the vascular bundle because the Calvin cycle that you are familiar with is happening in the bundle sheath cell, not in the mesophyll cell, remember. You're not dealing with C3 now, this is C4. So, the situation is special, it's a bit unusual.
What happened in the bundle sheath cell? When the original C4 compound, malate or aspartate, has entered the bundle sheath cell, it needs to undergo a decarboxylation event. This four-carbon compound, the CO2 in its body needs to be taken out. Why? Very easy, so that the CO2 can proceed with the regular Calvin cycle. You still have your Calvin cycle, but it is now specialised in the bundle sheath cell.
Malate here will release its CO2, and this CO2 proceeds in the regular Calvin cycle. This is the critical step here because malate here will incessantly release CO2, to Rubisco with a carboxylase mindset, there is no room for oxygen for you. So, that's the whole idea. Use a compound, ask the compound to go in, crowd towards Rubisco, and all these compounds bombard Rubisco with CO2, so Rubisco has no reason to do oxygenation at all. In fact, the efficiency of this thing is photorespiration, if it ever going to happen, only about 2% to 3% only. That's how good this carbon-concentrated mechanism is.
You can see the enzyme here, NADP-malic enzyme, the enzyme that dissociates the four-carbon compound into carbon dioxide, and pyruvate. Pyruvate is the three-carbon compound that can be shuttled back to the mesophyll cell. CO2 is released to proceed with the Calvin cycle, and then at the end of this, you get your pyruvate, the C3, this is not a carbon or sugar precursor, this is just a three-carbon compound. This three-carbon compound, pyruvate, or in some species, there's an exception here, alanine, depends on the species. Pyruvate or alanine go back to the mesophyll cell so that it can be converted to phosphoenolpyruvate, PEP, and this PEP can redo the four-carbon fixation again, and the cycle continues.
Pay attention to this enzyme here. There are three enzymes here: NADP enzyme, NAD-malic enzyme, and PEP carboxykinase. These three enzymes are actually also the name of three variants of C4 plants. C4 plants have variants as well, depending on the species. Depending on the species, it will decide which enzyme to use, whether it is NAD-malic, NADP-malic, or PEP carboxykinase.
Yes, three variants. About corn, corn, so it uses the first one, the 5A, NADP-malic enzyme. For the millet, millet, use the 5B, NAD-malic enzyme. Some grass, like the guinea grass, people use guinea grass for the pasture, that uses number six, PEP carboxykinase. So, these enzymes, despite of all these different names, the function is only one, decarboxylation. Meaning that the moment it is going to do its work, it will remove CO2 from the compound. That's all its job, despite all these names.
At the end of this reaction, you get your pyruvate or alanine again, this depends on the species as well. It will be transported back to the mesophyll cell, recycled into phosphoenolpyruvate, and then come back again to do the C4 carbon fixation.
Keywords: C4 photosynthesis, Mesophyll, Bundle sheath, CCMs, Calvin cycle
Reference book: Plant Physiology and Development 7th Edition
by Lincoln Taiz, Ian Max Møller, Angus Murphy, Eduardo Zeiger
Watch full video: https://youtu.be/0rBZkk8AYRg
Watch Introduction of CCMs: https://youtu.be/e3pR9WMA8yU
Attribution 4.0 International — CC BY 4.0 - Creative Commons
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