Imagine you're a plant. You're chilling out in the sunlight, absorbing some carbon dioxide through these tomato pores, absorbing some water through your roots, and you're just chilling. But then you realize you need to make food too. And how do you do that?
We all know that plants make their own food by the process of photosynthesis. In photosynthesis, what happens? Carbon dioxide, water in the presence of sunlight is used to produce glucose.
That is the main source of energy and oxygen of course is released as a byproduct. And then plants use this glucose to produce energy by breaking down this glucose. But if you take a look at this light reaction, which is what is actually taking place when plants are absorbing this sweet sweet sunlight. If you take a look at that light reaction, it involves water, ADP+, NADP+. So, in the presence of sunlight to yield ATP, NADPH and oxygen.
And ATP as you all know is the energy currency. This is what gives the plant energy basically and NADPH is an electron carrier. Oxygen of course is the byproduct.
Where in this reaction is glucose? Because photosynthesis involves using carbon dioxide and water to produce glucose. Where is glucose?
There is no glucose here, only energy. and NADPH are produced. Well, that's because this reaction, rightly called the light reaction, involves just the production of ATP and NADPH.
Later, in a reaction known as the light independent reaction, the products of the light reaction, which are the ATP and NADPH produced, are used along with the carbon dioxide that is absorbed to produce glucose. And this process is known as carbon fixation. Fixation because whatever carbon dioxide is there in the atmosphere is fixed in the form of glucose, into the form of glucose. That is why it is known as carbon fixation. And this is rightly termed the light independent reaction because it doesn't directly depend on light to take place.
Of course, it needs the product of the light reaction which is ATP and NADPH, right? The products of the light reaction are needed but it doesn't need light energy directly for this process to occur. Some people also call it the dark reaction which is a misnomer because this doesn't happen or take place during the night when it is dark, when there is no sunlight. It takes place during the day but it just doesn't need direct sunlight like the light reaction does.
So this light independent reaction is also known as the Calvin cycle in honor of the person who discovered it Calvin Calvin and it's a cycle because of course it's a cyclical process so the process takes place in this manner. So in this video we're going to learn about the Calvin cycle and how it takes place. So let's start and let's just focus on the first step of the Calvin cycle.
Now Calvin cycle involves one of the most important enzymes known as ribulose, bisphosphate, carboxylase, oxygenase. Quite a mouthful. It is abbreviated as Rubisco.
RU for this ribulose, bis for this bisphosphate, carboxylase C for carboxylase and O for oxygenase. So this enzyme which is coincidentally the most abundant protein on earth is what begins the process of Calvin cycle. Now from the name itself you can say that it has two functions. It can catalyze the carboxylation reaction which is addition of carbon dioxide or the oxygenation reaction which is the addition of oxygen.
In this video we are going to focus on just the carboxylation aspect of Rubisco. When we talk more about C4 plants in upcoming videos, we'll talk about the oxygenation function of Rubisco as well. So what Rubisco does is it carboxylates something.
And what does it carboxylate? It carboxylates this 5-carbon molecule known as ribulose bisphosphate or RUBP. That's where it gets its name from, the enzyme, because the substrate it acts on is the ribulose bisphosphate.
So one... carbon dioxide molecule, one carbon reacts with ribulose bisphosphate which is a 5 carbon molecule to give a 6 carbon intermediate. This 6 carbon intermediate is very short-lived, it's very unstable and it immediately breaks down into two molecules of 3-phosphoglyceric acid otherwise abbreviated as PGA. So, six carbons splits into two and each of that has three carbons. This process where the carbon dioxide reacts with RuBP to give this 3-phosphoglyceric acid or PGA, this is known as the carboxylation phase of this cycle.
This is the first phase. carboxylation phase. And this process is catalyzed by the enzyme Rubisco.
Now what happens? What happens to this 3-phosphoglyceric acid? This 3-phosphoglyceric acid is then converted to glyceraldehyde 3-phosphate. And this step requires NLG and NADPH. So we learnt that in the light reaction ATP and NADPH are produced, right?
They are used up exactly here at this part of the Calvin cycle. So, the two molecules of 3-phosphoglyceric acid with the help of ATP, NLG and NADPH are converted to two molecules of glyceraldehyde 3-phosphate. Again, this glyceraldehyde 3-phosphate is also a 3-carbon molecule.
It is abbreviated as G3P and they are also 3-carbon molecules. So, two such 3-carbon molecules are formed. It makes sense, right? There are two 3-carbon molecules here.
There are two three carbon molecules here. And now this step where phosphoglyceric acid is converted to glyceraldehyde 3-phosphate with the help of NLGN and NADPH is known as the reduction phase. This is the second phase of the cycle.
First is the carboxylation phase. This is the reduction phase. So then what happens to this glyceraldehyde 3-phosphate?
One of this glyceraldehyde 3-phosphate or G3P goes ahead to form glucose. The other G3P is used to regenerate this ribulose bisphosphate. And this process also requires energy.
I have not mentioned it here, but it also requires ATP be used and converted to ADP. So this process where glyceraldehyde 3-phosphate is used to regenerate RUBP. is known as the regeneration phase and this is the third phase.
So there are three phases in the Calvin cycle. One is the carboxylation phase where with the help of ribisco carbon dioxide and ribulose bisphosphate RUBP are converted to a six carbon intermediate. In the next step, the six carbon intermediate is split into two three carbon molecules PGA phosphoglyceric acid.
In the next phase of the Calvin cycle in the reduction phase phosphoglyceric acid is converted to glyceraldehyde 3-phosphate or G3P with the help of ATP and NADPH. One of this glyceraldehyde 3-phosphate goes ahead to make glucose and the other glyceraldehyde 3-phosphate is used to regenerate RUBP in the regeneration phase of the Calvin cycle. That also requires the use of energy.
Now, this reaction is not balanced because if you see here, there are three carbons, 1, 2, 3. Somehow, these carbons need to make 1, 2, 3, 4, 5, 5 carbon RuBP. How is that possible? So, let's next focus on balancing this Calvin cycle reaction.
So, instead of just one molecule of carbon dioxide, three molecules of carbon dioxide enters the Calvin cycle and the three molecules react with three molecules of RuBP. So, 3 CO2. plus 3 RuBP. Now you're going to get 3 of the 6 carbon intermediates, right?
And they're going to split into 6 molecules of phosphoglyceric acid because you have 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 18 such carbon molecules here. They're going to split to give 6 molecules of PGA. And the 6 molecules of PGA are going to get converted to 6 molecules of G3P.
out of which one, just one again is gonna go ahead and make glucose. But now you are left with 15 carbon atoms here. 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 carbon atoms here.
That is used to regenerate the 3 molecules of RUBP. Because here also you have 15. 1, 2, 3, 4, 5, into 3, 15. So this is the entire Calvin cycle reaction where 3 molecules of carbon dioxide react with 3 molecules of RuBP to give 6 molecules of phosphoglyceric acid which are then converted to 6 molecules of G3P. One then goes ahead to make glucose and the remaining 5 of the G3P are used to regenerate the RuBP.
But even now, if we see, we are left with only one G3P molecule for the production of glucose and it has only 3 carbons. But the chemical formula for glucose is C6H12O6, which means essentially 3 more carbons are required. How does that happen? Well, the Calvin cycle essentially occurs 6 times.
Then, each time does it take 3 carbons? No. Each time the Calvin cycle turns, one carbon dioxide molecule is fixed. So by the end of six turns, you will have six carbons.
So if you were to balance the number of ATPs as well, so the 6ATP converting to 6ADP is for three carbons. So if you were to take for one carbon dioxide, you will have two ATPs. Similarly, if you took a look at the NADPH, it is 6 NADPH for 3 carbon molecules. So for 1 carbon dioxide, you will have 2 NADPH here in the reduction phase. In the regeneration phase, 3 ATPs are used for 3 carbon dioxide molecules.
So for 1 carbon dioxide molecule, you will have 1 ATP. So totally for every turn of this Calvin cycle, 3 ATP molecules and 2 NADPH molecules are So, at the end of 6 turns, where 6 carbon dioxide molecules are used, 6 x 3, 18 ATP molecules and 6 x 2, 12 NADPH molecules are totally consumed to give 1 molecule of glucose. And that is the Calvin cycle.