Hi everybody and welcome back to Miss Angela's Biology Classroom. In today's video we are going to be looking at photosynthesis. We are going to be looking at the light dependent phase and the light independent phase also known as the Calvin cycle. If you're new here make sure to subscribe and give this video a thumbs up and turn on your notifications because I post a new video every Thursday.
It's important for us to familiarize ourselves with the photosynthesis equation so that we know what components are required for photosynthesis. And then what are the ultimate products that we're trying to get? So it's important that we look at the balanced equation. And that's what we see here, that the left side of the equation balances to the right. Many of you may have different kinds of textbooks where it may or may not be balanced.
And what I'm referring to when I say balanced is these numbers sitting in the front of like the carbon dioxide and the water. Some textbooks don't have those numbers. Neither of them are incorrect or. more correct. It's just the way in which the textbook has chosen to describe the information.
But I would personally say that the balanced equation, as you can see here, is the most correct answer to give. So let's quickly look at the components. So in order for photosynthesis to occur, we already know we need carbon dioxide and we need water.
Now we need two important, and I want to call them materials slash substances, but they're not technically part of the equation. substances that we need that we can absorb. For example, we need sunlight.
Technically, sunlight is not a material. It's an energy that we need. And likewise with chlorophyll, yes, it's a substance, but it's not something that the plant is necessarily absorbing.
But you need those two things in order for photosynthesis to occur. And at the end of this whole process, we want to make glucose, which is a sugar for energy. And of course, we're going to make oxygen as a byproduct. So now let's take a look at the physical locations of our two reactions that we're going to cover in this video.
If you are not very familiar with the structure of a chloroplast, you really need to go back and look at what is a chloroplast, thylakoids, etc., and cover the basics so that you have a very good understanding of where these two reactions are taking place. And we've got two. We've got the light-dependent reaction, which takes place in the thylakoids.
If you remember, these are those stacked, sort of, they look like... coins on top of one another. They sit inside our chloroplast.
And so that's where the light dependent reaction is going to take place. And then the Kelvin cycle, also known as the light independent reaction, is going to take place in the stroma, which if you've forgotten is the liquidy filling inside of a chloroplast. Now you'll notice in this diagram that there is an important flow of, let's call it, substances and products.
And we always start on the light-dependent side. And that's because no photosynthesis can actually take place without fulfilling the dependent reactions first. And a misconception is that sugars and things are made during the day, but they're actually not. Sugars are made at night during the Kelvin cycle.
So think of the light-dependent phase as a phase where we are charging the batteries up. full of energy and imagine the batteries or the solar panels being the thylakoids. And then at nighttime, we take all that energy and we use it during the Calvin cycle to produce our sugars. And so we circle through and that means we go through the thylakoids, we make these products, which I'm going to point out now in the next part of the video.
We then go into the Calvin cycle, which is step two over here. And then of course we circulate back during the day and it's a continuous process. between the two reactions.
So let's look at the first phase, which is the light-dependent phase. This is the phase that happens during the day. And essentially what we're going to do is we're going to take sunlight and we're going to divide a water molecule into its two components, hydrogen and oxygen. And we do this through a process called photolysis, which photo means light, and lysis is to cut or to break up. And so we're literally cutting and breaking with light.
So it's a very simple process. Sunlight is going to land on the plant. It's going to be absorbed into the chlorophyll. And this whole process takes place inside the thylakoids. Remember, those are the disc-shaped structures that I spoke about earlier.
And what happens is the sunlight comes along and it splits our water molecule into its two components, one of them being hydrogen, which is a really important component of what we are discussing now in terms of... where we're going to use it. So we need that hydrogen. We don't want to lose it.
I'm going to tell you what we do with it next. And the other component that we divide here is oxygen. Now let's talk about oxygen quickly, because that's an easy one to discuss where it goes.
Once we split the hydrogen and the oxygen from one another, the oxygen is given off in the form of gas. And that is where we get oxygen from plants. That's where animals then use it to breathe and to respire. Now let's get back to that hydrogen.
This hydrogen over here has two components that it gives us. One, it's the actual hydrogen ion itself, but something else that comes off of this hydrogen is its very excitable electron. Now, this electron is really important because it's going to help us produce the substances that we need for the light independent phase or the Kelvin cycle.
Now, these two substances, the hydrogen and the electron, cannot be left as is because both of them are very excitable. If they're left alone, they're causing too many chemical reactions and we don't want that. So first things first, what we do is we take our hydrogen.
And we're going to attach it to a hydrogen acceptor called NADP. And NADP is going to pick up our hydrogen. And it is literally like a little wheelbarrow, as I've used as an example before. And it becomes NADPH.
And the purpose of this is so that NADPH will then deliver the hydrogen to the light independent phase at night. Now, what are we going to do with this electron? Now, where did this electron come from? It is from the hydrogen. And I want you to imagine the electron being highly energized, right?
It's got a lot of energy coming off of it. Likewise, with the hydrogen, very excitable, lots of energy. So they have to be looked after. And what we do with that electron is we use it to phosphorylate, which means we're going to add a phosphorus.
In this instance, we use the term photophosphorylation, which is where you take a ADP molecule, which if you remember is an energy carrier, we add a phosphate, and we produce the substance ATP. So now what we have done is we have created two products at the end of this phase. We have created NADPH.
which is our hydrogen acceptor and carrier. So it's carrying our hydrogen because we need that hydrogen to make glucose. And we have made a molecule of ATP, which is our energy carrier. In other words, it's got lots of energy stored inside of it. And we're going to use that to also make our glucose.
So essentially what we have is we have the energy we need for the glucose and we have a building block, which would be that hydrogen. These substances are then carried into the next phase, which is the Calvin cycle or the light independent phase. Now we move into the light independent phase, also known as the Calvin cycle. Now, depending on the level of grade that you're in, or if you're doing an AP bio course, you might be doing a slightly more advanced version of the Calvin cycle, but this is the basics so that you can really understand it quite well.
And this phase requires no light. So no light is essential for this. This mostly happens at nighttime or when there are low levels of light. Now, I'm going to explain to you substances that perhaps you're not sure where I got them from. But once I go through the full cycle, you'll see where a substance has just sort of magically appeared.
Now, the light independent phase takes place inside of the stroma. So this is the liquidy part, the liquidy filling of our chloroplast. So we start off with a substance called.
ribose biphosphate, and it has five carbons. So that's its backbone. And you're going to see where I get this five carbon compound very, very soon. Now, what we do, remember, this is photosynthesis. So that means we've used the water component in the beginning of the equation, but we haven't used the carbon dioxide component yet.
And that's where this comes in. And so what happens is our ribose biphosphate, which you can abbreviate to the RUBP in an exam or in a test, that is going to combine with a carbon dioxide molecule. Now, that carbon dioxide molecule would have been brought in via the stomata of our plant.
And so that would have been pulled in by the leaf. And so now what you have is you have a six carbon compound, which I would like to point out is unstable. And that's a good thing because stability in molecules makes it really hard for us to access parts of them and to use them.
But this is a very unstable six carbon compound and we want it to be unstable. Please don't think that this six carbon compound is glucose. It is not.
Just because it has six carbons doesn't automatically mean that it is glucose. So. Now, because this is a six carbon compound that is unstable, it actually almost automatically breaks down into two identical molecules called PGAs, and they both have three carbons each.
Now, for simplicity in my video, I'm only going to talk about one of the PGA molecules at a time, just because it would be difficult to keep track of both of them in the picture. So I'm going to talk about one. Now, this PGA molecule that we have here, we need to do a few things to it before it can become a glucose. As we know, glucose has six carbons, 12 hydrogens, six oxygens.
Now, right now, we only have three carbons. And we don't definitely have enough hydrogens. And we definitely don't have enough oxygens either. So we need to add some of those components first.
So to do that, we are going to use some of our previously made substances from the light-dependent phase. And the first one that we are going to use is our ATP molecule. And our ATP molecule is going to come along. And it's going to deliver some energy. And that energy...
is going to assist us in fusing together our molecule, making it more stable, and allowing our hydrogens and our oxygens to attach successfully. And when that ATP comes along, it delivers some energy, and it breaks apart into ADP and P, which is phosphorus. That phosphorus or phosphate does not attach to our PGA molecule.
The second component that comes along, which we made earlier, if you remember, is our NADPH molecule. So along that comes, and what it delivers is its hydrogen. As I mentioned to you earlier, the NADPH comes along, it delivers its hydrogen, and that hydrogen is left behind, and we are left with a substance called PGAL3C. So it's got three carbons still. Except this time, you'll notice I've added a little blue ball on the end to represent the hydrogen.
We've now made a component that can become glucose. So now here's the thing. We have two options.
This PGAL molecule has the ability to go on to become a glucose molecule. So I'm going to add that in on my diagram so you can see that. So there is a possibility that the PGAL...
can become glucose, right? And that would mean we need to add a few more things, right? Because glucose has six carbons. So we need to add three more carbons and more hydrogen and oxygen, which is possible.
We can combine two PGAL molecules together to do that. But technically, most of the PGALs don't actually become glucose the first time around. They've got to go around a couple of times before that happens. And so, and this brings me to the beginning of the cycle, some of these three carbon PGAL molecules are used to reform the five carbon ribulose biphosphate that we see up here.
Now, I know you're thinking, but then how do we go from the three to the five carbon? Well, for this simplistic example and explanation, I'm not going to explain how you go from the three to the five. I just want you to know that that's what happens. Depending on the level of schooling you're at or whether or not you're doing advanced biology, you would then need to learn what happens in between the three-carbon and the five-carbon compound.
And now at the end of the light independent phase, you have produced glucose. And that glucose is obviously stored as sugars and starches. And let's not forget what happens to our two components that we are consistently cycling again.
And that's ADP and it's phosphate and NADP. They are not lost because where do they go once they've delivered the goods? They go back to the light dependent phase and they will be able to re-energize themselves. They'll be able to pick up another hydrogen and they'll be able to do this whole process again. And that's why you can see.
A plant has to have some photoperiod. It must have some period of time in the sunlight. Otherwise, it cannot pick up a hydrogen and it cannot form ATP.
As always, I like to sum up our lessons with a quick terminology recap because terminology is so important for biology and this is a great way to also make flashcards on these terms. So we started off with looking at the light dependent phase. This is the phase that happens during the day. It is where we collect sunlight in the thylakoids and we go through a process called photolysis, which is to cut with light.
What are we cutting? We are cutting water into hydrogen and oxygen. And the hydrogen itself joins with a carrier called NADP to form NADPH. I'm going to use that component at night. And then we use that excited electron that we get from the hydrogen in a process called photophosphorylation.
Yes, I know, very long word. Basically, it means photo, light. And then phosphorylation means to attach a phosphate, meaning I use light to attach a phosphate.
Who am I attaching the phosphate to? Well, I have an ADP molecule, which is an energy carrier, and it carries energy. And so the only way to carry the energy is to add a phosphate. So we go from ADP to ATP, so diphosphate to triphosphate.
And that is photophosphorylation. It's using light to attach a phosphate. Now, the ATP and the NADPH enter the Kelvin cycle, also known as the light-independent phase. It is either at night or when light levels are very low.
And this occurs in the stroma, which is the liquidy part of the chloroplast. Now, in that liquidy part of the chloroplast, we have a substance called RUBP, which is ribulose biphosphate. It has five carbons. carbon dioxide from the atmosphere joins with it, makes an unstable molecule, and that breaks down into a PGA molecule, two of them.
Those PGA molecules undergo an addition of energy and hydrogen, and it makes a PGAL. And some of those PGALs become glucose, the sugar, which can be used for energy or also to be stored as starch. But the majority, if you remember, go back into forming the ribulose to start the whole process once more.
As always, I hope you enjoyed this lesson and you learned a lot today. Make sure to subscribe, like, and turn on your notifications, and I'll see you next time. Bye!