In this video, we're going to begin our introduction to photosystems. Photosystems are defined as complexes of pigments, proteins, and other molecules found in the thylakoid membrane. Recall from our previous lesson videos that these thylakoids are the green pancake-looking structures that are found inside of chloroplasts. To refresh your memory, down below on the left-hand side, we're showing you the image of the chloroplast organelle. Recall that on the inside of these chloroplast organelles are these green-looking pancake structures without, and these green-looking pancake structures are again called thylakoids. If we zoom into just one of these green-looking pancake structures here, that is where we will find the photosystem. Notice in this image here, this is representing the membrane of the thylakoid. We have the thylakoid membrane here. On one side of the thylakoid membrane, we have the thylakoid space, which is the inside of the thylakoid, the inside of the green pancake itself. Above the thylakoid membrane, we have the stroma, which is just outside of the green pancake but still inside of the chloroplast. Notice that embedded within the thylakoid membrane, this is where we find our photosystem, this complex of pigments, proteins, and other molecules found in the thylakoid membrane.
Photosystems are composed of several light-harvesting complexes that surround a reaction center. Notice that we have our light-harvesting complexes color-coded here in this purplish color, which color codes to the purplish colors that we see down below. We also have the reaction center up above that's in orange that color codes to the reaction center that we have down below. If we take it our look at our image down below, notice once again that here in purple over here and over here, we have this light-harvesting complex. So we can go ahead and label them as the light-harvesting complexes. Surrounding the light-harvesting complexes that you see here and over here on the right, notice that they are surrounding this center region here which has the reaction center. You can see the reaction center here in orange right in the middle, surrounded by light-harvesting complexes as we described up above.
Within the light-harvesting complex, you'll notice there are these teal circles, which represent accessory pigments, and these accessory pigments are going to be helpful in absorbing photons of light. You can see photons of light here interacting with the light-harvesting complex as its name implies, and it's able to interact with light because of these accessory pigments that absorb light. If we take a look at the reaction center once again here in the middle, notice that the reaction center has these two green molecules right here that are chlorophyll a molecules, the predominant pigment found inside of chloroplasts. Also, notice that in red, right here, there is the primary electron acceptor, which we'll get to talk more about later in our course. But for now, what we can see is that this photosystem here is going to be really important for absorbing light and allowing chloroplasts to absorb light. That's again because it contains accessory pigments and it contains the predominant pigment chlorophyll a.
As we move forward in our course, we'll get to talk more about photosystems and how they're involved in what's known as the light reaction, which again we'll talk about more in a later video later in our course. But what's important to note now is that most plants are actually going to have 2 photosystems. These 2 photosystems once again are going to be involved with performing the light reactions of photosynthesis, which again we'll get to talk about these light reactions of photosynthesis later in our course. But for now, this is our brief introduction to photosystems, how photosystems are found inside of the membranes of thylakoids, these green pancake structures within chloroplasts, and how photosystems are important for absorbing light and are involved with the light reactions. We'll be able to get some practice applying some of these concepts as we move forward in our course. So, I'll see you all in our next video.