Photosystem II: The Light-Fueled Engine of Life's Oxygen Factory

The world that we see around us has all sorts of different life forms, some are like giant structures that come out of the earth and have umbrella-like foliage, yes, the trees! some are so tiny whose existence we might not know if not for the advent of the microscope, bacteria, viruses and the list go on and on. Some creatures are ginormous like the blue whale, through the artery of whose, a man can crawl through. There are birds, reptiles, fishes of all kinds, and how marvelously diverse it is. 

Because of such a thriving biodiversity, a lot of attempts have been made for a long time to classify the organisms based on a variety of criteria like if it's an animal or a plant, what sort of cells it is made up of, etc. The popularly accepted classification was given by R. H. Whittaker in 1969 which divided the life forms into 5 kingdoms called Monera, Protista, Fungi, Plantae, and Animalia. Then another widely accepted classification was given by Carl Woese who gave 3 domain classification in the late 1970s in which he divided the organisms into Archaea, Bacteria, and Eukarya.

But all this life wouldn't have evolved or existed without the existence of plants. And yes, this may seem very cliché point out due to the contemporary rise in the ''save the environment, save the earth, save trees'' movements, but indeed it is a fact. Thanks to Photosystem II that made our environment habitable by generating oxygen and releasing it out, though it made it as a byproduct but our world almost would be swept out of life without oxygen, and we all know this so obviously. 

But what is photosystem II? We can break down the word to understand it better. The word 'photo' came from the Greek word "phōs" (φῶς) which means light! We use words like Photograph which means an image created because of the action of light on a light-sensitive material. And photon which represents the quantum of light or any other electromagnetic radiation. And now if we connect the word 'system' to it, then we can understand that Photosystem is like a machine that operates only when there is light and it indeed is the case. Photosystems are like one of the many other greatly engineered projects of the nature that we aren't able to fully understand but have been fascinated by, for a long long time. 

So, when I said that photosystem II makes oxygen and mentioned plants, something that naturally comes to us is photosynthesis. Well, yes, again photosynthesis means synthesizing starch or other products by using light. So, Photosystems are like functional units of chloroplasts, which are one of the cell organelles that we have previously talked about in 'What is chlorophyll and why does it impart only green colour?'. Photosystems are present in the thylakoid membranes which are the chloroplast membrane invaginations that are chipped off to form a flattened sac-like structure. They can stack up to form granum, which is present in the stroma. The Stroma of chloroplasts is similar to the cytoplasm of the cell. 

Chloroplast (SOURCE: Ullmann, 2001)

Photosystems are present in these thylakoid membranes, and they are majorly divided into two sections, mainly to distribute the workload. One of the sections is the Light Harvesting Centre, which is also abbreviated as LHC, and a reaction center (RC). The structure can change in photosynthetic bacteria, algae, and higher plants along with the pigments that are present in the light-harvesting centers.

Upon looking at the major two sections of the Photosystem, the word starts to make a little more sense, the LHC is catching the sunlight, and the RC processes the energy for further processes. So, it is indeed a Photo-system. They can be of two types, Photosystem I and II. they both function in different wavelengths of light and have variations in their structure and function. 

Photosystems harvest light energy to split water molecules into Oxygen, make energy-rich molecules, and use them further to make useful products like starch. No other organisms than algae, photosynthetic bacteria, and plants have this capability. But why is this important? 

SOURCE: https://en.wikipedia.org/wiki/File:PhotosystemII.PNG
This is an image of Photosystem II in which the spring like structures are the alpha helices of the proteins it is made up of. We can also see the beta sheets in cyan in lower left side. The photosystem has proteins and pigments and only the proteins are visible here.  

Based on those fancy physics equations which we will not go into, but I am sure are very interesting to study, say that the randomness increases with time. This means that the room we are in is likely to get messier with time, and not neater than it is right now without our efforts. So, similarly, energy is required to reverse this dis-order. Now, if we remember the equation of photosynthesis that says that carbon dioxide and water molecules mix to give oxygen and glucose, we can make an observation here that carbon dioxide and water both are very low in energy compared to glucose and oxygen. This means that the plants can do the reverse of the process than what naturally happens and how are they able to drive this uphill reaction? using the light as their energy source, that they harvest in the LHC! Fascinating.

In the upcoming short essays and stories, we will dive deeper into the intricacies of the beautiful process of photosynthesis.