 We know that in photosynthesis, carbon dioxide reacts with water to form carbohydrates and oxygen. We know how important this oxygen is to us. Most organisms on earth would die off had it not been for this oxygen gas in the atmosphere. And most of the oxygen that is found in the atmosphere has come there because of the oxygen released in the process of photosynthesis. But did you know there is a type of photosynthesis where no oxygen is released? What am I talking about? In order to understand that, first let's recall what we already know about photosynthesis. We know that in green plants, an algae and cyanobacteria, in the light reactions there are these things called photosystems, photosystem 1 and photosystem 2 and what happens is light directly falls on these photosystems. And when it does so, the electrons in the photosystem gets excited. The electrons get excited and they are picked up by these round things called electron acceptors which form a chain called the electron transport chain. So when electrons are lost by photosystem 1, an electron gap is created which is filled by the electrons coming from photosystem 2 which also lose their electrons because of the photons coming from the sun. So the electrons eventually end up in going to NADP which gets converted to NADPH. What happens to the electron gap in photosystem 2? In order to fill that gap, water is split into electrons, protons and oxygen. The electrons that come from water, they go to the photosystem 2 directly. The protons are used to make a proton gradient which ultimately results in formation of ATP and this oxygen, this oxygen has no use whatsoever in photosynthesis. So it's just released in the environment. The ATP that is made by the proton gradient and the NADPH at the end, these two are the reactants in the next step of photosynthesis, the dark reactions. And in the dark reactions is where the carbohydrates are finally produced, the end goal of photosynthesis. All this happens in this type of photosynthesis called oxigenic photosynthesis which produces oxygen. Now let's see what happens in the other type of photosynthesis. The other type of photosynthesis happens in other bacteria not in cyanobacteria and some other bacteria which are also autotrophic. For example purple bacteria, green sulfur bacteria. In these bacteria as you can see there is only one photosystem not two and this photosystem can be of the type one or the type two and it acts as the same way as photosystems to light falls on it and then the electrons get excited, they are lost to electron acceptors which finally reach NADP sometimes it's just NAD. So it can result in either NADPH or NADPH and the rest is the same, it goes on into the dark reactions and then carbohydrates are produced. But what happens in the electron gap that is produced in the photosystem is what is interesting. No water is split in this case instead hydrogen sulfide H2S is split into protons and electrons and sulfur. So electrons go into the photosystem, the protons go to form the proton gradient, the ATP is formed, ATP and NADPH are formed are the end products of the light reactions even here. But the sulfur, this is different, no oxygen is released but sulfur particles are released instead by these bacteria when they conduct photosynthesis. So since no oxygen is released this type of photosynthesis is called an oxygenic photosynthesis and most often the electron donor in this case is hydrogen sulfide but sometimes there are other electron donors in some bacteria like for example hydrogen could be an electron donor, ferrous ions, thiosulfate ions or even some organic molecules, a lot of things but not water. Now this type of photosynthesis, the anoxygenic photosynthesis actually is the more primitive one. It is the more primitive variety of photosynthesis. It was there even before oxygenic photosynthesis came into the picture when there wasn't a lot of oxygen around in the atmosphere that's when the bacteria whoever could photosynthesize would do anoxygenic photosynthesis. So we can kind of tell that this is more primitive than the other type of photosynthesis by looking at the two of them side by side. So there is only one type of photosynthesis system in the anoxygenic type and there are two photosystems and see how much more complicated and more sophisticated it is the oxygenic photosynthesis. So this shows that this is more of course there are other genetic studies analysis have been done which shows that this is more recent oxygenic photosynthesis in fact has evolved from an oxygenic photosynthesis but thankfully the oxygenic photosynthesis did evolve otherwise life as we know it would not exist.