 Welcome back to the NPTEL lecture series on bioelectricity. So, in the previous class while talking about the photosynthesis, we distinguished between the light reaction and the dark reaction. And I mentioned that I will be only talking about the light reaction in the electron transport phenomena, which essentially is an inspiration to develop next generation of solar cells where we will be able to replace silicon based panels with some kind of molecules which are much more cheaper, much more easy and much more sustainable. So, in the light reaction section we I enumerated that there will be six point initially I will be taking care. First one will be the structure of the chloroplast which we have discussed in our previous class and the basic reaction of water plus carbon dioxide making carbohydrates plus oxygen. And in this class we will be discussing two topics. One is all those historical landmarks where this simply simple reaction of carbon dioxide and water forming carbohydrate and oxygen. Who are the people who set the ball rolling for us wherever we are today. It is because of those seminal contribution made by these individuals who ensure that you know follow the track right. And after that we will be talking about the structure of the chlorophyll and the different types of chlorophyll and their absorption spectrum and how the slight structural differences change their spectral properties. So, let us start with it where our first slide will be the one which we enumerated in the last class that is we will be talking about the structure of the chlorophyll which is done. We are done with the we are done with the basic reaction and let us now move on to the discovery of the basic reaction CO2 plus H2O forming CH2O N plus oxygen. As back as 1700 plant releases oxygen discovered by Joseph Pristley. The experiment which was done by Pristley was fairly simple. What he did is something like I will show you. So, for example you have a plant out here some kind of indoor plant this and so you grow this plant inside a chamber like this. So, where light is falling and it is connected with whatsoever gas is being released it is sealed inside you there is another chamber where you have an animal. So, we know that if one individual is confined inside a room without supplying oxygen this individual will eventually die because we need continuous supply of oxygen and that holds true for any animal which survives on earth it depends on oxygen. So, this was the experimental setup where it was proved by Joseph Pristley that essentially during the process plant are releasing something called oxygen and that was the beginning of the journey of understanding what we understand today as the modern photosynthesis. So, let us highlight this. So, this was basically coming back. So, this was this component that it is evolving oxygen was by Joseph Pristley and this was back in 1780 almost three centuries back. Next one to follow was the discovery that this process requires light was by a gentleman called Jan Eigenhaus. He discovered that this process can only happen in the presence of light. So, essentially in the last class while I was telling you people that photosynthesis could be divided into light reaction and dark reaction. So, this light reaction is dependent on light was also discovered. So, let us look at this point. So, here is the light which is so adding the new component each new light and this discovery was made by Jan Eigenhaus that light is involved in it. Followed by this is the next discovery of carbon dioxide. This process is being regulated by carbon dioxide without carbon dioxide suddenly regulated this the essential component of this or the reactant of this whole machinery is carbon dioxide. This was done by another scientist called Joe Senebier carbon dioxide this credit goes to. So, now we are left with who discovered water that water is involved in it. There was another gentleman called Theodore Sausser. Theodore Sausser is credited with the discovery that water is involved in this process was Theodore and this whole reaction that carbon dioxide plus water in the presence of light forming carbohydrate plus evolving oxygen was completely put in place by Meyer. So, just this whole thing that there is a transformation of light energy into chemical energy by Julius Robert Meyer essentially solar energy to chemical energy. So, overall if you look at it this is pretty much is the scheme of photosynthesis where there is a carbon dioxide sequestration taking place and if you currently see this is one of the challenging problem in nature how we could reduce the pollution by you know sequestering carbon dioxide. So, the complete vegetative cover of the on the fluid of earth essentially. So, carbon dioxide sequestration. So, during this whole process of photosynthesis almost 10 to the power 10 tons of carbon is forming organic matter. In other words 10 to the power 17 kilo calorie of free energy is made by plants globally. So, globally the plants are producing that much energy by ensuring the carbon dioxide available along with water in the presence of light transform it into biomass and even if we could you know mimic part of it this will be a big benefit to mankind and there is enormous effort in several countries across the world on carbon dioxide sequestration going on. Somewhere or other we have to reduce the pollution how we can reduce the pollution we could sequester carbon dioxide. People are using different kind of algae different kind of other mechanism to sequester it. So, now coming back after this. So, with this brief historical perspective about how this simple reaction has evolved since 1700 which is almost now 78 almost 400 years nearing 400 years. So, now what we will do we will talk about the fundamental molecules. So, if you go back here we go back to the structure where we were kind of here. So, this is where the light is falling you see this black and the first molecule which gets activated by it is chlorophyll. So, now what we will do is we will talk about the structure of chlorophyll. So, before I get into the structure of chlorophyll just for a revision sake I will just go through the electromagnetic radiation who stands where where the IR is standing and all those things I will do that first. But even before that let us give you an overall outline what exactly is happening in photosynthesis. So, essentially what happens is when the light falls on the plants and it hits upon the chlorophyll molecule chlorophyll molecule ejects an electron fair enough as soon as a molecular species ejects an electron it gets oxidized. Now this electron which is getting ejected travels through a cascade of molecules. So, what it does this electron goes and attack another molecule. So, as soon as that molecule accept the electron it gets reduced. But then it comes back to its ground state by donating that electron to the next molecule and from there it donate to the next molecule. So, likewise this electron travels or hop through a series of such molecules and then eventually in that process it creates a proton gradient across the thylakoid membrane. And this proton gradient essentially leads to the synthesis of a energy rich molecule called adenosine triphosphate and NADP which further acts in generating glucose molecules which is part of the dark reaction. But you remember when I started I told you when the light falls on the chlorophyll it ejects an electron. So, that is chlorophyll molecule becomes oxidized. So, but if that is the case in no time all the chlorophyll molecules will become oxidized eventually the plant will die because the machinery cannot function further. But that does not happen this molecule which gets oxidized is put back into its ground state how it is being done. So, what happens this is supplied with another electron this electron comes from another part of the photo system which is called photo system 2. So, the part what I just now describe it photo system 1 I will be coming into the real molecular details. So, do not worry about it I am just telling the global scheme of things ok. Now, from photo system 2 exactly the same event happens light falls electron gets ejected that electron through a cascade goes and brings back the oxidized electron into its reduced state or into its ground state. But in that process there is another photo another chlorophyll molecule which is getting oxidized because it has donated the electron. So, now that second molecule of chlorophyll. So, say for example, let me just draw it for your understanding sake say for example, this is one chlorophyll molecule CHL or the light falls on it as soon as the light falls it ejects an electron once it ejects an electron. So, this is then oxidation reaction O. So, then of course, I told you the electron travels through a cascade and does its job do not worry about it, but how to bring back this chlorophyll back to its ground state. So, this is supplied with another electron out here from another chlorophyll molecule if I call this as chlorophyll 1 then this is chlorophyll 2. Now, as soon as this chlorophyll 2 donates this electron this chlorophyll 2 also gets oxidized. So, it has to be brought back to its ground state. Now, how it is being brought back to its ground state? What nature has designed is something most abundant electron donor and the most abundant molecule on earth is water. So, it is there the water molecule gets a split up. So, what essentially happens is something like this you have the water molecule of an electron. So, this is pretty much what is happening and these electrons what you are seeing are the ones which eventually bring that chlorophyll back to its ground state. So, essentially a photosynthesis what we will be talking about we will be talking about three things. We will be talking about this chlorophyll which forming photosystem 1, this chlorophyll which for and this chlorophyll molecule is coupled with this wonderful water splitting machinery, water splitting machinery is essentially a manganese cluster. This is the sum total of this thing and very interestingly this manganese cluster which is splitting the water is fairly conserved all over the photosynthetic species which has evolved on the flow of earth. It is fairly very well conserved and one by one we will take up all these things, but in order to understand this whole process we have to first of all understand the structure of the chlorophyll. So, this is the whole thing and you might wonder that why there are what are the catches of this game. So, just to again to give you a global perspective on this thing if we could essentially split water the way a plant does which is probably the most efficient means by which a plant does it then we can produce a lot of hydrogen out here and we could use this hydrogen because this is one of the challenging problems. If this hydrogen could be used for plus enumerate where all we get could be used for fuel cell this is what we are going to study once we will finish the manganese cluster fuel cell. Now, if we understand these if we could really mimic these kind of a structure we can make efficient plant cheap solar panels and coupling this with these kind of machineries we can really have a sustainable energy sources. So, this is overall the scheme of things what we are trying to understand that is the sole reason why we are trying to understand each one of these electron transfer which are happening in biological machines which if we could emulate even one or two percent of it of course, maintaining the efficiency we can really solve some of the major energy related issues across the world. So, now coming back where I took the detour try to you know give you a global perspective of this whole subject why we are kind of you know intensely across the world people are trying to understand these some of these bioelectrical phenomena and the chemicals involved in it. So, next what we will do I will just give you an overall outline of the spectrum just draw it for recap and then we will move on to the structure of the chlorophyll molecules. Now, coming back to the spectrum structure of the spectrum where what lies where. So, put it like this visible you have the visible then you have the IR then you have the microwave then you have the radio wave on the other side you have UV x rays and gamma rays. So, if you look at the zone we are talking about the visible spectrum is around 380 nanometer to this is all in nanometer 380 nanometer to 750 nanometer you have your UVs around 280 nanometer x rays 100 and these are less than 1 nanometer this is all in nanometer whereas, sorry you cannot see the values let me draw the values again. So, visible spectrum we are talking about visible spectrum into 380 nanometer to 750 nanometer in the IR spectrum you have more than 1 millimeter then in the radio you have sorry the microwave you have more than a meter and the radio waves which are around 1000 meters and the UV we are talking about 280 nanometer the range of 280 nanometers and then you have the x rays which are around 100 nanometer and gamma rays which are less than 1 nanometer. So, this is the kind of a spectrum and whatsoever we will be talking about we will be talking about the visible spectrum. So, within the visible spectrum starts with 380. So, you have violet, indigo, blue, green, yellow, orange and in the green you are around 560 and yellow around 600, orange and of course, red and red around 750. So, this is pretty much is the spectrum where we will be talking about the absorption of the chlorophyll molecules. So, now coming back to the structure of the chlorophyll molecule. So, if you remember the structure of the hemoglobin molecule which is involved in carrying blood all over your carrying oxygen in the blood all over your body. So, it has a poly pyrrole ring and in the center you have a iron. In the case of chlorophyll the molecular architecture is fairly the same almost the same if not exactly, but only difference is that in the center there is a magnesium instead of iron there is a magnesium. So, I will draw the structure and the chlorophyll are of two types type A and type B and type A and type B has some small molecular differences I will highlight that and that molecular differences leads to the difference in their spectral properties. Let us draw the structure of the chlorophyll now. So, I told you. So, there is a manganese magnesium cluster out here. So, you have the manganese in the center which is all fine the nitrogen, nitrogen, nitrogen and nitrogen. Then up to this it was all right and the mistake what I did was out here. So, basically another bonding which is taking place out here which ok. So, here you have oxygen out here you have a hydrogen here you have 2 CH 3 and then out here you have slightly moon modified and hydrogen here is a CH 2 there is a CH 2 and you have this R this R is important because this is the R where the difference are starting ok. We will come back to this weight CH 3 says H here you have CH 3 you have you have CH CH 2 and then you have R group out here the 2 R groups. So, I will tell you which one is actually that causes the second you have to redo this again ok. We have R group out here you have CH 2 CH 3 and you have another CH 3 molecule out here and it is almost like this then the double bond here double bond here ok. So, this R group let me just highlight this R group what you see this R group is actually I have to go to the next straight. So, this R group is equal to CH 2 CH 3 is a long side chain out there CH 2 CH and CH 3 CH 3 it is this R group which is attached there ok. But I showed you another R group which is out here which is in green this R group is very important ok. Now coming back to the. So, if you have seen this structure this is a very very very symmetrical structure it is really I myself cannot remember this structure all the time I kind of I have to refer to this structure ok. But it is really easy to draw this structure as long as you do not have to write those huge side chains which are there. But with this basic structure there are there are two forms of chlorophyll which are formed and that change in that form comes in that other R group go back to the structure now if you look at this out here sorry this one and no circling this R group this R group varies what happens is this that R group what is there could be two types either that R group could be a CH 3 group methyl group or a CH O group and if it is a CH O group then this is called chlorophyll B and if it is a methyl group this is called chlorophyll A this is the basic difference between chlorophyll A and chlorophyll B and because as I was telling you because of that one simple change at one of the R groups it is a spectral properties slightly varies and that is really helpful because that way there is no rigid single spectrum there are staggered spectrum which happens because of this in terms of the absorption. Now let us look at the absorption spectrum of chlorophyll A and chlorophyll B back to the spectrum part let us get the access right absorption coefficient which is as a matter of fact the absorption coefficient of chlorophyll molecule is among the highest found in nature and that is why nature has probably selected chlorophyll as the molecule of choice for its functions ok. Now chlorophyll B chlorophyll A with red so chlorophyll A is something like this so chlorophyll A the spectrum which starts to taper down like this and then again starts to pick up and we have another peak out here and then it tapers down. So this is chlorophyll A which is in red now chlorophyll B I will be drawing it in blue chlorophyll is slightly staggered and picks up has its peak out here and then again it also tapers down like this and then starts to take up somewhere out here and one second just one second let me let me draw this spectrum again the chlorophyll B starting like this here and it comes back and somewhere out here. So if you look at both the spectrum very carefully we will observe something as if chlorophyll B is in between the two extremes of chlorophyll A so that way what is happening is that this if the total number say for example if a cluster has say 5 chlorophyll B molecule and 5 chlorophyll A molecule they will be absorbing a certain amount of light within a certain way but if I keep on changing the numbers if chlorophyll A is number is higher than chlorophyll B or something so your total absorption is going to change. So that is what gives it an edge by having not having a single chlorophyll molecule but instead having two chlorophyll molecules which ensures that you can you know tweak or play around with the absorption of the molecule at a specific gain. So this is about the structure of the chlorophyll molecule. Now next pertinent question what we are going to answer or going to kind of you know try to understand is something called a reaction center. So if I take you back to the some of the previous slides okay so again coming back to this slide where the light is falling and you could see that sun light is falling and then thylakoid membrane there are chlorophyll molecules which are getting oxidized okay and then again coming back to the ground state okay. So now does so now let us imagine a situation okay so suppose this is a part of the leaf and there are lot of chlorophyll molecules these are all the C's stand for the chlorophyll molecules okay when the light falls here or does all of them simultaneously start ejecting electron and gets activated it has been observed that the story is really not like that what happens is that so within a pool of chlorophyll molecules there are some very specific centers called the reaction centers say for example this one say for example this is the reaction center which I am circling with with the red. So what happens when the light falls so there is a transfer of electron like this taking place vibrational energy transfer they are getting excited and eventually they reach to that unique center which is called a reaction center where the whole process of electron emission starts other than that this whole energy is being transferred. What we do not know is that who determines the reaction center and does the reaction center changes as the plant is living its life we do not know this but this is one of the very unsolved mysteries because I will come to that how it has been discovered because in the next class that is what we are going to do how this reaction center itself was discovered but it is not all the chlorophyll molecules are taking part in you know this whole cascade. One of the thing which you could speculate is that probably because of this reaction center concept the longevity of the leaf is increasing because it is not all the molecules are at excited stages because of the light. So with this concept I will close in on this class and in the next class what we will do is let us go back where we were. So we have done with the discovery of the basic reactions we have done with the structure of the chlorophyll to trap solar energy and now we have just initiated with the reaction center and from the reaction center we will talk about the different experiments which have been performed and from there we will move on to the next part of it and how these will be translated in terms of energy production. Thanks a lot.