 Welcome back to the lecture series on bioelectricity. So, today we are on the 38 lecture. So, we are finished with the animal bioelectricity. We have talked about the plant bioelectricity and in the plant bioelectricity we have talked about the Venus fly trap and touch me not plant. We have talked about in depth about photosynthesis and electron transport and how light harvesting pigments are used by nature to transmit signals and thereby you know synthesizing molecules for their survival. Then we talked about the disensitized solar cells and pretty much now we have a clear overview about several life forms which are involved in transforming energies and in that process generating electricity. So, now these last three lectures 38, 39 and 40th one will be talking about some unconventional systems which are not really well explored in terms from the biological perspective. So, as of now we have all talked about some living systems where we see say whether it is a plant, whether it is an animal, whether it is an insect. So, now we will be talking about a specific kind of ancient molecules which has evolved over the period of time and has some very interesting role in information transfer or in terms of you know energy transduction as well as they have a significant role in the whole evolutionary process to start off with. So, those who have just even a very minimalistic idea about biology know that we it is being believed that you know life has evolved from the genetic materials or from the proteins. So, this is where it all started some form of genetic material either they translate those genetic material into proteins or vice versa there are proteins something some unknown chemical reaction took place which led to the genesis of what we call about life. Yet there are this is not the only theory about life evolution of life there are other theories. So, if you have to broadly classify these theories. So, there is one theory which is called the inorganic evolution of life which essentially believes that the present day life forms what we see has evolved over billions of years from very very simple elemental or you know very simple compounds somewhere or other these simple compounds may have self assembled by some unknown reaction which we at least have an deciphered yet and they form what we call the modern day membrane. And in that whole process those self assembled molecule acted as a template for series of chemical reactions which led to the evolution of organic molecules. And one such theory which exist whenever you will study evolution is called iron sulfur theory. So, if I had to broadly classify the evolutionary theories which are currently kind of debated upon then you will see. So, one is the nucleic acid low by and proteins of course, we do not know this evolution that who came from what and within the nucleic acid we talk about ribonuclease acid and deoxyribose nucleic acid this is one theory the other theory is iron sulfur theory. So, this is the theory which believes life evolved from simple inorganic molecules and as a matter fact the whole idea is very simple what it claims is there is some very specific inorganic molecules very early in the evolution which consist of iron and sulfur which acted as a which self assembled this iron sulfur compounds self assembled forms something what we called as membrane like structure or you can call it an inorganic membrane. Whereas, if you look at the other side you will see here we talked about the membranes which are in very initial classes I have taught you about lipid bilayer. So, these inorganic membranes acted as disbelief is again these are all believed acted as a template for synthesis of nucleic acid proteins and all these kind of different molecules. So, this is what essentially these two different theories kind of proposed. So, there are a lot of warring factions between these theories and no one can say with 100 percent certainty that which one is right or which one is wrong because we cannot really go back 4 billion years back in time to figure out what is true what is right what the reason why I picked it up this topic is totally different perspective. So, if you look at it when I started this course I told you that we are talking about the movement of current across the lipid membrane. So, there are lipid bilayers on those lipid bilayers you have these proteins which are embedded which are called the ion channels and there are several other proteins which are involved in it and it is a potential difference across this lipid membrane in a cell which ensures that there is a flow of current in and out depending on what kind of ions we are talking about this is the cell and this is the lipid membrane and there is a potential drop across the membrane and what we called as RMP resting membrane potential which is approximately minus 18 millivolt depending on the cell type of course now by this time you guys are well burst with it. So, now if this is the case and we will be interested to know about if assumption. So, there is an assumption. So, if we assume that these inorganic molecules leads to the formation of you know nucleic acid proteins and other macromolecules which includes lipids and carbohydrates then it may be a good idea to understand what are the electrical properties of these kind of inorganic molecules if they are the ones which are involved in formation of these kind of lipid bilayer and what we call as modern day cell. So, one of the molecule which has kind of remain in the center stage of the iron sulfur theory is FeS2 which is also commonly called as iron disulfide because there are two sulfur or iron pyrite. So, today's class see the slide to start off with the slide. So, today's class will be on iron pyrite and ancient molecule with rich heritage. So, what I will essentially do I will expose you to the iron pyrite and I will in the class with what are the possibilities where we are heading. So, again I am not going to get into the device development or anything in this section because this is not really meant for it, but I will just give you a glimpse of what really we know about it. So, historical perspective of iron pyrite like if you look at it, it is a ubiquitous molecule as a matter of fact iron pyrite is available all over the place specially if you are walking by the road and there is a drain you know. So, if you just you know go little like you know if you go slightly underneath where there is a lack of oxygen you will see there is iron pyrite present there. So, it is a ubiquitous molecule it is a versatile material which is probably evolved very very early in the evolution very early. So, what we assume is when probably these molecules are evolved this was at least 4 to 6 billions of years a condition when earth was in a very very hostile environment in the early days of modern chemistry it was used as a source of vitro and sulphuric acid this was used to manufacture sulphuric acid and vitro. So, this is overall about the material what is very interesting to know that it has very amazing semiconductor property and it is one of the very potential material for photovoltaic as matter of fact if you look at the development of semiconductors in last 150 years the most landmark is 1940s and 50s when Bardin, Britain and Choclet discovered the crystalline silicon, but story of semiconductor is way before that it was during the time of Michael Faraday there are lot of sulphides which were being suspected and they were showing because it was Faraday who observed that there is a non-linear property current and voltage he observed and he really could not explain it that why there is a non-linear because there is no word called semiconductor it was about insulator or resistors that is it either sorry insulators or conductors there was nothing called semiconductor in between. There after it was Bose who worked on galanacrystals likewise and so forth, but it was known as early as 1876 if you could see Brown it has semiconductor properties and documented as early as 1876 earlier very early in the development of semiconductor and this is I am talking about much before crystalline silicon was even on the scene it was used in the rectifier circuits in the radios, but the problem there is a problem with this soon it was realized that it has inherent stoichiometric problem what does that mean it means that whenever we write something like this if he has to our assumption is that there is one molecule of iron and two molecules of sulphur, but in real life that does not happen it is always 2 minus x and this x could be anything of course, it cannot be greater than a 2, but it could be anything it could be 0.5 it could be 0.1 it could be 0.2 likewise and this is the problem of these kind of molecules there is always a defect it is never stoichiometrically a balanced one, but that leads to a problem since very soon it was realized that there is a stoichiometric problems. So, it was extremely challenging material to develop solid state electronic devices yet this is the most important part yet due to its excellent light absorbing and optoelectronic properties it has gained enormous research attention in the field of photovoltaic material. Though this material is defective yet and we will come later in the lecture that it has some absolutely amazing optoelectronic properties. So, an ancient molecule which probably has revolved from 4 to 6 billion years ago has the ability to absorb sunlight and transform it into some kind of electrical current. So, in the history of the molecules these will be very very first molecule which we should consider as evolution of autovoltaic or p v molecules which has the ability to you know translate light to electricity and very interesting part if you see the spectrum where it absorbs this molecule can absorb a spectrum very far red light. So, it means on your left hand side if you have the u v which is starting from 200 nanometer and it is going all the way to 700 nanometer odd. So, you will see the maximum absorbance is on towards the right which is red far red likewise. So, if we could develop a solar cell out of such materials which are biologically evolved over a period of billions of years chemically evolved this could harness low light situations it is not dependent on 200 or 300 or 400 nanometer more on the left side of the spectrum. So, this interesting property is the reason why there always remain an interest to develop devices out of it and especially in the modern day when we realize that crystalline silicon is reaching it is limit we are unable to minimize the cost. So, we needed a cost effective system which could harness maximum amount of solar energy because it is the solar energy which has been involved in the evolution of life form and most likely would the way solar energy must have worked is what you have understood in photosynthesis. So, the sun light is falling on a light trapping molecule and from that light trapping molecule it is leads to some form of you know electron hopping and leading to is eventually leading to the evolution of energy rich molecule and in that process evolving oxygen and transforming carbon dioxide into biomass. So, one common feature or one common thing what you can see about our evolution is some of the molecules have played a critical role. So, one of them if one of them is chlorophyll and definitely in that same line comes Fe S 2 they all have one commonality if you look at the evolution they all should have ability to cross talk with the solar energy because it is the solar system which is responsible for evolving life. They all are light dependent systems and this is very very important for us to understand that because sunlight is ample in nature. So, if you could harvest sunlight that the same way plants have evolved so successfully they could harvest the sunlight and you know they could make lot of energy rich molecules. Same way if you could harvest the sunlight using these kind of molecules you can generate sufficient amount of power which could you know sustain our energy requirements across the globe. So, coming to the next slide moving on evolutionary theory. So, as I was telling you Fe S 2 holds the central stage in one of the two existing evolutionary theories on the origin of life on earth. The two set of theories trying to explain origin of life on earth is the pre biotic broad theory which is essentially your nucleic acid and protein that RNA and DNA starting point of life this is one. The second one is the hydrothermal iron sulfur world theory and those of you who are keen to understand more about this you can refer to of course, I will be giving in your hand out seminal work which was done by scientist called Wachter Hauser. Wachter Hauser has done significant work on iron sulfur hypothesis as a matter of fact is one of the pioneer who proposed the origin of life in the iron sulfur rich environment. So, having said this let us look at it what are the arguments in favor of iron sulfur evolution of life. So, if you look at evolution so according to the hydrothermal origin. So, I will come to why it is called a hydrothermal origin of life if you used to might have evolved in the pre Archean era in an extremely harsh anoxic sulfur rich environment by spontaneous reaction of a f e s and h 2 s having said this let me tell you this is the condition when possibly this whole iron sulfur world was evolving out here this is the world where there was no oxygen was absolutely no oxygen there was no way that there could be any u v protection. So, the atmosphere is getting formed and temperature was very high there is all over the place there were sufficient amount of hydrogen sulfide in a very harsh and in a very very you can say where for us even to simulate or even to imagine how the life would have been it is really tough. So, under those conditions possibly in the harsh anoxic sulfur rich environment by the spontaneous reaction of f e s and h 2 s possibly it leads to a reaction where we are having f e s 2 and h 2 got evolved. And of course, this reaction is thermodynamically feasible because it has minus 41 kilo joule per mole, but the hydrogen evolved during the formation of f e s 2 is used to derive the carbon dioxide fixation reaction by coupled reaction taking place across the proposed semi permeable iron sulfur colloidal membrane. So, coming back I highlighted this part to you the self assembled inorganic membrane. Now, what this inorganic membrane is doing out here or at least what is being proposed it is doing out there. So, imagine somewhere or other it is self assemble and forming something called you know f e s 2 membrane on these membrane and the f e s 2 is getting formed. So, it has inner side I am just putting the inner side somewhere or other just imagine if this kind of thing has formed and in that process there is evolution of h 2 h 2 the existing c o 2 and this is environment which is filled with you know sulfur dioxide h 2 s c o 2 of course, there was water iron likewise. So, very harsh environment with a very high temperature u v protection. So, there is enormous amount of u v which was reaching the substrate then this it is being really not known how it has happened if at all it has happened c o 2 h 2 this is how c o 2 started getting fixed or exactly what you have seen in photosynthesis where c o 2 is getting fixed. Same thing was possibly happening here where c o 2 fixation reaction was taking place on top of the iron pyrite membrane coming back to the slide if you look at it the hydrogen evolved during the formation of f e s 2 is used to drive c o 2 fixation reaction by a couple reaction taking place across the proposed semi permeable iron sulfur colloidal membrane. So, this is that proposed iron sulfur colloidal membrane and it is believed that these membrane has lot of nickel these kind of ions which are present there which were catalyzing this reaction containing nickel as a catalyst if you see the slide you will see this. So, it was the series of nickel ions or maybe few other ions which we do really do not know which one which all were present there. So, those ions which were present there if you look at the slide now containing nickel as a catalyst. So, what essentially happened is that c o 2 plus h 2 you are getting the first carbon fixed molecule a c o h, but for that reaction the delta g was 30 kilo joule per mole. So, now if you add these reaction 1 and 2 f e s plus h 2 s plus c o 2 you are getting. So, now you have f e s plus h 2 s plus c o 2 this is what is happening you are getting f e s 2 plus h c o h. So, this is your carbon fixation. So, this is how possibly again this is all possibly this is how possibly again this is all possibly this is how possibly this evolution of the first membrane has happened and as a matter of fact if you try to correlate this with the lipid bilayer membrane which is on this side of the board look at them then you will realize all the lipid bilayer membranes have lot of iron sulphur clusters and there are lot of iron sulphur proteins. So, it has been argued by the proponent of these theories that possibly those iron sulphur clusters in the in those proteins have evolved from that proposed f e s 2 colloidal membrane what is true what has happened nobody can tell with certainty, but this is one of the possibilities and next as we are looking and mind it as I am repeatedly telling you this is the situation where there was no oxygen absolutely devoid of oxygen. But then arrow of evolution was moving in a totally different in one direction such a process is a sulphide as the major source of electrons. So, it was your h 2 s which was the major source of electron now earth was cooling down this is the situation when this temperatures were now falling as time was passing by temperatures were falling down what exactly is the nature of the catalyst catalyzing the formation of f e s 2 from f e s iron sulphur word is not precisely clear it is believed that during arcane era several primitive microbial life forms evolved using f e s 2 as a direct source of energy and even this day some of these microbes survive the test of changing time. So, what is important here to highlight is f e s 2 has been used as a source of energy by a series of microbes and probably they have evolved in the pre arcane era or soon during that time. But as the earth was cooling down things were started changing the raw material like h 2 s was getting depleted. So, now we there is a depletion of this h 2 s as we are evolving out there and nature needed another molecule as a donor of electron because it is our whole evolution if you look at it the evolution of molecules will be accepting and donating electrons. So, who will be the next electron donor. So, only other molecule which was present there in abundance was water and now carbon dioxide fixation. So, forces the nature to search for newer abundant material to fix carbon dioxide nature slowly evolved to use more efficient work stations where h 2 can be used as a prime source of electron to provide necessary energy to drive the fixation of carbon dioxide thereby replacing h 2 s. Now, if you replace in this reaction h 2 o. So, what will be evolving is here your sulphurose evolving now what will be evolving will be oxygen. So, we move on to the next slide f e s 2 and the evolution of oxygen consequence of such transformation from hydrogen sulphide to water is paramount it was the time when anaerobic environment. So, this was all anaerobic environment of nature started to experience the evolution of free oxygen from water. So, now the situation from anaerobic environment is moving towards anaerobic environment it was the time when anaerobic environment of the nature started to experience the evolution of free oxygen from water. While on one hand water evolved as a new source of electron it simultaneously leads to the evolution of oxygen in the environment. Nature wanted to combat the surge of oxygen where most of the reduced form of metal ions started to oxidize most relevant in this context is most of the f e 2 plus which were present which is present in iron disulfide became f e 3 plus. So, they all got oxidized and in that process giving away an electron and the second thing happen is sulphur started forming sulphates. So, all the inorganic sulphurs moving towards so this was the major transformation which was taking place and slowly and slowly as this reaction was progressing environment was there was a steep or there is a slow rise of oxygen in the environment. So, oxygen pressure was now increasing as this reaction was prolonging on the floor of earth most of the primitive anaerobic microbes could not withstand the oxygen pressure and finally, perished. So, all those microbes which evolved in this pre-archaean era or the archaean era started perishing because they were in no position to withstand the oxygen partial pressure because they are not used to with it they were all developed under this kind of situation oxygen pressure was totally alien to them, but with evolution of now oxygen taking the central stage into this and all the iron are now getting oxidized and sulphurs are getting oxidized. It was a totally new world which was about to evolve and what essentially happened most of the primitive anaerobic microbes could not withstand oxygen pressure and finally, perished, but some of these survived even this day and remain hidden beneath the earth crust where the partial pressure of oxygen is low. These surviving group of microbes include a group of acedophilic chemo-autotropic bacteria thio bacillus ferro oxidants, thio bacillus thio oxidants ferro bacillus ferro oxidants and other related species. These microbes found refuge in the oxygen deficient environment of coal mines, hydrothermal vents and that is the word what I was trying to tell you why it is called the hydrothermal hydrothermal origin of life I was kind of trying to mention you that iron sulphur theory is called as the hydrothermal origin of life because these life forms now survive in the hydrothermal vent underneath the muddy sewage ecosystem that is why I told you if you walk by the street or under there is a drain on the other side if you just go rule on the depth tightly under the mud where this is a lack of oxygen you will see iron sulphur or iron pyrite present out there and there will be bacteria which will eat upon those iron pyrite back coming to in these anaerobic and micro aerobic conditions they sustain their critical biomass. So, this is what we really know about the story of iron pyrite a molecule which has evolved as I was telling you a molecule which is evolved which is a very rich heritage I mean it is one of the most amazing molecules and as I will closing in inching towards the end of this lecture you will realize I mean what are the applications of it which has intimately involved in the evolution of course, if you are a supporter or if you are convinced with the iron sulphur theory of evolution. But irrespective of that whether you like iron sulphur theory whether you are a proponent of nuclear like you know prebiotic broth theory whatever whatever one thing cannot be taken away from the chemistry of it is iron sulphur iron pyrite is a very ancient molecule and it has absolutely amazing properties and iron sulphur clusters are pretty much ubiquitous in the life forms all over the life forms iron sulphur clusters are very integral part of the lipid miliar these pieces of information cannot be taken away from it one more thing which cannot be taken away from iron sulphur is it is amazing potential what I was trying to tell you out here out here it has absolutely amazing potential to absorb sunlight just like the chlorophyll has of course, not as good as chlorophyll, but definitely it is close to that. Now coming back to the slides so that there is a changing landscape of evolution the class of microbes uses a p s 2 another metal sulphide as a sole source of energy for their survival and maintaining their critical biomass they derive energy from metal sulphides by an exothermic reaction where well metal sulphides are oxidized to metal sulphates yielding approximately 200 kilo calorie per mole of energy which is utilized by microbes to increase their critical biomass. So, as the landscape was changing these microbes got lost into oblivion, but that where they are currently where f e s 2 is most prominent f e s 2 formation in hydrothermal wind. So, if you look on the floor of earth if this is the earth if you are looking at the floor of earth and if you go down the sea pretty much at the base of it where this you know molten magma and all those gases are coming out from the core of the earth it is out there you are having this hydrothermal winds. If you look at the slide and I will show the real picture of hydrothermal winds where there is a kind particularly stable source of iron sulphide bearing nanoparticles in the ocean. So, this is where a series of iron pyrite nanoparticle are getting the form continuously from because earth's core is iron core. So, there is no dearth of iron on the floor of earth and this is the real picture of hydrothermal wind and iron pyrite is also called fools gold or and this is the fertilizer this is considered as a fertilizer in the ocean which helps in you know helping the life form to survive deep deep inside the sea where there is no light there is no oxygen. So, situation is even more worse. So, now there is no oxygen in the situation in these hydrothermal winds it is pretty much the similar situation like this what you see here iron water CO 2 H 2 S is a 2 and there is no sunlight like you know sunlight cannot penetrate out there and these are called black smokers from the mariner winds side of the pacific ocean. This is a picture from the pacific ocean and the inset what you see is a FeS 2 nanoparticle which is present there and life without light imagine a world of perpetual light where life flourishes without sunlight. And these are the life forms what you see you know rick tia pachypitala pachypitala are some of the life forms which has evolved deep inside the sea which utilizes all these inorganic molecules you know to derive energy for its survival. So, what essentially means that these molecules are there because of the oxygen environment they cannot survive I mean they cannot when they get oxidized at once they found their niche they indeed survive and if you see this particle they look like this. This is a typical FeS 2 particle this is also called fools gold because it had a shiny you know gold like luster. So, it is used for the photovoltaic material in solar cell application bacterial biomass and it is used as a chemical fertilizer in the ocean life forms. Now, if you look at the solar energy conversion ability of FeS 2 this is a comparison of saying if we assume in this graph the equal amount of say for example, you want to produce a x unit of energy x unit of conversion you wanted to see how much relative material you will be needing. So, if you look at it FeS 2 compare it with copper indium Se then if you compare it with CSI which is crystalline silicon ASI is amorphous silicon gallium arsenide is GAS cadmium telluride at CDTE. If you compare all of them you will see comparatively for producing same amount of output using these kind of photovoltaic material you need minimum amount with iron pyrite. This is its phenomenal ability to transform sunlight into electrical energy into photovoltaic energy and this is the inspiration point for generation of scientists that in a era when we are really running out of the petrol and all other sources what will be our other alternative when sun is there your option is. So, there is lot of research going on all these iron sulphide because it is ubiquitous it is not going to really you know it is non-toxic and by this time you must have wondering that it has remain a integral part of our whole evolution. So, coming back how they look like these are some of the crystals of iron pyrite if you look at them. So, now here I will close in actually, but before I close in I will leave something for you guys to think over it why I kind of you know started this an ancient molecule with rich heritage is indeed a very ancient molecule and if you believe the iron sulphur theory of evolution then possibly this has remain an integral part of the evolution itself and just like chlorophyll FES2 also has the ability to transform solar energy. So, could these molecule which are biologically benign and they are not going to you know create pollution could these kind of molecules be the answer for tomorrow's energy requirement and that is why I took up a totally different molecule and I am not getting into the solar energy conversion and all those things in its n and p type and all those things I will leave it I will give you certain references you can go through it, but to give you an idea why people have interest on a molecule and what is its genesis from the biological perspective, because we are talking about bioelectricity. So, this is the whole biological perspective biochemical or you know chemical perspective which over you call which over you love to call it is how FES2 a molecule which is ancient molecule as such a rich heritage that you know this could be one of the answer to our ever increasing demand for energy. Thank you.