The Science of Everything Podcast - Episode 102: Photosynthesis Part II

Episode Date: March 11, 2020

Continuing the discussion of photosynthesis from the previous episode, here I outline the importance of the oxygen evolving complex in oxidising water molecules, the importance of photosystem I in extracting additional energy, and the role of light-independent reactions of the Calvin cycle in carrying out carbon fixation. Recommended pre-listening is Episode 101: Photosynthesis Part I. If you enjoyed the podcast please consider supporting the show by making a paypal donation or becoming a patreon supporter. https://www.patreon.com/jamesfodor https://www.paypal.me/ScienceofEverything

You're listening to The Science of Everything podcast, Episode 102, Photosynthesis Part 2. I'm your host, James Fodor. So in the previous episode, we began talking about photosynthesis. In particular, I discussed an overview of what photosynthesis is, and I talked about photosystem 2 and how that absorbs light in the chlorophyll molecules and how the electrons are excited and pass through different components of the photosystem 2, eventually passed on to Plastic quinone which diffuses through the thylacoid membrane and is passed on to cytochrome C complex. And I explained how that process is used to pump hydrogen ions across from outside to inside the

thylacoid membrane and how that is used to generate a proton gradient. So that's what we got up to at the end of the previous episode. And if that's all very unfamiliar or you've forgotten, recommend relisten to that because I'm just going to pick up from where I left off there. In this episode, I'm going to start by Explanation. how the oxygen evolving complex provides electrons that are subsequently energized by the absorption of light and then pass through photosystem 2 and so on. I'm going to explain how these electrons are sourced from, ultimately from water via the oxygen evolving complex. So this is a critical

component to the process that I'll start by explaining and then I'll proceed by finishing off the story of photosynthesis by talking about photosystem 1 and then ADP reductase and then also looking at the light independent reactions and some other aspects of photosynthesis. So then let's begin by talking about the role of the oxygen-evolving complex. So you may have wondered, or at least I hope that you did, because there's a bit of a hole in what I've been saying so far. So what I've said is that electrons are excited by photons, and these excited high-energy photons are passed through the different components of photosystem 2,

Photosystem 2 and then onto the Plasterquinone molecule and from the Plasterquinone molecule onto the Cynacrome complex. And from there, although I haven't got to this yet, they're passed onto Plaster Cyanin and then onto Photosystem 1. And then ultimately they're actually donated to NADP and combined with the hydrogen antiform NADPH. So anyway, the point is that the electrons don't come back to photosystems. They don't come back.

They're gone forever. Now this is a problem in a sense because you can't just keep losing electrons because then you'll become positively charged and that will act as a break on the whole process. Electrons aren't going to want to leave if the region that they're living from is already positively charged. They'll be attracted and that attraction will prevent the electrons from leaving and will shut down the whole process. So this isn't going to work in the way that I've described it. What we need is a source of replacement electrons to replace the electrons that are being excited and then passed along the chain.

So where do these replacement electrons come from? Well, they come from the oxygen evolving complex. Well, they don't actually come from the complex. They're generated by the oxygen evolving complex. And, I mean, as the name indicates, ultimately they come from oxygen atoms, so the electrons effectively are ripped from the oxygen atoms and passed on a few intermediaries

to the chlorophyll molecules that originally lost the high-energy electron when it was excited by the... incident photons. But where does the oxygen-evolving complex get its electrons from? Well, I mean, I said they came from oxygen, but like, how does that work? Well, the answer is what the oxygen-in-envolving complex does is it takes water and basically splits the water up into its component parts into oxygen and hydrogen. And so this is why it's called the oxygen-evolving complex, because it takes water, H-2O molecule, and splits it, so it takes, think of it as two water molecules, so that's

four hydrogen atoms and two oxygen atoms, and splits that up into two oxygen atoms, which combined together into an O2 molecule, that's the oxygen, and then four hydrogen atoms. So it evolves or produces oxygen, O2 molecules, which are the oxygen we breathe in the air. These oxygen molecules don't serve any further purpose in photosynthesis, so they're actually just released as a waste product. Almost all of the oxygen that exists in the air, in fact, essentially 100% of it, is produced, what was produced fairly recently, by photosynthesizing, plants and algae and other organisms because oxygen molecules are highly reactive and therefore

they don't tend to stick around in an atmosphere very often for very long. The only reason we have such a high oxygen proportion in the atmosphere, about 20%, is because of the continuing huge volume of photosynthesis that occurs on the Earth. If that stopped, the oxygen volume would fairly quickly diminish down. And in fact, if we did find high oxygen content, high O2 content on the atmosphere of any extra terrestrial planets, then that would be a strong indication that life existed, at least photosynthetic life existed on those planets, because it's hard to see how else it could stay there, because it's so reactive, it tends to, you know, react with metals, say, and turn it into metal oxide, essentially rust or other types of reactions like that.

Anyways, this is very interesting because we don't typically think of oxygen as a waste product, but that's essentially what it is. In fact, there was a catastrophe early on in the evolution of life when lots of organisms started photosynthesizing and pumping out all of this oxygen to the atmosphere and over a period of millions of years, the oxygen concentration increased dramatically, and because oxygen is so reactive, it's effectively toxic, or at least was toxic to these early forms of life and led to extinctions. And then, of course, new organisms came on, which came into existence which are able to, well, as we would breathe in or utilize, use the oxygen for respiration and so on. But anyway, that's getting a bit far afield.

But I guess the point to emphasize here is that oxygen is a waste product of photosynthesis. It is generated by photosynthesis but serves no purpose either for the plant or for the photosynthesis reaction itself. So the only reason that oxygen is needed is to serve as a source of electrons, basically because oxygen in the form of water, H2O, is in what we call reduced form. That essentially means that it has relatively high number of electrons. If you recall from previous chemistry episodes that I've done or otherwise, reduction refers to the gain of electrons. So if a chemical species is reduced,

that means it's got a lot of electrons relative to the number it could possibly hold. You can also think of reduction as a gain in hydrogen atoms. That's not exactly the definition, but it's helpful because electrons, obviously, often, in order to balance out the charge, right? If you have a proton, then you have one electron and they neutralize each other. So often exchanging an electron takes the form also of exchanging a proton to go with it to balance out the charge. So gaining an electron and gaining a proton or vice versa, losing an electron and losing a proton or hydrogen ion, the same thing as a proton. They often go together.

And so if a species is reduced, you can think of it as having lots of electrons and often at the same time has lots of hydrogens. If you oxidize a species, then it loses its electrons and also often loses hydrogen atoms. and that's exactly what happens to water. It starts off in reduced form with lots of electrons. The chlorophyll molecules need those electrons to replace the ones they've lost, and so the oxygen evolving system grabs those electrons out, takes them away from oxygen, and essentially passes them on to the chlorophyll molecule.

Now, as you may recall, again, from previous chemistry episodes that are done, taking electrons away from oxygen is not an easy thing to do, because oxygen has a very high electronegativity. That means it has a very strong pulling power, attractive power for electrons. You have to look at the structure of the periodic table to explain why that is, but it's basically, apart from fluorine,

I think it has the highest electronegativity of any element, which means that it's very difficult to pull electrons away from oxygen. It represents a very low energy level for electrons to get to. So they kind of like being with oxygen. It's very stable. If you try to take electrons away from oxygen, oxygen, then you're going to have a hard time, basically, because they're very stable there. Or in other words, you need a lot of energy input in order to do that.

Now, of course, in the case of photosynthesis, you do have that energy input. It's coming from the photons that are being absorbed and exciting the electrons. So basically the way you can think of this is that the dearth of electrons that results from the electrons being excited and then passed off away, that reduction in the number of electrons present in the reaction center drives the splitting of water and the ripping away of electrons to replace those that are lost. But it's only this constant source of energy from the sun that rips those electrons away. They're my effectively generating a charge differential, which then enables the, which serves as an attractive force essentially to rip the electrons away

from water and pass them on to the reaction center. It's only this source of energy that allows this to happen. You can't get energy by taking away electrons from oxygen and splitting water in the process. That takes energy, but of course the energy is available because it's coming from the sunlight. Now, of course, it's not just so simple as, oh, you split the water because the electrons are needed by the reaction centre. There is a complicated process by which this occurs, and it's actually this intricate, bonded network of manganese ions, four of them, which are, connected together with oxygen atoms. And basically what happens is there's a sort of a process that occurs

whereby each magnesium ion in turn changes its oxidation state, which is effective there's a number of electrons that it has. And as it does so, the electrons are then passed onto a tyrosine residue, which is just a type of amino acid, basically, that's part of the protein complex in Ferdistam 2. And then from the tyrosine residue, it's passed onto the chlorophyll molecule. Anyway, the details of this are not super important, although the way in which this happens, it's sort of like a five-state machine that has got state, one, two, three, four, and then, you know, back to state zero,

and then, you know, back to state zero and so on sort of iterates around where the oxidation states of these magnet ions changing as the electrons come in from the splitting of the water and then are passed on to the tyrosine residue and then onto the chlorophyll molecule. So it's got quite an intricate network here, and it actually takes several photons worth of energy, in order to drive this around through one cycle. So it's not just one photon, it sort of ticks around. One photon's absorbed, and you get one increase in the oxidation state. And one of the magnetis cells, and then another one is absorbed,

and then another one of the magnetase changes oxidation state. And as this process occurs, the electrons are gradually passed on to the chlorophyll molecule. So they're replaced as needed as a result of this oxygen evolving complex. So the point of all this is that the splitting of water and the extraction, of the electrons from the water and passing them through to the chlorophyll in the reaction centre is a catalyzed process, catalyzed by the oxygen

evolving complex. And so it helps this to occur and reduces the energy barriers associated with this process. But again, the whole point is that the water here is serving as a source of electrons which are needed to replace the electrons being excited and then passed off through the change through Ferdosystem 2 and the

plastic quinone and to the cynochrome complex. So those need to be replaced. They come from water. This process also generates, well, for each oxygen atom that is split up in this way, or for each water molecule that's split up in this way, this generates two hydrogen ions, two protons, which exist in internal membrane space of the phylochoid. And so this adds to the proton gradient that, you know, the relatively high concentration of protons that exist, hydrogen ions that exist inside the thylacoid space. Remember that the whole point of the cytocrine complex was to pump

hydrogen ions from the outside to the inside. But this oxygen evolving complex by producing hydrogen ions also adds to this concentration. So it further serves as a means of increasing the effectively storage of energy in this chemical gradient. So these protons then, like the protons generated by the cytochrome complex, pass through the ATP synthesis and thereby are used to generate ATP. So this is an extra source of energy, an extra source of energy. ATP molecules. So this essentially completes most of the aspects of the behavior of photosystem 2 and the cytochrome complex. The last bit that we need to describe now is what happens to the high energy electrons, or formerly high energy electrons, by this stage most of

their energy has been depleted, although as we'll see, they'll be boosted up again in Photosystem 1. But we have to describe what happens to them after they leave the cytochrome complex. And as I mentioned, after they exit the cytochrome complex, They don't really exit there. They're passed on from the last component of the cytochrome complex, which I think is a heem group. So ultimately they end up at a heem group. Then they're passed on to a protein which is called Plastercyonin. This is another one of these electron carrier molecules, but instead of existing inside the membrane, like actually within the membrane itself between the two phosphorylipid layers, which is where Plasterquinone exists. This is a soluble protein, so it exists inside the thylacine.

space. So it's much larger than the Plasticinone molecule, which is just a non-polar molecule, which is able to diffuse throughout the phospholipid membrane. Plasticinin is quite a bit larger because it's a protein, but the key part of it is that it has a number of residues which surround a copper atom, and it's the copper atom that houses the high energy or formerly high energy electron. So it's basically passed at this copper atom, which is coordinated by a number of surrounding residues in the protein, and it holds on to the formally high-energy electron as it's passed from the cytochrome complex onto Photosystem 1. Okay, but what happens when the formerly high-energy electron gets to Photosystem 1? Well, as you might have imagined,

it's passed onto a series of chlorophyll molecules which exist inside a reaction center, and therefore is able to be excited again by the absorption of an additional photon. So remember I said that the electrons are zapped twice, there's zapped once, by absorbing a photon in photosystem 2, and then once again, by absorbing a second photon in Photosystem 1. Well, this is that second photon, the second increase in energy that the electron receives,

after having depleted most of its energy passing through Photosystem 2 and Plastic Winone and the Cytocrine complex. So now it gets another boost in energy because it's been passed to the chlorophyll molecules in the reaction center in Photosystem 1, which has its own antenna complex and works more or less the same

as that in photo system 2, although the wavelengths absorbed by the photosystem 1 into a slightly different, you know, because the complexes are set up slightly differently, but it's conceptually the same thing. So we're getting another boost in energy from the same process. So after the electron gets its boost in energy from the chlorophyll molecules in Furtisystem 1, it's then passed on through another chain of intermediary components of Ferdicystem 1, so particularly through a number of heme groups, and also through a number of pherodoxin groups. A ferrodoxin is a molecule that,

at its center has a iron sulfur cluster, so this is an iron bound to two solfers, which are then bound to another iron molecule, and then there are four sulfur molecules, which also bound to the iron atoms. So it's basically a clustered combination of ions and sulphurs, and it too is able to hold onto the electrons, the high-energy electrons. Once again, we've got the high-energy electron which has passed through various intermediary compounds, all part of photosystem 1, and eventually it reaches the active site of an enzyme which is called the ferrodoxin NADP plus reductase. So this is the actual enzyme that catalyzes the chemical reaction that produces NADPH from NADP and the hydrogen ions. So the ferrodoxin NADP plus

reductase is, I'm not actually sure if it's a component of photosystem 1 or attached to it, but either way, it receives its electrons from the other components of photosystem 1 and then is able to use the energy from the high-energy electron that it has received to catalyze the formation of NADPH from NADP plus H-plus. And again, the reason it needs a high-energy electron is because this is an energetically unfavorable process. So basically, we're pressing that hydrogen ion onto a spring and forming a high-energy bond there. That's an energetically unfavorable process. So you need a high-energy electron in order to be able to do that. And that then generates the NADPH molecule, which is the second source of energy that's produced by photosynthesis.

And this process also consumes that high-energy electron because the reactants of this reaction are NADP and hydrogen, or in other words, a proton. So the proton is positively charged, so when it combines with the NADP, it needs something to neutralize that charge because NADPH is a neutral molecule, and so the electron does that. So the electron is effectively consumed and forms part of the NADPH molecule. So that's why these electrons need to be replaced. They're ultimately replaced by the oxygen-evolving complex, which is ripping them out of water molecules and taking them away from oxygen.

And so that effectively completes the story here. We've gone right through the process of generating the proton gradient, which is the ultimate source of the energy, or the immediate source of the energy for the ATP synthase enzyme, which is the immediate source of the ATP-Rectase enzyme, which is the other energy-rich. compound that is produced by photosynthesis. So these all ultimately derive their energy from the photons that are absorbed either by FOTO system 2 or FOTO system 1 and have that energy transduced through a series of complex mechanisms into, in the case of NADP, it's a high-energy electron which is then absorbed and contributes to the formation and catalysis of NADPH by the NADP reductase, or in the case of photosystem 2, the high-energy electrons deplete their energy by causing protons to be pumped across the membrane into the thylacoid space,

which then are able to float back down their concentration gradient and through the ATP synthesis enzyme to produce ATP molecules. So this is quite a remarkable process here. But at this late stage, we've still only described the light-dependent reactions. This is all the first stage of photosynthesis, which, if you recall, is all the energy production stage, the stage that takes water, NADP and adenosine diphosphate, plus amino-organic phosphate, and turns it into NADPH and ATP. So that's the energy-producing light-dependent reactions. We still haven't talked about the light-independent reactions, which

take all of these energy-rich molecules, particularly the NADP and the ATP, plus carbon dioxide, and fix that carbon into a biologically accessible form, which is then used to feed further metabolic pathways and produce the actual biological substance and matter that makes up the plant. So I won't talk about these processes in nearly as much detail, partly because they're much less interesting. It's usually just a series of enzymes producing one thing which then feeds into the next process, which produces another thing, and eventually, you know, you get the molecule you're interested in. But I will just talk briefly about the enzyme Rubisco. So this enzyme is critical because it is what actually does the carbon fixation. It captures carbon dioxide from the

atmosphere, so plants have to take that in through their pores that are literally holes in the leaves through which carbon dioxide flows. And in a process called the carbon cycle, this carbon dioxide, which is then accessible to the enzyme Rubisco, uses the newly formed NADPH and ATP to produce three carbon molecules. I'll just call them C3 molecules for three, the three carbon. So these are actually built up sort of one carbon at a time. So it kind of has to go through the process a number of times to keep adding on carbon dioxide in a sense. But through this cyclical process, the Rubiscoe enzyme produces the three carbon molecule, which then, and then there's a process of progressive regeneration of the substrate to which the carbon dioxide is added. Because basically what

happens is just, is the process starts off with a five carbon compound. A carbon dioxide is added, or a carbon from the carbon dioxide is added to produce a six carbon compound, which is then broken into two. One of them is a three carbon compound, which is the output of the process, and that heads on to the central metabolic pathways. The other one is also a three carbon compound, but it feeds back into the cycle and eventually regenerates the five carbon compound, which then goes on to have the carbon dioxide added and form. Form the six carbon compound, which splits into the two, three carbon compounds, one of which is the output, and one of which feeds back into the cycle, which goes around, around, around. So this cyclical process

called the Calvin cycle. It is the process by which Rubisco fixes carbon dioxide into a metabolically accessible form. So it produces a three-carbon compound product, which then feeds into the central metabolic pathways and is used to produce often six carbon sugars like glucose and galactose, which are then combined into longer chains of sugars, which are used in turn to either store energy or produce the actual physical structure of plants, like cellulose, for example, is just a long polymer of sugar. So that's made up of, ultimately, of mostly, I think, six carbon sugars, monomer components, which in turn are produced from these three carbon units that are ultimately pumped out by Rubisco, using the energy, ATP and NADPH, that was in turn produced

by the light dependent reactions of photosynthesis. So these light independent reactions central on this enzyme Rubisco, because it's what does the actual carbon fixation actually takes carbon dioxide and combines it into a form that is accessible and turns the carbon into organic carbon, basically, because carbon dioxide is not really considered an organic molecule, it's not directly accessible to the metabolic pathways of plants. It needs to be incorporated into a form can be accessed. One final point I wanted to make here

before going through a brief summary of everything we've talked about is that there are two types of plants, well, you know, there's a lot of types of plants, but two types that are relevant here. so-called C-3 and C-4 plants. Most plants are C-3 plants, and I really want to say C-3-P-O then, but C-3 plants because Rubisco produces a three-car compound, which then is passed on to the central metabolic pathways.

C-4 plants are in the minority, although some quite a number of prominent crop plants are actually C-4 plants. And the reason for that is because C-4 plants use a different system which is more efficient under hot-dry conditions. And so the basic issue here is that when the carbon dioxide concentration inside a leaf drops down, or inside the chloroplasts actually drops down too low, the catalyst Rubisco, that enzyme that's critical for fixing carbon, instead of fixing carbon into a metabolic process,

instead of incorporating it into the compounds that's building, it actually puts oxygen in instead, which is not what the plant needs. And so that's really wasteful. it uses up these sort of hard one ATP and NADPH molecules for something that's entirely pointless. So this is a sort of a defect in plant design essentially. But C4 plants have been able to overcome this. And the way they do this is by having special cells called bundle sheath cells that actually separate out. They physically separate by having membranes in between the light dependent and the light independent reactions.

So the NADPH and the ATP and so on are produced in. in one type of cell, the mesophile cell, and then they're passed on to the bundle sheath cells, and it's only these bundle sheath cells that contain the Rubisco enzyme necessary for fixing carbon. And in this modified case, they produce four carbon compounds instead of the three carbon compounds that are usual,

and that's why it's called the C4 photosynthesis instead of C3 photosynthesis. The big advantage of this separation is that it allows the oxygen that it's, remember, produced by the light dependent reaction, because they have to rip the electrons out, rip the electrons away from water, and thereby they produce oxygen as a byproduct, as a waste product. This oxygen, well, eventually it's released through the pores of the leaves,

but it still accumulates the thumb concentration inside the chloroplasts, right, before it's diffused out. And it's this oxygen that actually causes this problem with the oxygen being fixed instead of the carbon dioxide if the carbon dioxide falls too low. This process, by the way, is called furorospiration. It's a waste of the resources of the plant. C4 plants get around this problem by physically separating out the carbon fixation step from the energy production step. So that you don't have, the oxygen will still be produced in the energy production step in the light dependent reactions,

but they occur in a physically different cell to the light independent reactions. So there will be no oxygen present or in the very low oxygen levels present in the bundle sheath cell where the light independent reactions occur, where the carbon fixation actually occurs. and so the Rubisco enzyme is able to fix carbon dioxide without oxygen getting in the way because it's been physically separated away in a different cell. So this is a really big advantage that the C4 cells, sorry, the C4 plants have, but it's only actually an advantage in certain conditions. It's only an advantage in conditions where photorespiration tends to occur, which is in hot, dry conditions.

Because under other conditions, photo respiration doesn't tend to occur, And in those situations, this separation is pointless because it doesn't achieve anything. All in fact does is use up extra energy because what you have to do is pump these ATP and ADP molecules from the cell where they are created into the bundle sheath cell where they'll actually be used to fuel carbon fixation. So that takes energy to pump them across. And that's fine if pumping them across is necessary in order to prevent further respiration. But if it's not necessary, then it's kind of a waste of time, right? So in conditions in which photo respiration is an issue, hot dry conditions, then C4 plants do better. But in conditions in which photo respiration is not really an issue, then C3 plants do better because they don't waste this energy having to pump the energy molecules into a new cell.

So this is sort of the reason why we see C4 and C3 plants both existing is because there's no overall better system. It just depends on the environment in which it exists. That said, a number of important crops, crop plants are C4 plants, including sugarcane, millet, and maize. So it's relatively uncommon within the plant kingdom, but still some important species are known to be C4 plants. Now, in researching for this episode, I just found out something quite interesting, because there is apparently a project funded in part by the Bill and Melinda Gates Foundation, in which scientists are attempting to produce a genetically engineered form of rice to convert rice, which is naturally a C3 plant like most plants are, into a C4 plant.

Now, in many environments in which it grows, this would allow it to be more efficient, and some of it saying it could produce up to 50% more grain, basically because in many environments, the rice plants would not be incorporating oxygen, would not be fixing oxygen instead of carbon dioxide and therefore wasting all of that energy. So that could significantly improve the yield of rice crops around the world and contribute to food security. So that's a really cool application, I think. And interesting, because it takes a reasonable amount of knowledge about how these plants work, why how photosynthesis works, in order to understand why this could potentially be a really useful thing.

So anyway, I just thought that was pretty nifty. Right, so in closing, I just wanted to go through and briefly review, sort of step back a little bit and look at the bigger picture and revise the basic story that we've been telling throughout the episode. So the basic story is this. Photosynthesis is a series of chemical reactions by which sunlight, the energy from sunlight, is converted into chemical energy which the plant stores in the form of chemical bombs. It proceeds via a two-stage process, the light-dependent reactions and the light-independent reactions. The light-dependent reactions, the purpose of these, is to produce energy carrier molecules

or high-energy intermediary molecules, NADPH and ATP, which serve as a temporary stores of energy, and then the second series of reactions or the second phase of photosensis, the light-independent reactions, use these energy intermediaries, the ATP and the NADPH,

in order to fix carbon dioxide, to fix the carbon from carbon dioxide into a biologically accessible form, and thereby make this carbon available to the central metabolic pathways of the plant, where it can be used to produce biomatter and store energy in various mostly long, sort of long polymer chains of long polymer carbohydrates.

So the light dependent reactions occur through a series of protein and co-factor complexes which are embedded in the membrane of the thylacoids, which, remember, are basically just sort of big hollow sacks, which are stacked up into granite, that in turn sit inside the chloroplast double membrane, which in turn sit inside the plant cells. Now, these series of protein cofactor complexes, which sit bedded in the thylacod membrane, are called photosystem 2, the cytochrome complex, and photosystem 1.

There's also ATP synthase, although I didn't talk too much about that today because I focused on, I talked about that in the episode 75 on cellular respiration. The basic process is that light is absorbed by molecules called. called chlorophyll, and it's able to absorb the light molecules because of the conjugated series of bonds that exist in the aromatic ring of carbons that surrounds the central magnesium ion. And this is likewise how many of the other molecules or compounds or co-factors that the electrons are subsequently passed onto are able to hold onto the electron as well, because they have these conjugated bonds, either in rings around an emetal ion or as a long tail of these carbon

atoms, or in other cases it's just by passing the electron onto a metal atom or ion itself, which then can change oxidation state by accommodating an additional electron or losing an electron. Light is absorbed by these chlorophyll molecules. It's then passed on through a series of intermediary complexes. As I mentioned, these include heme groups, chlorophyton molecules, which are similar to chlorophyton molecules, which are similar to chlorophyll but lack the central magnesium ion, quinone molecules, and ion sulfur clusters, depending on the complex, we're talking about. It's passed through the series of intermediaries, passed on out of the photosystem 2 complex to a Plasticinone molecule, which is a non-polar molecule which diffuses

through the membrane, passing the high-energy electrons on to a further series of intermediary compounds existing in the Cytochrome C complex, and ultimately then passing on out of the cytochrome C complex onto a protein called Plastasionin, which in turn passes the electrons onto photosystem 1, where they're passed through a series of intermediaries, and finally are absorbed effectively by the ferrodoxin NADP reductase enzyme, which combines NADP and a hydrogen ion into NADP H. During the process of being passed through all of these intermediary components in the Psytrimc complex, four hydrogen atoms, for every two photons, are pumped from outside the thylacoid membrane to the inside of the internal

thylcoid space. Thereby generating a proton gradient and the protons move down their concentration gradient back out to the outside, but in doing so they pass through ATP synthase, which uses the stored energy of the concentration of the hydrogen ions to turn the rotors of ATP synthase and thereby generate adenosine triphosphate ATP, which is the other main energy storage molecule that's produced by photosynthesis. So effectively, photosystem 2 produced uses ATP via the proton gradient, whereas the photon absorbed by photosystem 1 generates the

NADPH molecule, and it doesn't use any proton gradient. It just does that sort of more directly. And as I mentioned, these two processes can actually be separated by different forms of photosynthesis, which just use one or the other of these two photosystems in slightly modified variations in different species, because effectively each is sort of independent of each other. They can be linked, but they don't have to be, and they can generate energy independently of each other in the right setup in different species. All of these electrons that have come through, pass through the chain of photosystem 2, Cynochrome complex and the Photosystem 1, and ultimately end up at NATP reductase. These electrons ultimately come from water molecules, which are split as a result of

ultimately the energy coming in and absorbed by the chlorophyll molecules. in photosystem 2, that high energy of the photon enables them to split the water molecules, separating the oxygen and the hydrogen atoms, or hydrogen ions, and by oxidizing the oxygen in this way, the electrons are able to be released and replace the electrons that are lost by being excited by the photons and then being passed on down the chain and out and out through the sequence of membrane complexes. So the electrons that are lost are ultimately replaced by the splitting of water aided by the oxygen-evolving complex,

which sits inside the thylacoid membrane space. And ultimately, this is all possible because through every stage in the process, all that's happening is that electrons and or protons are moving in accordance with achieving their lowest energy state in the given configuration they exist. This is all possible because things have been set up through evolutionary processes in just such a way so that when the electron is first excited, then it becomes energetically favorable for it to jump to this co-factor and then to that other co-factor, and then it becomes energetically favorable to move over to the plastic quinone molecule, which then defuses, and then it becomes energetically favorable for it to move to this hem group and that hem group and so on.

and it's all set up so that each stage of the sort of electron falling down the stairs is energetically favorable. You don't have the electron falling back down to the lowest energy stage in one fell swoop. The point is to be able to force it to fall down in the right way so that you can extract useful work in the process, in particular pumping the hydrogen ions, the protons across the membrane and thereby generating a proton gradient which is able to pass through the ATP synthase. So that's the whole point of this elaborate mechanism of passing, past the parcel of the electron, both in photosystem 2 and Photosystem 1 and also the Cynochrome Complex. It's all about cleverly extracting the energy as the electrons fall back down into lower potential energy states. And even, and this is also the case in the oxygen evolving complex, even though splitting up of water into oxygen and

hydrogen is not itself energetically favourable. It's energetically favourable in the context of the relative dearth of electrons generated by the loss of electrons as they are sort of pulled out by this process. They're excited by the lighter, then pulled out by the process of being passed from one intermediary to the other. If they weren't replaced by something, this would lead to a buildup of negative charge. And so that attractive force is what enables the water molecules to be split. And so in the context of what's occurring, it's still energetically favorable for the electron to move away from the oxygens in the water molecule, which is where they come from,

and to move instead to, or first the magnetase atoms in the oxygen-in-complex, and then ultimately into the chlorophyll molecule in the reaction center of Ferdistam 2. So all this is is electrons and protons moving down their concentration and potential energy gradients and try to achieve a lowest energy state. It's only possible because they are initially excited to high energy levels by the absorption of photons. If that didn't happen, of course, this whole process would be impossible because fundamentally what we're doing is taking electrons from a source

where they're already at a very low energy, that we're taking them from the oxygen in the water molecule, where they're very low energy, so we need to absorb energy to do that, and we're putting them in a high-energy state, in the NADPH molecules where they're at a high-energy state. So taking from a low-in-putting them to high-energy state requires an input of energy, and that, of course, comes from the photons. And it's through this magnificent process that effectively all of the food that we eat is generated directly or indirectly, either through autotroes which produce their food, their energy and build their biomass through this process, or by heterotroves which eat some other organism, which itself directly or indirectly gets its energy through photosynthesis.

So this is an absolutely central process for all of life on the planet Earth. And I hope that this episode has helped you to understand it somewhat better. If you would like to ask a question about the podcast or suggest an episode topic or just give some feedback, always love hearing from listeners, you can send me an email. My address is Fods12 at gmail.com. That's FODS12 at gmail.com. You can also visit the podcast website. That's Fods12.podbean.com. If you would like to support the podcast, you can also go to our new Patreon page. Go to Patreon and type in The Science of Everything podcast, and you can see, support the show at varying amounts per episode. This is, as I always say, completely voluntary.

This is just if you would like to search support from the show and help cover some of the monetary and time costs, and I greatly appreciate everyone who is able to support the show in this way. So, thank you very much for listening, and I'll talk to you next time.