Photosynthesis Lecture Notes

hello everyone welcome back to a s and a level biology with dr demi i am dr demi and in today's video i will be telling you about photosynthesis photosynthesis is chapter 13 of the a level biology syllabus so we're making progress very slowly but surely and i hope that you have found many of the videos i've made very helpful i've seen some of the requests for past papers to be analyzed so i have done a random selection of some questions from past papers and i will be doing a video on that very soon so just stay tuned and turn on your notifications so that you can know when i post that video but for today i want to zoom in on photosynthesis now i know that this is a chapter that many students find challenging as a matter of fact what i've noticed with many biology students is that they tend to find things that have to do with plants very unattractive for some reason they just seem like oh it's not as interesting as when you're discussing something that happens in the human body but i want you to know that photosynthesis is not difficult it's really easy and the purpose of this video is to demystify it for you so like i said my videos are mostly for revision but they can also help you is pre-exposure so you can watch the video before you go to the class and you would have had some sort of foundational understanding so when you get to class you can ask lots and lots of questions i i love that when my students do that so i always encourage them to watch videos before they come to the class so that we can have more of a conversation so i hope that you can also see the videos in the same way i have split this chapter into two videos so this is the first of two and in this particular video i'm going to be telling you about the process of photosynthesis the structure of the chloroplast and just telling you about all the things that happened during photosynthesis it might be a bit long maybe 25 minutes but the point is at the end of it you will have a very good understanding of what happens during photosynthesis so let's get into it shall we so starting off with the structure of chloroplasts when we started this biology syllabus from the very first chapter we started with the different organelles that you would find in a typical animal or plant cell and we said that in the animal cell we have the mitochondria and in the plant cells we have chloroplasts and what you will learn or what you will see about chloroplasts and mitochondria is that they both have double membranes and so they are double membrane organelles and they are believed to have been existent as independent organisms on their own before they were engulfed by eukaryotic cells and that process of them being engulfed is called the endosymbion theory so if you haven't read about that please make sure you just do a quick check or a quick scan in your book um to get that so if you if you look at a chloroplast under the microscope you will find that they look like biconvex disks you they very small in size about three to ten micrometers in diameter and inside of them which is where we're really interested inside of the chloroplast you will find these um arrangements here i like to think of them as coins just stacked on top of each other and um these are called granum when you have a stack like this it's called granum and when you have many stacks they're called granam that's the plural of granum so i'm just going to use my red annotation tool here to sort of spell that out as gently as i can i'm good okay so many of these granum would be called grana and each granum is made up of a stack of thylakoids the thylakoids are the site of light dependent reactions of photosynthesis so you've probably heard before if you have some biology foundation that photosynthesis happens in two stages you have the light reactions and the dark reactions while the dark reactions are not exactly dark but i'll explain that later uh what this means basically is that you have a stage of photosynthesis that is highly dependent on the presence of light and you have the second stage that is independent of the presence of light which means that even if there's light or there's no light that stage will proceed as it should so we'll zoom into the thylakoid on a different slide but i would i want you to know that that is where the light reactions happen you also have in here what we call the stroma um and the stroma is like the cytoplasm of the chloroplast and it's where you have the light independent reactions of photosynthesis so i want you to just bear that in mind that the light dependent reactions happen in the thylakoids and the light independent reactions happen in the strummer now when you zoom into a thylakoid this is typically what you would find a thylakoid has what we call a photosystem and what a photosystem simply does is that it helps with the harvesting of light so that the photosynthetic reactions can happen so if you look in here for example again using my annotation tool you can see that there are these little rectangles um that are inside the funnel so this is like a funnel arrangement and what these rectangles do is that they simply trap light so when sunlight shines on the photo system they trap the light and they pass the light down to each other until it gets to what we call a primary pigment the primary pigment is usually something like chlorophyll a or chlorophyll b and i'm sure you've heard of chlorophyll it is what gives plants the green color it is said and chlorophyll also is very important in the process of photosynthesis as you will see as we go along we also have what we call the accessory pigments so these is that this is usually the primary pigment at the bottom of the funnel and we have the accessory pigments that trap the light from the surface and pass it down to the primary pigment so this is what happens in a thylakoid when you zoom in and this is why it is the site for the light dependent stage of photosynthesis because it simply allows for the harvesting of light it is structured in a way that it harvest lights and allows the primary pigment um to partake in photosynthesis the stroma like i said is a colorless fluid it's um the site for the light independent reactions it's colorless it surrounds the granum or the grana and it also has enzymes and these enzymes help with the light independent stage of photosynthesis which is also called the carving cycle so you'll see the cycle once we get to the end of the slide it's the very last slide um the carving cycle is what the light independent stage is called in the stroma you would also find things that are typical of prokaryotic cells because it is believed that the chloroplast used to be a prokaryote before it was engulfed by the eukaryotic cell so you would fight these like the 70s ribosomes you would find dna secular dna you would find some lipids and starch grains and all of these other things that you typically find in a prokaryotic cell so now let's get into photosynthesis shall we this is the reaction for photosynthesis and something that i ask students notice immediately is that photosynthesis looks like the opposite of respiration so if you remember when we were discussing respiration in detail i said that if you want to get the reaction for respiration it's basically glucose which is that plus oxygen and then you make co2 and water as your product in photosynthesis it is the reverse you take co2 with water and then you make glucose and oxygen um so if you bear that in mind it just makes it very easy for you to be able to remember light is often used in this process like it said photosynthesis photo meaning light synthesis making um so light is used to split water into hydrogen and oxygen and the oxygen is given off but i'll show you how that happens when we discuss the process of phosphorylation also this lends to the fact that when you think about photosynthesis we always say that plants take co2 from the environment and they release oxygen while we take oxygen from the environment and we release co2 and that just tells you why it is very important for us to exist in the balance with the plants around us because we are simply contributing to the atmosphere in different ways and very dependent on each other now like i said earlier on the previous slide you have two types of photosynthetic pigments you can have your primary pigments and you also have your accessory pigments accessory pigments are things like your carotenes that give carrots their yellow color and your primary pigments would typically be your chlorophylls you also have two different photosystems in photosynthesis they're called photosystem one and photosystem two i wouldn't stress too much about that but photosystem one is typically called p680 while photosystem ii is often called p700 you also have what i explained earlier that the accessory pigments would absorb light and pass that light energy down um so the primary pigments and the primary pigments would then carry out some excitation that results in the production of atp and also the production of glucose as we go along so now let's look at the light dependent stage of photosynthesis in the light dependent stage we have what we call the splitting of water the splitting of water by light is referred to as photolysis which is the words that i am underlining here and just from the word photolysis you know that it's using light to split something and water is then used to produce hydrogen ions and electrons that are then passed around in some sort of electron transport chain and proton transport chain that would then result in the production of nadph and atp to say that in a different way so that it just doesn't sound confusing the point of the light dependent stage of photosynthesis is to make nadph and atp these two products are passed on to the light independent stage where they are then used to produce the sugar that you need to make from the plant and so something to also be in mind here when we discussed respiration we often said that in respiration we make nadh and this nadh is passed to the last stage of respiration which is oxidative phosphorylation and that is how we make our atp from the hydrogen that is removed from nadh in photosynthesis that is not nadh it's nadph it's also like uh nadh in the sense that it is an electron carrier and the easiest way to remember which one belongs where is to always know that the one with photosynthesis has a p in it p for photosynthesis so that would help you remember it better so in the light dependent stage there are two key things that happen you have phosphorylation and fertilizers i explained in the previous slides that photolysis is the splitting of water in the presence of light and you see how that feeds in very soon but phosphorylation what exactly is that phosphorylation is the addition of a phosphate which means that what phosphorylation is referring to it is the production of atp by adding a phosphate group to adp and we have two different types of phosphorylation you have either cyclic phosphorylation or non-cyclic phosphorylation so in cyclic phosphorylation this is typically what happens the light comes from the star from the sun and it would shine on the thylakoid you know that inside the thylakoid we have this funnel arrangement that comes with different organisms um not organisms different pigments rather that are arranged in what we call the light harvesting clusters this is typically a photosystem and in cyclic phosphorylation this is photosystem 1 which is also referred to as p618 once the light hits the pigments these are the accessory pigments the circles that i have included here they would have a stat light from the sun and they would pass it down all the way to the primary pigment once the light reaches the primary pigment the primary pigment will then release an electron so that electron is basically excited and we call that a photo activation the electron is released and once it gets to the top it's released and captured rather by an electron acceptor the electron acceptor will then pass the electron to an electron carrier the electron carrier would then pass the electron through an electron transport chain until it goes back to the photosystem the point of all of this is that as electrons travel they release energy and what that energy is then used for is to couple adp and phosphate together in order to make atp now i know this can be a bit frustrating because it is so technical well not too technical but it just seems like very detailed chemistry stuff uh but bear in mind that just by summarizing that the way that i have done you would be able to make it through the exams so let's think back again i'm just going to undo the animation so that we can sort of go back if we can for some reason my animations are stuck so let's just go on but i'll use my annotation tool here so we start here where the light shines on the thylakoid inside the thylakoid we know that we have what we call a system and in that photo system we have accessory pigments such as these ones that help to harvest light and as they harvest the light they pass the light down to the primary pigment so i'm just going to call this primary pigment one okay and there's no space to write one okay primary pigment one so once the light hits primary pigment one primary pigment one will release an electron that electron is excited and passed on to an electron acceptor here at the top that is called photoactivation the electron acceptor will then pass the electron to an electron carrier which will then take it through an electron transport chain and as it does that the electron provides the energy just by its movement provides the energy that is able to then allow the coupling of adp and phosphate in order to make atp the reason why this is called cyclic phosphorylation is that the electron that is excited is passed back down to the photosystem it doesn't go anywhere else it goes to the electron carrier releases energy and comes back down to the photosystem that is why we call this cyclic phosphorylation in non-cyclic phosphorylation things are a little bit more complicated and also very interesting but not that complicated that you can't get it oh i was wrong photo system two is p680 and photo system one is p700 um that was a mistake i made on the previous slide sorry i mixed that up so please bear in mind that photo system two is p680 photo system one is p 700 not that you're often asked questions about this but it's just important to get the right information out there so now let's look at what happens in non-cyclic phosphorylation first things first is i want you to notice over here what is happening with water all right water is coming in here and you can see that it is being split if you follow the arrow over here it is being split into oxygen and hydrogen i hope you can see that so let's just go through this i just wanted you to notice that that that is photolysis happening and then you also have phosphorylation but in a non-cyclic style so what happens in this case is that light will come in so i'm just going to draw something that looks like the sun over here perhaps i should use a yellow marker for that you know just because we always look at the sun as something that's yellow so there we go so light comes in here and it hits this photosystem okay this is for the system two the light hits photosystem two and photosystem two would release an electron the same way it happens in cyclic phosphorylation it releases an electron and what then happens is that that electron goes through an electron transport chain and obviously as it goes through the electron transport chain you already know that that provides energy for adp and phosphates to be bound together so that they can make atp and so they basically take the energy and then make atp so that then happens but look at what happens here the electron that travels from p680 which is photosystem 2 would then go to p700 which is photosystem one and at photosystem one what happens here also is that when light hits it it also has um an excited electron so let's think about it this way let me just re-explain it from the beginning i just noticed here now that we actually have lights that's just being hit here so here's our light okay let me go back to my red marker um so here's light and light is hitting our photo system all right i'm just erasing some of the other stuff that's not necessary so here's our light here and it's heating the photo system okay so this photo system releases an electron as a result of the light hitting it which means that there is photo activation because the electron gets excited but it doesn't stop there on the second photo system which is for the system one p 700 light also hits that photo system and it also releases an electron now look at what happens here the electron that is released from p680 will travel through an electron transport chain and replace the electron from p700 that has been excited the electron that was excited from p700 is then donated to nadp in order to make nadph all right so think about it the electron that goes from here p680 if you follow the arrows this makes it a lot easier the electron that's excited from p680 goes through an electron transport chain and replaces what was lost from p700 remember that p700 was also hit by light so it has also released an electron and that electron is used in the formation of nadph the question students usually ask me is that okay then what happens to p680 because it is missing an electron that electron is replaced by water so when you have photolysis you release electrons and proteins and so the electrons will then replace whatever has been lost from p680 if that makes sense and just by this movement of electrons that we have here in a continuous fashion we have energy in the form of chemiosmosis so i hope you remember this from photos um from respiration chemi osmosis that is then used to couple adp and phosphate in order to make atp um so i'm just spelling kenya's muscles here painfully careful um about writing without any pens but just using annotation tools um so this is called chemistry so it provides the energy for adp to then bind with phosphate so we have adp plus phosphate and that is what makes atp and the atp and nadph that we make from this phase of the reaction this is nadph over here the adp atp and nadph that we make is then transferred to the light independent stage in this case over here the oxygen that is given off and that is produced from fertilizers is given off as gas and that is the oxygen that we take in from the environment and while the hydrogen ions obviously are used also in making nadph this doesn't show that very clearly but that is typically what happens so that is the light dependent stages you can see nothing much happens there it's really just an electron transport chain where electrons move around and nadph is produced as well as atp those two things nadph and atp then move to the next stage which is the light independent stage the light independent stage is also called the calvin cycle and this is where things might be a bit complicated for students but typically it's not first of all i'll tell you something you are not expected to know the molecules or the number of molecules involved in the carving cycle as a matter of fact i think it says in the syllabus for 2018 to 2020 that you don't have to know the biochemistry so you don't have to know the number of molecules or the arrangements but you do have to know the names of certain important intermediates and that is what i'm going to show you with this slide so this image that i got from lumenlearning.com i use it in the classroom to explain the stage of photosynthesis so i hope that it is also easy for you to follow so think about it we've made atp and we've also made nadph then we come to the calvin cycle in the calvin cycle co2 which we know is very important in photosynthesis comes into the picture co2 enters the calvin cycle and it combines with a compound called rubp the full name for rubp is ribolus bisphosphate i will put that in the description so that you're able to follow um just be able to get the spelling so most of the things that i'm unable to say in detail or that might be confusing i'm just going to put in the description so you can easily just check that um so co2 comes in it combines with rubp if you look here at this image our ubp has five carbons so there's one there's two there's three there's four and there's five so if you add this five carbon compound with a one carbon compound you're going to make a six carbon compound that six carbon compound is highly unstable so it's often not shown in the carving cycle it's highly unstable and it immediately splits into two molecules of a three carbon compound again ignore the number of molecules here i'm dealing with this as if i'm working with just one molecule that is why it looks the way it does so um don't be afraid if you hear me say one molecule but you're looking at it you're saying this is three um so if i'm dealing with it like just one molecule of co2 with one molecule of rubp would we make one molecule of a six carbon compound which then splits into two molecules of a three carbon compound those two molecules of a three carbon compounds then undergo a series of reactions and as you can see here we make atp over here we use atp rather over here so atp is used wherever you see atp going to adp that means atp has been used and this is the atp we made from the light dependent stage of photosynthesis and nadph is also used as you can see there it goes from nadph to nadp which means the hydrogen ions are also used and that goes on through there so we have a reduction reaction of this intermediate here that reduction reaction results in the formation of this molecule called glycerate 3-phosphate i'm going to put that again in the slide so don't stress too much about it so from one molecule of rubp combined with co2 we've made these intermediates and we end up with glycerite phosphate one-fifth or one-sixth sorry um one-sixth of the glycerate three phosphate molecule is converted to half a molecule of glucose or is converted to glucose you don't have to worry about the number of molecules yet again um it's converted to glucose while the remaining five of the six molecules would be taken through a series of reactions um in order to remake our ubp so if you think about it just pay attention here there are different stages three key stages in the carving cycle the first stage is what we call the carbon fixation which means co2 comes in it combines with rubp and that is what we call the carbon fixation reaction something else to bear in mind here is that the enzyme that catalyzes the formation of the six carbon intermediate formed after co2 and rupp combine is called rubisco so rubisco catalyzes the reaction between rubp and co2 over there right once that is added together once those two add together and they form a sixth carbon intermediate the intermediate splits into two molecules of a three carbon compound which is this over here we then go through a series of reactions when you use atp and nadph from the light dependent reaction and we form glycerate three phosphate from glycerate three phosphate one sixth of that is used to make glucose so we take one sixth of that and we use glucose and the other six is then used to and the other fifth rather the fifth well the five out of six is used to make our ubp so we're basically saying that from the six carbons um from what's called now from the three carbons if we combine the three carbons uh what's called together um into two molecules we'll make a six carbon sugar um which would be glucose but then the other carbons that have been made the other three carbon compounds that have been made are used to regenerate rubp um don't be too stressed that this is a little bit confusing just always think of it in terms of the steps you have carbon fixation the co2 combines with rubp catalyzed peribiscule to make a six carbon intermediate that is highly unstable and that then splits into two molecules of a three carbon compound atp and nadph from the light dependent reactions are used to reduce that three carbon compound to glycerate three phosphate and from glycerate three phosphate we use some of these molecules to make glucose while the rest of it is used to regenerate rubp and so the cycle continues in that way if this has been confusing please just post questions in the chat i know that the carving cycle can stymie students a lot so i'll totally understand but post any questions that you have in the chat anything you'd like me to just re-explain perhaps in words that i can type to you and i would be happy to answer them this is the end of this very first part of photosynthesis and i will continue in the next part where i tell you a bit about c4 plants and then about the factors that affect photosynthesis until then have a good time and thank you so much for watching

hello everyone welcome back to a s and a level biology with dr demi i am dr demi and in today&#39;s video i will be telling you about photosynthesis photosynthesis is chapter 13 of the a level biology syllabus so we&#39;re making progress very slowly but surely and i hope that you have found many of the videos i&#39;ve made very helpful i&#39;ve seen some of the requests for past papers to be analyzed so i have done a random selection of some questions from past papers and i will be doing a video on that very soon so just stay tuned and turn on your notifications so that you can know when i post that video but for today i want to zoom in on photosynthesis now i know that this is a chapter that many students find challenging as a matter of fact what i&#39;ve noticed with many biology students is that they tend to find things that have to do with plants very unattractive for some reason they just seem like oh it&#39;s not as interesting as when you&#39;re discussing something that happens in the human body but i want you to know that photosynthesis is not difficult it&#39;s really easy and the purpose of this video is to demystify it for you so like i said my videos are mostly for revision but they can also help you is pre-exposure so you can watch the video before you go to the class and you would have had some sort of foundational understanding so when you get to class you can ask lots and lots of questions i i love that when my students do that so i always encourage them to watch videos before they come to the class so that we can have more of a conversation so i hope that you can also see the videos in the same way i have split this chapter into two videos so this is the first of two and in this particular video i&#39;m going to be telling you about the process of photosynthesis the structure of the chloroplast and just telling you about all the things that happened during photosynthesis it might be a bit long maybe 25 minutes but the point is at the end of it you will have a very good understanding of what happens during photosynthesis so let&#39;s get into it shall we so starting off with the structure of chloroplasts when we started this biology syllabus from the very first chapter we started with the different organelles that you would find in a typical animal or plant cell and we said that in the animal cell we have the mitochondria and in the plant cells we have chloroplasts and what you will learn or what you will see about chloroplasts and mitochondria is that they both have double membranes and so they are double membrane organelles and they are believed to have been existent as independent organisms on their own before they were engulfed by eukaryotic cells and that process of them being engulfed is called the endosymbion theory so if you haven&#39;t read about that please make sure you just do a quick check or a quick scan in your book um to get that so if you if you look at a chloroplast under the microscope you will find that they look like biconvex disks you they very small in size about three to ten micrometers in diameter and inside of them which is where we&#39;re really interested inside of the chloroplast you will find these um arrangements here i like to think of them as coins just stacked on top of each other and um these are called granum when you have a stack like this it&#39;s called granum and when you have many stacks they&#39;re called granam that&#39;s the plural of granum so i&#39;m just going to use my red annotation tool here to sort of spell that out as gently as i can i&#39;m good okay so many of these granum would be called grana and each granum is made up of a stack of thylakoids the thylakoids are the site of light dependent reactions of photosynthesis so you&#39;ve probably heard before if you have some biology foundation that photosynthesis happens in two stages you have the light reactions and the dark reactions while the dark reactions are not exactly dark but i&#39;ll explain that later uh what this means basically is that you have a stage of photosynthesis that is highly dependent on the presence of light and you have the second stage that is independent of the presence of light which means that even if there&#39;s light or there&#39;s no light that stage will proceed as it should so we&#39;ll zoom into the thylakoid on a different slide but i would i want you to know that that is where the light reactions happen you also have in here what we call the stroma um and the stroma is like the cytoplasm of the chloroplast and it&#39;s where you have the light independent reactions of photosynthesis so i want you to just bear that in mind that the light dependent reactions happen in the thylakoids and the light independent reactions happen in the strummer now when you zoom into a thylakoid this is typically what you would find a thylakoid has what we call a photosystem and what a photosystem simply does is that it helps with the harvesting of light so that the photosynthetic reactions can happen so if you look in here for example again using my annotation tool you can see that there are these little rectangles um that are inside the funnel so this is like a funnel arrangement and what these rectangles do is that they simply trap light so when sunlight shines on the photo system they trap the light and they pass the light down to each other until it gets to what we call a primary pigment the primary pigment is usually something like chlorophyll a or chlorophyll b and i&#39;m sure you&#39;ve heard of chlorophyll it is what gives plants the green color it is said and chlorophyll also is very important in the process of photosynthesis as you will see as we go along we also have what we call the accessory pigments so these is that this is usually the primary pigment at the bottom of the funnel and we have the accessory pigments that trap the light from the surface and pass it down to the primary pigment so this is what happens in a thylakoid when you zoom in and this is why it is the site for the light dependent stage of photosynthesis because it simply allows for the harvesting of light it is structured in a way that it harvest lights and allows the primary pigment um to partake in photosynthesis the stroma like i said is a colorless fluid it&#39;s um the site for the light independent reactions it&#39;s colorless it surrounds the granum or the grana and it also has enzymes and these enzymes help with the light independent stage of photosynthesis which is also called the carving cycle so you&#39;ll see the cycle once we get to the end of the slide it&#39;s the very last slide um the carving cycle is what the light independent stage is called in the stroma you would also find things that are typical of prokaryotic cells because it is believed that the chloroplast used to be a prokaryote before it was engulfed by the eukaryotic cell so you would fight these like the 70s ribosomes you would find dna secular dna you would find some lipids and starch grains and all of these other things that you typically find in a prokaryotic cell so now let&#39;s get into photosynthesis shall we this is the reaction for photosynthesis and something that i ask students notice immediately is that photosynthesis looks like the opposite of respiration so if you remember when we were discussing respiration in detail i said that if you want to get the reaction for respiration it&#39;s basically glucose which is that plus oxygen and then you make co2 and water as your product in photosynthesis it is the reverse you take co2 with water and then you make glucose and oxygen um so if you bear that in mind it just makes it very easy for you to be able to remember light is often used in this process like it said photosynthesis photo meaning light synthesis making um so light is used to split water into hydrogen and oxygen and the oxygen is given off but i&#39;ll show you how that happens when we discuss the process of phosphorylation also this lends to the fact that when you think about photosynthesis we always say that plants take co2 from the environment and they release oxygen while we take oxygen from the environment and we release co2 and that just tells you why it is very important for us to exist in the balance with the plants around us because we are simply contributing to the atmosphere in different ways and very dependent on each other now like i said earlier on the previous slide you have two types of photosynthetic pigments you can have your primary pigments and you also have your accessory pigments accessory pigments are things like your carotenes that give carrots their yellow color and your primary pigments would typically be your chlorophylls you also have two different photosystems in photosynthesis they&#39;re called photosystem one and photosystem two i wouldn&#39;t stress too much about that but photosystem one is typically called p680 while photosystem ii is often called p700 you also have what i explained earlier that the accessory pigments would absorb light and pass that light energy down um so the primary pigments and the primary pigments would then carry out some excitation that results in the production of atp and also the production of glucose as we go along so now let&#39;s look at the light dependent stage of photosynthesis in the light dependent stage we have what we call the splitting of water the splitting of water by light is referred to as photolysis which is the words that i am underlining here and just from the word photolysis you know that it&#39;s using light to split something and water is then used to produce hydrogen ions and electrons that are then passed around in some sort of electron transport chain and proton transport chain that would then result in the production of nadph and atp to say that in a different way so that it just doesn&#39;t sound confusing the point of the light dependent stage of photosynthesis is to make nadph and atp these two products are passed on to the light independent stage where they are then used to produce the sugar that you need to make from the plant and so something to also be in mind here when we discussed respiration we often said that in respiration we make nadh and this nadh is passed to the last stage of respiration which is oxidative phosphorylation and that is how we make our atp from the hydrogen that is removed from nadh in photosynthesis that is not nadh it&#39;s nadph it&#39;s also like uh nadh in the sense that it is an electron carrier and the easiest way to remember which one belongs where is to always know that the one with photosynthesis has a p in it p for photosynthesis so that would help you remember it better so in the light dependent stage there are two key things that happen you have phosphorylation and fertilizers i explained in the previous slides that photolysis is the splitting of water in the presence of light and you see how that feeds in very soon but phosphorylation what exactly is that phosphorylation is the addition of a phosphate which means that what phosphorylation is referring to it is the production of atp by adding a phosphate group to adp and we have two different types of phosphorylation you have either cyclic phosphorylation or non-cyclic phosphorylation so in cyclic phosphorylation this is typically what happens the light comes from the star from the sun and it would shine on the thylakoid you know that inside the thylakoid we have this funnel arrangement that comes with different organisms um not organisms different pigments rather that are arranged in what we call the light harvesting clusters this is typically a photosystem and in cyclic phosphorylation this is photosystem 1 which is also referred to as p618 once the light hits the pigments these are the accessory pigments the circles that i have included here they would have a stat light from the sun and they would pass it down all the way to the primary pigment once the light reaches the primary pigment the primary pigment will then release an electron so that electron is basically excited and we call that a photo activation the electron is released and once it gets to the top it&#39;s released and captured rather by an electron acceptor the electron acceptor will then pass the electron to an electron carrier the electron carrier would then pass the electron through an electron transport chain until it goes back to the photosystem the point of all of this is that as electrons travel they release energy and what that energy is then used for is to couple adp and phosphate together in order to make atp now i know this can be a bit frustrating because it is so technical well not too technical but it just seems like very detailed chemistry stuff uh but bear in mind that just by summarizing that the way that i have done you would be able to make it through the exams so let&#39;s think back again i&#39;m just going to undo the animation so that we can sort of go back if we can for some reason my animations are stuck so let&#39;s just go on but i&#39;ll use my annotation tool here so we start here where the light shines on the thylakoid inside the thylakoid we know that we have what we call a system and in that photo system we have accessory pigments such as these ones that help to harvest light and as they harvest the light they pass the light down to the primary pigment so i&#39;m just going to call this primary pigment one okay and there&#39;s no space to write one okay primary pigment one so once the light hits primary pigment one primary pigment one will release an electron that electron is excited and passed on to an electron acceptor here at the top that is called photoactivation the electron acceptor will then pass the electron to an electron carrier which will then take it through an electron transport chain and as it does that the electron provides the energy just by its movement provides the energy that is able to then allow the coupling of adp and phosphate in order to make atp the reason why this is called cyclic phosphorylation is that the electron that is excited is passed back down to the photosystem it doesn&#39;t go anywhere else it goes to the electron carrier releases energy and comes back down to the photosystem that is why we call this cyclic phosphorylation in non-cyclic phosphorylation things are a little bit more complicated and also very interesting but not that complicated that you can&#39;t get it oh i was wrong photo system two is p680 and photo system one is p700 um that was a mistake i made on the previous slide sorry i mixed that up so please bear in mind that photo system two is p680 photo system one is p 700 not that you&#39;re often asked questions about this but it&#39;s just important to get the right information out there so now let&#39;s look at what happens in non-cyclic phosphorylation first things first is i want you to notice over here what is happening with water all right water is coming in here and you can see that it is being split if you follow the arrow over here it is being split into oxygen and hydrogen i hope you can see that so let&#39;s just go through this i just wanted you to notice that that that is photolysis happening and then you also have phosphorylation but in a non-cyclic style so what happens in this case is that light will come in so i&#39;m just going to draw something that looks like the sun over here perhaps i should use a yellow marker for that you know just because we always look at the sun as something that&#39;s yellow so there we go so light comes in here and it hits this photosystem okay this is for the system two the light hits photosystem two and photosystem two would release an electron the same way it happens in cyclic phosphorylation it releases an electron and what then happens is that that electron goes through an electron transport chain and obviously as it goes through the electron transport chain you already know that that provides energy for adp and phosphates to be bound together so that they can make atp and so they basically take the energy and then make atp so that then happens but look at what happens here the electron that travels from p680 which is photosystem 2 would then go to p700 which is photosystem one and at photosystem one what happens here also is that when light hits it it also has um an excited electron so let&#39;s think about it this way let me just re-explain it from the beginning i just noticed here now that we actually have lights that&#39;s just being hit here so here&#39;s our light okay let me go back to my red marker um so here&#39;s light and light is hitting our photo system all right i&#39;m just erasing some of the other stuff that&#39;s not necessary so here&#39;s our light here and it&#39;s heating the photo system okay so this photo system releases an electron as a result of the light hitting it which means that there is photo activation because the electron gets excited but it doesn&#39;t stop there on the second photo system which is for the system one p 700 light also hits that photo system and it also releases an electron now look at what happens here the electron that is released from p680 will travel through an electron transport chain and replace the electron from p700 that has been excited the electron that was excited from p700 is then donated to nadp in order to make nadph all right so think about it the electron that goes from here p680 if you follow the arrows this makes it a lot easier the electron that&#39;s excited from p680 goes through an electron transport chain and replaces what was lost from p700 remember that p700 was also hit by light so it has also released an electron and that electron is used in the formation of nadph the question students usually ask me is that okay then what happens to p680 because it is missing an electron that electron is replaced by water so when you have photolysis you release electrons and proteins and so the electrons will then replace whatever has been lost from p680 if that makes sense and just by this movement of electrons that we have here in a continuous fashion we have energy in the form of chemiosmosis so i hope you remember this from photos um from respiration chemi osmosis that is then used to couple adp and phosphate in order to make atp um so i&#39;m just spelling kenya&#39;s muscles here painfully careful um about writing without any pens but just using annotation tools um so this is called chemistry so it provides the energy for adp to then bind with phosphate so we have adp plus phosphate and that is what makes atp and the atp and nadph that we make from this phase of the reaction this is nadph over here the adp atp and nadph that we make is then transferred to the light independent stage in this case over here the oxygen that is given off and that is produced from fertilizers is given off as gas and that is the oxygen that we take in from the environment and while the hydrogen ions obviously are used also in making nadph this doesn&#39;t show that very clearly but that is typically what happens so that is the light dependent stages you can see nothing much happens there it&#39;s really just an electron transport chain where electrons move around and nadph is produced as well as atp those two things nadph and atp then move to the next stage which is the light independent stage the light independent stage is also called the calvin cycle and this is where things might be a bit complicated for students but typically it&#39;s not first of all i&#39;ll tell you something you are not expected to know the molecules or the number of molecules involved in the carving cycle as a matter of fact i think it says in the syllabus for 2018 to 2020 that you don&#39;t have to know the biochemistry so you don&#39;t have to know the number of molecules or the arrangements but you do have to know the names of certain important intermediates and that is what i&#39;m going to show you with this slide so this image that i got from lumenlearning.com i use it in the classroom to explain the stage of photosynthesis so i hope that it is also easy for you to follow so think about it we&#39;ve made atp and we&#39;ve also made nadph then we come to the calvin cycle in the calvin cycle co2 which we know is very important in photosynthesis comes into the picture co2 enters the calvin cycle and it combines with a compound called rubp the full name for rubp is ribolus bisphosphate i will put that in the description so that you&#39;re able to follow um just be able to get the spelling so most of the things that i&#39;m unable to say in detail or that might be confusing i&#39;m just going to put in the description so you can easily just check that um so co2 comes in it combines with rubp if you look here at this image our ubp has five carbons so there&#39;s one there&#39;s two there&#39;s three there&#39;s four and there&#39;s five so if you add this five carbon compound with a one carbon compound you&#39;re going to make a six carbon compound that six carbon compound is highly unstable so it&#39;s often not shown in the carving cycle it&#39;s highly unstable and it immediately splits into two molecules of a three carbon compound again ignore the number of molecules here i&#39;m dealing with this as if i&#39;m working with just one molecule that is why it looks the way it does so um don&#39;t be afraid if you hear me say one molecule but you&#39;re looking at it you&#39;re saying this is three um so if i&#39;m dealing with it like just one molecule of co2 with one molecule of rubp would we make one molecule of a six carbon compound which then splits into two molecules of a three carbon compound those two molecules of a three carbon compounds then undergo a series of reactions and as you can see here we make atp over here we use atp rather over here so atp is used wherever you see atp going to adp that means atp has been used and this is the atp we made from the light dependent stage of photosynthesis and nadph is also used as you can see there it goes from nadph to nadp which means the hydrogen ions are also used and that goes on through there so we have a reduction reaction of this intermediate here that reduction reaction results in the formation of this molecule called glycerate 3-phosphate i&#39;m going to put that again in the slide so don&#39;t stress too much about it so from one molecule of rubp combined with co2 we&#39;ve made these intermediates and we end up with glycerite phosphate one-fifth or one-sixth sorry um one-sixth of the glycerate three phosphate molecule is converted to half a molecule of glucose or is converted to glucose you don&#39;t have to worry about the number of molecules yet again um it&#39;s converted to glucose while the remaining five of the six molecules would be taken through a series of reactions um in order to remake our ubp so if you think about it just pay attention here there are different stages three key stages in the carving cycle the first stage is what we call the carbon fixation which means co2 comes in it combines with rubp and that is what we call the carbon fixation reaction something else to bear in mind here is that the enzyme that catalyzes the formation of the six carbon intermediate formed after co2 and rupp combine is called rubisco so rubisco catalyzes the reaction between rubp and co2 over there right once that is added together once those two add together and they form a sixth carbon intermediate the intermediate splits into two molecules of a three carbon compound which is this over here we then go through a series of reactions when you use atp and nadph from the light dependent reaction and we form glycerate three phosphate from glycerate three phosphate one sixth of that is used to make glucose so we take one sixth of that and we use glucose and the other six is then used to and the other fifth rather the fifth well the five out of six is used to make our ubp so we&#39;re basically saying that from the six carbons um from what&#39;s called now from the three carbons if we combine the three carbons uh what&#39;s called together um into two molecules we&#39;ll make a six carbon sugar um which would be glucose but then the other carbons that have been made the other three carbon compounds that have been made are used to regenerate rubp um don&#39;t be too stressed that this is a little bit confusing just always think of it in terms of the steps you have carbon fixation the co2 combines with rubp catalyzed peribiscule to make a six carbon intermediate that is highly unstable and that then splits into two molecules of a three carbon compound atp and nadph from the light dependent reactions are used to reduce that three carbon compound to glycerate three phosphate and from glycerate three phosphate we use some of these molecules to make glucose while the rest of it is used to regenerate rubp and so the cycle continues in that way if this has been confusing please just post questions in the chat i know that the carving cycle can stymie students a lot so i&#39;ll totally understand but post any questions that you have in the chat anything you&#39;d like me to just re-explain perhaps in words that i can type to you and i would be happy to answer them this is the end of this very first part of photosynthesis and i will continue in the next part where i tell you a bit about c4 plants and then about the factors that affect photosynthesis until then have a good time and thank you so much for watching

Transcript for:Photosynthesis Lecture Notes

Transcript for:
Photosynthesis Lecture Notes