in this video we're going to be talking about the concepts of absorption spectrum and also action Spectrum in photosynthesis but before we talk about that I just want to do a little bit of revision you see in the chloroplasts they have these things called tilid and inside the tilid they have these very important structures called photosystems and if you remember I told you that a photo system is just a structure that contains photosynthetic p pigments and this pigments what what they're supposed to do is they're supposed to absorb light for example in this photosystem over here it is made out of something called the reaction Center and the reaction Center is consisting of two chlorophyll a molecules uh next to each other and of course uh chlorophyll a is not the only pigment inside the photo system for example in this particular photo system I'm drawing out three other photosynthetic pigments of different colors and these are known as accessory pigments and if you remember the reason why the photos system should have different type of pigments is because it is to allow the photosystem to absorb as much light as possible because light is not just a single thing it's it is actually an electromagnetic wave consisting of different wavelength in fact visible light falls under the range of 400 nanom to 700 nanom right so there are many different um I guess you can say varieties of the wavelength of light so having different types of pigments will allow the photos system to absorb different types of light so what do I mean by that you see as an example here we will have to talk about a very important concept known as the absorption spectrum and the absorption spectrum is just a single it's basically a graph that shows how well each photosynthetic pigment absorbs different wavelengths of light and they may ask this definition in the exam all right so some students will look at that definition and go I don't understand what exactly this means what it just means is you see the photos system has different types of pigments the absorption spectrum is just a graph to tell you which pigment absorbs which wavelengths of light best so as an example I'm just drawing out the four pigments over here you don't have to memorize the names of the pigments don't worry so I'm just going to call them pigments a b c and d and I'm also going to draw out a graph here and the graph here on the y- axis is the absorption capacity or just the absorption and on the x-axis is the wavelength of light I told you before that wavelength of visible light falls under the range of 400 to 700 nanometers now some students will ask the question do I have to memorize that 400 to 700 nanometers well you don't have to but it's good to know that light visible light at least uh Falls within that range now I'm just going to draw out the absorption spectrum of pigment a pigment B pigment C and pigment D I do not need you to memorize this graph at all it is not important for you to memorize the patterns you just need to understand what this graph means all right so you have four lines there each line represents the absorption capacities of each pigments and we're going to look at each of them one by one so as you can see here we're just focusing on pigment a and I'm just throwing out you can see the Peaks over there I'm putting an arrow down so what this means is pigment a absorbs a lot of light when the light is in the wavelengths of about 420 to 520 nanom that's what that Peak actually means like that okay what about pigment B if you look at pigment B the absorption of pigment B if you shine 400 to 500 nanom does it absorb a lot of those uh wavelength of light no it doesn't it's good at absorbing light at around 600 to about 650 nanom so that's what Pikman B is specialized for pigment C however if you see that in the graph it absorbs a lot of light at about 600 nanom give or take uh to about 700 yeah I know it's actually about 600 20 nanom but I'm just going to put it at 600 nanom to 700 nanom just to give it a kind of round ballpark figure but pigment D however is a little bit more interesting okay when you look at pigment D over here it absorbs it's quite good at absorbing about 450 nanometers of light but what about 500 to 600 does it absorb a lot of light there no it's bad at doing so it sucks uh but it's also good at absorbing light at around 650 nanometers so pigment D over here is good at absorbing light at the wavelengths of 450 nanom or also 650 to 660 nanometers this is what the absorption spectrum is saying so so what I'm just trying to tell you here is the graph tells you which pigments absorb which wavelengths of light best it's as simple as that now why do we have to care about their absorption Spectrum well the reason is as follows the more light the pigment absorbs it will increase the rate of photosynthesis because you know it drives light dependent reaction it produces ATP and reduced nedp which will then be used in light independent reaction it is as simple as that so the next concept we have to see is something called the action spectrum and it's just essentially a graph showing the overall rate of photosynthesis over a range of wavelengths of light so the Spectrum in this case here means the range of wavelength of light that's the Spectrum the range and action what action are we talking about the action is just the rate of photosynthesis so as you can see here and you don't need to memorize the action Spectrum as well uh you you do need to memorize the definition but you don't need to memorize the graph patterns but you need to understand how the absorption Spectrum affects the action Spectrum now imagine in this situation here this is a plant land plant or a terrestrial plant and this plant has all four pigments it has pigment a pigment B pigment C and pigment D within the chloroplast or within the photosystem so is this good well this is quite good for the plant so if we were to draw out another graph which is the action Spectrum the action Spectrum will usually have the rate of photosynthesis on the Y AIS because it's the overall rate of photosynthesis against the x-axis which is the wavelength of light now let's imagine I shine 400 nanom of light on the plant which means it's just purely purple light will the plant do a lot of photosynthesis some students are like how the hell do I know that well you do have reference you can look at the absorption Spectrum graph so look at the 400 nanometers which I'm highlighting over there does it actually absorb a lot of 400 nanometers you can see the part where I've highlighted that they do have pigments but uh pigments um pigments a pigment B and also pigment D yeah they can actually absorb 400 nanometers but do they absorb a lot of 400 nanometers not really so will it be a high rate of photosynthesis in this case not exactly too there will be a bit of photosynthesis but not much what if we shine 450 nanom of Light which is like blue bluish light in this case will the plant do a lot of photosynthesis well let's look at the graph now so look at the parts where I've just basically highlighted in this case for 450 nanom yes pigment a and pigment D can actually absorb a lot of light in that case as you can see I'm circling over there so the overall rate of photosynthesis will actually be quite High what about 500 nanom well at 500 nanom you still have pigment a and pigment D they can actually absorb a lot of light yes if you notice pigment B and C they cannot absorb a lot of 500 nanometer I'm circling that in blue over there if you can see that but the plant still has pigment a and pigment D to compensate for that so a lot of absorption of 500 nanom of light will still be taking place so the overall rate of photosynthesis will be quite High not so high but still a little bit higher than usual okay so I've plot that in the graph of the action Spectrum what about 600 nanometers okay at 600 nanom the pigments are not so good at absorbing that particular wavelength of light so the overall rate of photosynthesis is low and 700 nanom you can see that pigment B and also pigment C are fantastic at actually absorbing that particular wavelength of light so in that case the rate so in that case the rate of photosynthesis will also be quite high so the logic is as follows if the plant has the pigment to absorb that particular wavelength of light the rate of photosynthesis will be high so that's how the absorption Spectrum affects the overall action Spectrum if you still don't understand let's look at a different one now we have a unique looking seaweed over here as you can see the sea the seaweed is just groups of plants that grow in the water and look at that particular seaweed it's not green in color that particular seawe is like a bluish purplish color the reason why that seawe is it has that kind of different type of color is because they have different types of pigments as you can see here for this particular seawe does it actually have all the pigments no they only have pigments B and pigment C in this situation so how does does it affect the rate of photosynthesis in this plant if I shine 400 nanom of light or 500 nanom of light will the plants have pigments to absorb this particular wavelength of light look at the graph as you can see the graph over here 400 500 nanometer pigments B and pigment C suck at actually absorbing that wavelength of light so in that case they will actually reflect it and when they reflect the light that's why these particular seaweeds are actually a bluish and greenish color because they cannot actually absorb that particular wavelength very effectively so will the plant actually have a high rate of photosynthesis in this case no they won't because the rate of photosynthesis is low the rate of photosynthesis is low because they cannot absorb that particular wavelength of light very effectively what about 600 nanometers let's look at 600 nanometers well 600 nanom pigment C is not so good at absorbing 600 nanom but pigment B the purple one is actually quite good it's not fantastic but it's okay so the rate of photosynthesis becomes slightly higher but when you give 700 nanom of light in this case 700 nanom will actually have more absorption pigments B and C are fantastic at actually absorbing that particular wavelength of light as you can see in the absorption spectral graph so what will happen to the rate of photosynthesis you bet the rate of photosynthesis in this case in the action Spectrum will be quite high so as you can see over here the reason why the action spectrum of the seaweed and the action spectrum of the land plant or the terrestrial plant they don't have the exact action Spectrum even if you give the same wavelength of light the reason is because the land plant has four different types of pigments to maximize light absorption but the seaweed only has two different types of pigments to maximize its light absorption so that is how the absorption Spectrum can actually cause an effect in the action Spectrum