this is the video for all of the standard level content in C 1.3 on photosynthesis when we think of photosynthesis what we really need to think about is the big picture of energy conversion and one of the awesome things that makes producers well awesome is the fact that they can take light energy and transform it into chemical energy and that is just super important because they form the base of the food chain so they're again taking light energy and converting that into chemical energy and that chemical energy is then passed along to Consumers and then through trophic levels and even though a little bit is lost at each step this initial transformation of light to chemical is essential for food chains now that chemical energy can be in the form of carbohydrates lipids proteins nucleic acids all kinds of things now we're going to focus in on glucose as the product of photosynthesis in these examples and to manufacture glucose producers are going to take water and carbon dioxide and convert that into glucose with an oxygen byproduct why is there an oxygen byproduct well it comes from this process called photolysis photo meaning light Lis meaning to break we're going to use the energy from light to break apart water this water and the reason that we're doing that is because we need hydrogen in order to convert this carbon dioxide into glucose so I'm not going to write out the balanced um form here but just so you know photolysis I'm going to be breaking this into its components so I'll have hydrogen ions or protons I will have electrons okay and I'm going to have oxygen and that'll be given off in the form of oxygen gas obviously this isn't um balanced here but when we think about this oxygen byproduct it's really from this photolysis of water okay so we're using all of these important components to make glucose and giving this off as a byproduct and when we think about evolution of life on Earth especially if you're studying this course at higher level um when living things first started producing this oxygen byproduct that really changed the composition of Earth's atmosphere and we saw a lot of big changes happening after that so not only are photosynthesizing organisms very important for energy conversion but also for oxygen production now in order for these photosynthetic organisms to do photosynthesis they have to be able to absorb light and they're going to absorb light using these things called pigments so pigments are going to absorb different wavelengths of light and they actually have many pigments we can separate those pigments and study them separately using a process called chromatography the separation of pigments you should definitely try this out on your own here's a quick summary of how this process would work you would transfer some pigments from a plant or algae or something like that onto a piece of paper so this is called chromatography paper you allow that pigment um to come into contact well not really the pigment but the paper you allow the paper to come into contact with some chromatography solvent down at the bottom something like propen known so here is my plant pigment that I transferred and it looks green what's going to happen is if I let this sit if I'm patient enough some of this this solvent will be absorbed by by the paper and it will start moving up the paper as it does that okay you can see the after photo here it's going to separate out some of the pigments that I transferred from my plant and so I need two measurements if I'm going to be able to identify these pigments I need the distance that the solvent traveled okay so how far up the paper did that solvent go and how far did each of these pigments go and once I know those two things I can calculate something called the RF value okay so the RF value can be calculated by measuring the distance that a pigment traveled so let's say my plant pigment started out here and one of the pigments I don't know ended up up here so I would want to measure that distance and I want to compare that to the distance that the solvent traveled so let's say the solvent started down here and by the end my solvent was up there so this RF value stands for retention Factor you don't need to know that what we do need to know is it's basically a ratio of the distance between um or the distance that the pigment traveled and the solvent traveled we are going to get different RF values for different pigments because different pigments are going to move different lengths okay okay so some of them will move um not nearly as far as the solvent others will move yeah closer to how far the solvent moved so each pigment will have a characteristic RF value and so what that means is that not only can we separate the pigments but we can identify the pigments if you're doing this on your own you can actually find a table of the known RF values for different pigments and once you've calculated yours you can use them to identify the name of each of these pigments now colors and pigments aren't necessarily the same thing colors are the way that we perceive in our brain the different wavelengths of light pigments are going to be things that absorb and reflect different wavelengths of light so the pigments that we are seeing is whatever something else is reflecting H so what does this mean so let's say I have a green leaf here okay so it looks pretty green to me well what's actually happening and this is my eye okay I don't know my eyeball what's actually happening is that light is bouncing off of this leaf and we're getting blue light and we're getting red light and we're getting green light and we're getting yellow light so white light is made up of a mixture of all of these colors this green plant is going to be absorbed abing all of these different wavelengths of light except for the green light and how do I know it's not absorbing the green light because it looks green so what we are perceiving with our eye are the wavelengths of light that are reflected by something it is actually absorbing all of those other colors so let's take a look at which wavelengths the pigment chlorophyll is able to absorb so chlorophyll is the main pigment in plants and these visible wavelengths of light run anywhere from about 400 nanm to 700 nanm these wavelengths of light in the 400 meter range or 400 nanometer range these are going to be my Blues okay so like blues and purples and then I'm going to have greens and I'm going to have yellows and I'm going going to have Reds up here in this 700 range now chlorophyll is very good at absorbing blue light and really terrible at absorbing green light and then very good at absorbing red light and if you look at an absorption Spectrum for chlorophyll um like on an internet or in the textbook you'll see a little bit more like dips and Peaks and valleys but in general here is the pattern that you need to know that chlorophyll is very good at absorbing blues and reds terrible at absorbing greens and that is why to our eye chlorophyll appears green it's reflecting that light that it does not absorb and so this is something that we call chlorophylls absorption Spectrum it looks like this so that absorption Spectrum shows us what chlorophyll is able to absorb in terms of wavelength that's the absorption Spectrum what we're going to work on next is something called an action Spectrum so an action spectrum is exactly what it sounds like not only what can it absorb but what can it do with it so for chlorophyll it's going to look very similar we're going to have wavelength and nanometers down at the bottom ranging from 400 to 700 our visible light spectrum and the pattern is going to be about the same okay so the pattern will look very similar the difference here is what we're measuring so in an absorption Spectrum you're measuring what percent of those lights can chlorophyll absorb here for the action Spectrum what we're really looking at is photosynthesis rates okay so when we say action we mean what is the plant doing with these different wavelengths of light and this is usually expressed as a percent of the maximum rate of photosynthesis so as you can see we're getting maximum rates or close to them um in this 400 nanometer range and 700 nanometer range but not so much here in the middle that makes sense these organisms can do a lot of photosynthesis when they can absorb a lot of light and they cannot do a lot of photosynthesis when they can't absorb the light now there's a couple of different ways that you can measure photosynthesis rates you could look at oxygen production okay the faster that oxygen is produced the more photosynthesis we'll talk more about these in a minute or you could look at carbon dioxide consumption but the important thing to note here is that the action spectrum and the absorption Spectrum may look very similar in their patterns but they are measuring different things now the next big learning in this topic is about how we actually go about measuring photosynthesis rates and we want to be able to test the effects of the different limiting factors on photosynthesis rates so limiting factors are exactly what they sound like they are things that can limit the rate of photosynthesis and there are three of them carbon dioxide concentration temperature and light intensity and we'll take a look at um all three of them of course okay so when we're thinking about carbon dioxide concentration that's one of the substrates for photosynthesis so the h higher the concentration the faster the rate will be until it reaches a maximum rate and then it starts to Plateau part of the reason for that is at some point all of the enzymes that we need for photosynthesis will be busy their active sites will be occupied so there's a maximum rate there we're going to find a very similar pattern with light intensity so we need light for photolysis and for other things if you're studying this at HL and so the more intense the light the faster the photosynthesis will go until it reaches a maximum rate and then even if you make the light brighter and brighter and brighter the rate will hit a plateau temperature looks a little bit like this so as you warm things up things will go faster and faster and faster and faster but after a certain point it doesn't level off it actually drops off quite sharply and that's due to the fact that photosynthesis is catalized by n enzymes and after a certain temperature those enzymes will start to denature so we want to be able to investigate the effects of these different limiting factors on photosynthesis so we need to look out for ways to a measure photosynthesis rates and B manipulate um our environment for these different limiting factors if you have fancy oxygen sensors or carbon dioxide sensors then land or terrestrial plants are great to use if you don't I highly recommend that you invest in some aquatic plants because they make measuring photosynthesis very simple with relatively little technology so here we're looking at something called pondweed pondweed is an aquatic plant and why is that fun for this well because plants give off oxygen as a byproduct and that oxygen is going to be oxy gas and oxygen gas when you have it underwater is going to be in the form of bubbles so as this plant is photosynthesizing it's going to get give off oxygen gas bubbles and you can even count these oxygen gas bubbles as a way of measuring the rate of photosynthesis so now that we know one way to measure the rate of photosynthesis let's talk how we might vary some of those conditions and we'll work first with the carbon dioxide concentration so if I want to create uh again an aquatic environment with no carbon dioxide then what I would want to do is boil that water so boil it and then allow it to cool you don't want to kill your plant and that boiling is going to remove the carbon dioxide from the water so if you're studying chemistry you know that hot liquids don't they're not great at dissolving gases so this is going to remove the carbon dioxide you can pour it from one Beaker to the other to oxygenate it that's fine um and then you can set up your testing apparatus when you're doing that okay counting your oxygen bubbles is a great way to measure the rate of photosynthesis you're probably not going to get a lot of photosynthesis from water that you've removed the carbon dioxide from but that's fine that's kind of like your control group if you want to then start measuring or adding more carbon dioxide to your water what you can do is add sodium hydrogen carbonate and that will disassociate into carbon dioxide it'll increase your carbon dioxide levels so you want to add a little bit at a time to test different increments remember what we should be looking for is this graph like this where if I have different carbon dioxide concentrations and I start with zero I should be getting a photosynthesis rate of zero but as I go up and up and up I should see an increased rate until I get to a certain concentration so what we want to do is repeat this until the rate of the bubble production is the same once we have found that point we have found this Plateau okay and so this would then be the optimal concentration of carbon dioxide in this next bit we'll kind of focus on a different way to measure the photosynthesis photosynthesis rate because varying the light intensity is pretty self-explanatory I would want to have a light that is either um a higher wated or something like that or I can move it closer to or farther away as long as I have something to mitigate the heat coming off of that light but anyways moving that light closer to or farther away is an easy way to vary light intensity a cool way to measure this is to actually use little leaf discs so if you take a leaf disc so just cut out a circle from a leaf it's going to have some air bubbles in it in that spongy mesophyll layer if you use a syringe to suck out all of the air bubbles then when you put it into some water it should sink to the bottom okay it doesn't have any air in there it'll sink as that plant photosynthesizes it's going to be producing oxygen gas as a byproduct that oxygen gas is going to alter the buoyancy of this leaf and it is going to start to float to the top so if you time how long it takes for that leaf disc to float to the top that can be an indicator of photosynthesis rates the less time that it takes the faster your rate of photosynthesis and again I can vary that light intensity until I find the point at which varying that light intensity doesn't result in a faster rate so to investigate temperature what you would want to do is set up your apparatus in a water bath where you can maybe have a heater or some kind of heating element that controls the temperature make sure that your other limiting factors like light and carbon dioxide levels are in abundance that they're not going to inhibit the photosynthesis in any way so lots of light lots of sodium hydrogen carbonate and you can add your culture of algae or whatever plant you're using there's a couple of different ways that you could measure this you could either um use an oxygen sensor to measure the oxygen gas that's produced here or and this is really cool you can actually use a pH sensor so here's how the pH bit works as this um organism is photosynthesizing it is going to consume carbon dioxide so it will be removing the carbon dioxide from the water now carbon dioxide causes water to be more acidic so if I am removing the carbon dioxide that means the water is less acidic and that means I should be noticing an increase in the pH so the bigger the increase in PH the higher the rate of photosynthesis and when I am um kind of manipulating that temperature I should notice that as it gets warmer and warmer and warmer that pH changes more and more and more until it gets to a certain point at which time it drops off due the due to the denaturation of the enzymes necessary for photosynthesis now what what do we need to get out of this well it's important to understand that we need to pick one independent variable and make sure that we control the rest so if you're choosing carbon dioxide concentrations you should be thinking sodium hydrogen carbonate and then controlling light and temperature again the same for the other two and you must have a reliable way of measuring the photosynthesis rates so we want to have a reliable way of manipulating and measuring whether that's counting oxygen bubbles letting leaves float to the surface using an oxygen sensor or a pH sensor now we can really see this play out on a really large scale in these F Ace free air carbon dioxide enrichment experiments and these are very cool so these are plots of land where certain things have been controlled um to investigate potential consequences of increased carbon dioxide levels so we think that um this may have an impact on photosynthesis rates not just in a tiny little experiment but in ecosystems as a whole due to the rising carbon dioxide levels from human activity so the hypothesis that is that it will actually increase photosynthesis rates so what these um people are doing is trying to control for other factors but manipulating the amount of carbon dioxide in the air and measuring things like plant growth looking at other limiting factors looking at other parts of the EOS system like other organisms and other abiotic factors um so a very cool connection here to not only photosynthesis experiments but interdependence within an ecosystem