all right welcome to the second and last video for chapter 8 in this part of the video we're talking about the light dependent and light independent reactions all right now that we know more about how plants absorb light in that visible region of the electromagnetic spectrum let's start to kind of put it together again we have our chloroplast here and again we're looking at the thid a thyo coid membrane and here is an example of one of those thids this is the thilo coid membrane inside we have the Lumen of the thilo coid and the outside is the stroma remember which is the liquid portion within the chloroplasts what we're going to see within the Philo coid membrane are photo systems we're going to see photosystems 2 and one photosystem 2 actually comes first and then photosystem one comes later the reason they're numbered weird like that is because photosystem one was discovered first and then photos system 2 was discovered later but when we put it all together we realized that within photosynthesis photosystem 2 actually functions first and then photosystem one comes into play later we're going to see that just like in chapter 7 for cellular respiration and the metabolism of glucose we're also going to have an electron transport chain in photosynthesis and then we're going to see that there are two enzyme complexes involved that are important and embedded in the thilo coid membrane as well one of these is called nadp reductase shown here and that is going to take nadp uh plus the oxidized version and turn that into the red uh reduced version nadph that high energy electron carrier we also see ATP synthes again like we did in chapter 7 we're going to use energy created by an electrochemical gradient to generate ATP let's look at the photos systems in more detail so we have photosystems 2 and one and they both are composed of a light harvesting complex which is shown in the purple areas these are proteins that have pigment molecules embedded within them that can absorb the energy from light in the center we also have a reaction Center complex and this includes a pair of chlorophyll a molecules and something known as a primary electron acceptor so what's going to happen is light will hit the light harvesting complex within the photosystem and again this whole thing is embedded in the Philo coid membrane and the energy from the light will be pass from pigment to pigment in the photosystem until it reaches the central pair of chlorophyl a molecules remember earlier we said when photons hit these molecules they can send electrons from a lower energy state to a high energy state so when this happens the electrons are excited and passed to a primary electronic sceptor this is a type of light driven redo reaction remember that something is reduced and something is oxidized in this case the primary electron acceptor is reduced because it's gaining those electrons and the chlorophyll a is oxidized so we have to replace those lost electrons from chlorophyll a and how this happens in photosystem 2 is from water so the reason we need water as a reactant in photosynthesis is to replace those missing electrons and oxygen is a result of this reaction it's a product of this reaction If This Were photosystem One electrons will come from the electron trans transport chain if we look at the light reactions in more detail again this is the thid and then we have the stroma out here and the lumen in here our photos systems are embedded within the thid membrane so earlier I mentioned light is absorbed by the pigments ultimately exciting the electrons from the chlorophyll a to the final electron acceptor here and those missing electrons from chlorophyll a are replaced by the electrons donated from water producing oxygen as a product those electrons will continue down the first electron transport chain shown here so the electrons are first passed to plastoquinone or C or PQ those electrons are then passed to the cytochrome complex and then they continue to plasto in and just like we saw in chapter 7 as electrons are passed from protein to protein down this electron transport chain it gives some of these proteins the ability to pump protons from the stroma into the Lumen of the ethoid so now we have an increase in protons within the Lumen so these electrons are going to continue down and they are going to enter photosystem one where something similar happens photosystem one absorbs more light and the chlorophyll a those electrons will be excited again and those electrons will pass through the second electron transport chain electrons will first be passed to ferado oxin and those electrons will be passed to nadp reductase and the final electron acceptor in this case is nadp+ which will accept those electrons along with the proton to generate our high energy electron carrier nadph all right so once more again we have our photosystems two and then one again remember electrons are excited from chlorophyll a and those electrons came from water those electrons will be passed down the electron transport chain and fill in the chlorophyll a electrons that were missing from the previous reaction those electrons are continually passed down to the second electron transport chain and UL Ely end up on the nadph molecule as the electrons move through we see that protons were pumped into the Lumin and that creates that proton gradient so there are more protons in the F liquid space that loom in compared to the stroma so these protons really want to go down their concentration gradient from high to low concentration and they cannot pass through the membrane because they are charged so they pass through ATP synthes and that releases a lot of energy to power the production of ATP from ADP and inorganic phosphate and that is through chemiosmosis remember the most common form or method of producing ATP since we have electron transport chains both in the chloroplast as well as in mitochondria as we saw in chapter 7 it's nice to see how they're similar so the electron trans chain in mitochondria were found in the Christi that inner membrane of the mitochondria in chloroplasts it's similar because they're found in the thilo quid membrane in both cases during the transport of electrons down the electron transport chain protons are being pumped from one side of the membrane to the other in the mitochondria they're being pumped from the Matrix from the inside of the mitochondria to that intermembrane space the dark gray region here in chloroplasts they're being pumped from the stroma which is the space here into the thilo Lumin into the dark region here that's going to generate a high concentration of protons in this upper region here the intermembrane space or the Philo quid Lumen and these protons really want to come back out going down their concentration gradient so when they're allowed to do so through ATP synthes that will generate ATP through CH osmosis so all of the previous section was part of the light dependent reactions this included the photo systems our electron transport chains and the generation of nadph and ATP these products nadph and ATP will be needed for the light independent reactions or the Calvin cycle and again remember even though we don't directly use light for the Calvin cycle it cannot occur without light because of the requirements for ATP and ndph all right so let's look at the light independent reactions or the Calvin cycle there are three stages here the first one is fixation where I fix carbon dioxide I take it from the air and I turn it into an organic solid carbon based molecule the second stage is reduction where I'm going to take electrons from nadph to reduce and produce a sugar a carbon based sugar compound and then I have to regenerate a product rubp that is continually recycled in the Calvin cycle for this to continue to happen in the stroma it looks like I have to go through the Calvin cycle three times in order to generate one glycer alide 3 phosphate molecule so what's shown here is just one cycle but I know to generate 1 3gp I would need to go through this three times so let's just pretend I'm putting in one carbon dioxide for now if I look at rulos spis phosphate that has five carbons and it looks like the main enzyme that goes through the carbon fixation process is called rubisco so rabisco has the ability to bind to carbon dioxide from the air and attach it to rubp to generate a six carbon compound so that's carbon fixation right after that it breaks it in half to generate two three carbon compounds called phosphoglycerate or three PG three phosphoglycerate it looks like there is some ATP that's used up to phosphorate both ends of this molecule and then my reduction happens where I get electrons from nadph and I reduce my carbon molecule into some kind of three carbon molecule g3p but I can't predict uce this unless I have more carbon dioxides coming in so ultimately I'm going to go through the Calvin cycle three times putting in three carbon dioxides and getting out one g3p glycer alide 3 phosphate molecule in order to ultimately produce glucose so usually what happens is these are put together to produce glucose or to produce starch or to produce even cellulose for our cell walls and the plants at the end of the cycle I have to consistently consistently regenerate rubp and that require some more ATP all right and just like chapter 7 you don't have to know every single step of the Calvin cycle I want you to know where it happens so this is happening in the stroma that liquid portion of the chloroplasts what goes in and what comes out so what goes in are our carbon dioxides and specifically we we need three carbon dioxide molecules to generate one glycer alide 3 phosphate product which ultimately will be used to form glucose or other compounds like starch or cellulose we need to know the phases as well or the stages the first stage is carbon fixation and that's completed by the enzyme rubisco which has the ability to take a carbon dioxide and bind it to rulos spus phosphate so this is carbons and one CO2 that's one carbon that will allow carbon fixation to happen turning carbon dioxide into an organic carbon compound I know the along along the way I have some phosphorilation of my molecules are my intermediates to trap them within within the stroma there is a reduction that occurs and that's why electrons from nadph are needed and then the Regeneration of rubp so that the cycle continue requires part of that phosphorilation from ATP and even though the Calvin cycle is sometimes known as the light independent reactions or the dark reactions remember that they cannot occur without the light reactions there's light that's needed because the light reactions are what generate these reactants ATP and AD n ADP that are required for the Calvin cycle to happen so rubisco what is RIS go stand for the enzyme name is really ribulose bisphosphate carboxylase oxygenase that's a big word you don't have to know what it stands for everyone pretty much just calls it rubisco the enzyme that fixes carbon so the nice thing about it is that it can fix carbon and that's when it's acting as a carox ASE for the Calvin cycle unfortunately rabisco also binds to oxygen and can act as an oxygenase which happens when it's going through photo respiration so photorespiration is actually a wasteful reaction that sometimes happens when the levels of o2 are too high and CO2 is low so in photorespiration you do not make sugars but you do use up energy luckily some plants like this Cactus shown here have evolved mechanisms to reduce the chance of photorespiration since rabisco can bind to both CO2 and O2 so we want rabisco to bind to CO2 but unfortunately it can also bind to oxygen when it binds to CO2 that's when we get the Calvin cycle when it binds to O2 that's when we get photorespiration which we don't usually want in Plants our book doesn't really get into the details of photorespiration so I'm going to cover it really briefly and you'll learn more about it in the next biology course in the series but most plants are called C3 plants and the reason for this is because rubisco when it fixes carbon dioxide one of the first products that's formed is three phosphoglycerate a three carbon compound in photorespiration since rubisco can also bind to oxygen what happens is that rubisco binds to oxygen instead of CO2 and that's going to produce a two carbon compound if you look it up on the internet which uses oxygen and uses up sugars and ATP and releases CO2 and it does not produce more ATP or sugar for the plant so for C3 plants photorespiration is seen as a wasteful reaction so most C3 plants what happens is since rubisco can bind to CO2 and oxygen and this is for C3 plants it really depends on what's more present or more prevalent in the environment that will determine if Calvin cycle happens or photorespiration happens luckily we have some plants that have adapted to their environment and can overcome this photorespiration wasteful reaction one example is uh known as or these are known as C4 plants like sugar cane so what happens is in C4 plants they have another set of cells called bundled sheet cells and rubisco is actually going to pushed over here so rubisco is found within the bundle sheath cells of these C4 plants so when they open their stomata and CO2 comes in it turns into a fourar carbon molecule first using a different set of enzymes this four carbon molecule is shuttled into the bundle sheet cells where rubisco is found and now we don't have to worry about rabisco binding to oxygen because it's in an inner lay layer of the plant and now we can have the Calvin cycle happen so in C4 plants they separate um the rubisco from its normal location and by doing this they reduce the chance for photorespiration happening we also have another set of plants called cam plants and this you don't have to know but it stands for culian acid metabolism and we see this in succulents like pineapples they're pretty neat because they separate the steps of the Calvin cycle through time so at night when it's cool and dry they open their stomata to let CO2 in and during the daytime when it's hot and they're closing their stomata to prevent the loss of water that's when rubisco comes into play to bind to carbon dioxide and allow the Calvin cycle to continue so chapter 8 from our book has a few really nice links to animations and videos that I recommend um the first one is from the California Academy of Sciences that I showed you earlier in the first video this one is a really nice way to visualize where photosynthesis happens the second one looks at an overview of photosynthesis including the light reactions and Calvin cycle it's more General but it's really good to look at to summarize your learning and the last one is from hhmi Howard Hughes Medical Institute and it's a really nice more detailed review of the light reactions in Calvin cycle so again I recommend all of these they're pretty good to review chapter 8 all right and that takes us to the end of chapter 8 on photosynthesis in the next chapter chapter N9 we're going to be talking about cell communication