all right so chapter 10 is going to focus on photosynthesis photosynthesis is the primary process by which organisms in the biosphere are able to get the nourishment that they need for their cells to be able to conduct their various processes the organisms that are able to conduct photosynthesis take solar energy and convert it into chemical energy organisms that are able to do this include your plants for algae some prokaryotes as well as specific protists autotrophs are organisms that are able to get their energy sources without ingesting those energy sources they are considered to be the producers when we talk about ecology they take organic molecule or make organic molecules from carbon dioxide as well other inorganic materials and plants specifically are photo autotrophs because when you are making a larger molecule you're going to need an energy source those are endergonic processes and they are considered photoautotrophs because they use the sunlight to do that heterotrophs get their energy sources their organic material from other organisms you can have primary consumers secondary consumers and so heterotrophs are going to either get their organic material directly from plants or they are going to get their organic material from other organisms that ingest plants and without our photoautotrophs we would not have enough oxygen in our biosphere so first section we're going to look at is just the pro overall what's happening with photosynthesis that it's taking light energy and using it to create chemical energy in the form of food chloroplasts are similar to mitochondria and that it is thought that they most likely evolve from bacteria in this case that are photosynthetic leaves are the primary location where photosynthesis takes place as we've talked about their color is coming from the pigment that's found within the chloroplast the chlorophyll we typically find the chloroplast primarily in the mesophyll cells those are the interior tissues of your leaf but there are microscopic openings that allow gases to move throughout the leaf and those are called stomata and you can have oxygen leave and carbon dioxide enter through those chlorophyll pigment is specifically found in the thylakoid membranes these are a series of connected stacks um and when they are stacked together they are known as an individual stack of thylakoid membranes is known as a grana or granum excuse me chloroplasts will also have stroma surrounding them which is just an interior fluid and we're going to talk about where the two parts of photosynthesis take place one will occur within the thylakoids and the other within the stroma so there's kind of looking at it zooming in on what's happening inside the leafs you've got your mesophyll cells which are filled with your chloroplasts you see the veins there you've got your stomata which are allowing the gases to enter and exit and then even within the chloroplast you can see the specific stacks of the thylakoid membranes you have a double membrane you have your outer membrane and your inner membrane inside the inner membrane is where you're going to find your thylakoids as well as your stroma so photosynthesis is the chemical reaction that's occurring is the exact opposite of what we saw with respiration you will notice with this one that you have light energy being your energy source as opposed to making energy in the form of atp but the reactants of photosynthesis are the products of respiration and vice versa other than the energy sources so it is a redox process the oxygen and water is oxidized it goes from negative two to zero and carbon dioxide the carbon is reduced it's considered to be an endergonic process because it needs energy to help to go through the process of forming that sugar molecule so it's an anabolic process we're making a larger molecule two main stages of photosynthesis and we're going to talk about each of these in more detail you have your light reactions and your calvin cycle the light reactions are considered to be the photo part of photosynthesis the calvin cycle is the synthesis part in the light reactions which take place in the thylakoids water is split oxygen is made the energy shuttle or the electron shuttle this time nadp plus gets reduced to nadph and atp gets made and because the energy source for the electrons to make atp is coming from light this is considered to be photophosphorylation the calvin cycle is where we actually get our sugar molecules made this takes place in the stroma using the co2 that has been taken in by the leaf and also using up atp and nadph made in the light reactions and there's a series of steps you go through in the calvin cycle the first one involves carbon fixation where the carbon dioxide molecules are incorporated into organic molecules that then can be reduced and subsequently modified into your glucose molecule so there you can see your two stages of photosynthesis and we're going to focus first on your light reactions within your felt light reactions you're making two sources of energy you've got your atp that's being made as well as nadph you are consuming water and you're generating oxygen so the chloroplasts are what allow this to happen light is a form of electromagnetic magnetic radiation also known as electromagnetic energy is able to travel in waves similar to like sine and cosine waves the distance it takes to go between crests is known as its wavelengths and we can use that distance to characterize what type of electromagnetic energy we have you can have energy that is you can have very short wavelengths that'll be very energetic and you can have very long wave leaks that will be not nearly as energetic the visible light kind of falls in the middle and those are wavelengths we can actually visibly see light does not just have this energy component to it it also has a mass to it very very small bits of mass but when it does have or when it is described as having these little tiny pieces of mass they're known as photons specific little particles that exist so there you can see your visible light and the range on the electromagnet magnetic spectrum from your shorter wavelengths which are your gamma rays to your longer wavelengths which are your radio waves so there are pigments in the chloroplast specifically and the thylakoids that are able to absorb certain wavelengths of light um and those pigments are typically we know talk about there's many different kinds in the leaves but the most common one we talk about is chlorophyll when the wavelengths are not absorbed if light is going to have all those different wavelengths available to it that's visible but not every wavelength is capable of being absorbed by those pigments certain pigments are only going to observe absorb certain wavelengths of light and if they aren't absorbed they get reflected and that's why we see leaves as being green because the wavelength that corresponds to the green color is not absorbed by chlorophyll it is reflected and so when you look at a leaf you see that green color as a result we can use photometers to measure how much absorbance is happening for different wavelengths for a pigment and so when that is done you can kind of get a better feel for what colors of light are going to be more effective for certain pigments and for chlorophyll it's going to be more in the reddish blue wavelength range so in the light reactions a photosystem is able to take the energy that is coming from light and is moved through these various pigments where the energy is transferred is able to use that to get electrons which can then be used to make atp and nadph so there are two photo systems that are in a part of photosynthesis or a part of the light reactions each of these photosystems has this reaction center complex it's a just a protein complex it has lots of light harvesting complexes surrounding it those light harvesting complexes are going to have the pigments like chlorophyll that are bound to proteins and they're able to move the energy from the light to that reaction center in the reaction center is a primary electron acceptor after it has after the electrons have been excited from the light energy this acceptor accepts those electrons and therefore gets reduced and so it does that by taking electrons from chlorophyll a and when chlorophyll a gives up its electrons it's oxidized and it needs more electrons to allow this process to continue and it gets those electrons from the process of water splitting so you can see in picture a the light photon is hitting those light harvesting complexes and they are bouncing that energy through each of those complexes eventually that energy is able to get to the reaction center complex where it hits the chlorophyll a molecules and excites them as it excites them the electrons get transferred and they eventually end up with the primary electron acceptor and at that point they can start to move down the electron transport chain and make atp and nadph the enzyme that catalyzes water splitting to get the electrons to re-energize that chlorophyll-a molecule is present on the photosystem itself so that the water that's in the thylakoid space is capable of being split so there are actually two types of photosystems photosystem 2 and photosystem 1. photosystem 2 is where water is split photosystem 1 is going to come at the end of your electron transport chain photosystem 2 will be at the beginning why they are named in that order is just the order in which they were discovered photosystem 2 is able to do a really good job of absorbing light at 680 nanometers while photosystem 1 does a better job of absorbing light at a slightly higher wavelength 700 nanometers but they both get those electrons excited in the same way and so how these electrons move primarily is through linear electron flow the light from the photons from light hit those pigments that are found in your complexes they allow the energy to be passed throughout until it hits that special chlorophyll a molecule in the case of photosystem 2 would be p680 the electrons that are then excited get transferred to the electron acceptor leaving behind p680 plus which really really wants to take on more electrons and that along with the enzyme that's a part of our photosystem helps to split the water and when it splits the water it's going to generate oxygen and some hydrogen ions and electrons which then get picked up by that p680 plus acceptor and reduce it as the electrons exit the photosystem they move down a series of proteins similar to what we saw with respiration um so that they can be the energy that is harnessed from those electrons can be used to make atp where the proton gradient is created just like we saw with respiration but the proton gradient is actually going to form in the thylakoid space and then the hydrogen ions will diffuse across that thylakoid space in to the stroma and atp will get made through atp synthase just like we saw with respiration so the electron transport chain works the same way it's just at the beginning of the process rather than the final piece like it was in respiration the same thing takes place that we saw with photosystem 2 with photosystem 1 but the light energy that's transferred excites a different pair of chlorophyll-a molecules p700 p 700 is able to take on electrons that have been moving through the electron transport chain and use those to be able to make atp the electrons get to move down a second electron transport chain to the protein ferridoxin and those electrons then get transferred to nadp plus allowing it to be reduced to nadph the electrons that are found with nadph and the atp molecules because they are found in the stroma are then able to be used in your calvin cycle and as a result of this process hydrogen ions do get removed from your stroma okay so there's your two photo systems you've got your light coming in exciting though the p680 molecules that's going to get water to split as the p680 loses a couple of electrons the electrons move through the proteins and the electron transport chain transferring those electrons creating that hydrogen gradient so that chemi osmosis can occur and you make atp as they move in to photosystem one some of the electrons are going to go straight to the p 700 after it's already been oxidized so that it can be reduced and take on more energy which we see from the light hitting those receptors and some of the electrons in the photosystem one and we'll talk about why it's just some and not all in a second are going to move out of that primary acceptor and move into ferridoxin allow the electrons to transfer via nadp plus reductase to make nadph so you've got photons come hitting both of the photosystems and they're both able to provide sources of electrons that will result in atp forming and nadph but i said that we were going to talk about why not all the electrons exit to make nadph you actually end up needing more atp than nadph for the calvin cycle and so not all the electrons go straight out into making nadph some of them cycle back through those different protein complexes and are able to make additional atp you don't get oxygen or additional oxygen but you do get additional atp which is what's going to be needed for the calvin cycle to work effectively and there's some thought that this might have happened because it doesn't require oxygen prior to a linear electron flow so this is perhaps a means to protect cells that have damage due to light and there are organisms who truly only have ps1 photosystems as opposed to having both ps1 and ps2 photosystems so here you can see the cyclic electron flow again the electrons are still going to be moving from photosystem 2 to photosystem 1 but once those electrons get into photosystem 1 they are able to cycle back through those protein carriers and generate more adp and then eventually some of those can go off from that primary acceptor and photosystem one to make nadph along with the ones that went directly to nadph so how you're able to do this without photosystem 2 remember you still have light hitting the chlorophyll pigments in photosystem 1 which can excite the electrons at your chlorophyll a molecule to move to your primary acceptor and since not all the electrons move out some of them move back through to make atp you would have additional electrons that could then reduce your chlorophyll a molecules so that they could continue to take on additional electrons and allow this process to continue so i've talked a little bit about the similarities and differences between chemiosmosis and chloroplast and mitochondria they both make atp using chemiosmosis but they get it from different energy sources mitochondria get it from the their chemical energy their shuttles from the electrons that they have acquired from breaking down their food molecules the chloroplasts are able to use light energy to get those electrons excited so that they then can make atp they are organized in a similar manner you have a double membrane with both chloroplasts and mitochondria but the process of the a of chemiosmosis is actually going to be inversed or reversed in the chloroplast as opposed to mitochondria in the mitochondria the protons are moving out of that inner matrix and then they move back in via atp synthase to make atp while on the chloroplast as i said previously the protons are moved out of the stroma into the thylakoid space and they move back into the stroma to make their atp part of it is that when the mitochondria is done making atp that's it it's ready to go but with the chloroplast they need to have that atp available to conduct the calvin cycle and so since that takes place in the stroma it's much more effective for the atp or much more efficient for the atp to end up in the stroma when all is said and done but the way that the proton motive gradient is formed and the way the atp is generated that is the same again the direction in which is happening and then the source of how the electrons are able to create that gradient is going to be another difference between these two organelles so here is a little bit more in depth of what's happening with the light reactions the thylakoid space is going to have your higher concentration of hydrogen ions your stroma is going to have your lower hydrogen concentration we go we excuse the light in photosystem 2 to excite those chlorophyll pigments that's going to cause water to split it's going to provide electrons that'll move down your electron transport chain that's going to help to create your proton motive gradient and that's going to help to make atp increase in your stroma as you move into your second photosystem photosystem one the light continues to hit those chlorophyll pigments some of the electrons come out and are able to generate nadph that is made in the stroma and then others go back through that cyclic flow to make more atp molecules and both of those are going to end up in this drama when all is said and done so we're getting everything that we need ready to be able to do the calvin cycle where we're actually going to make our sugar and just like the citric acid cycle the calvin cycle has to have the initial materials that are needed to get the cycle going always available so it has to regenerate those materials after they have been transformed by binding to carbon dioxide so the way this cycle is able to make sugars is by taking atp and taking nadph and using the carbon dioxide that has been absorbed by the leaf through its stomata and it's able to fix those carbon dioxide molecules onto a five carbon sugar that are five carbon molecule that's already present in the stroma and it does it via a ubiquitous enzyme we're going to talk about it in a second rubisco and as it goes through a series of changes it gets reduced it's able to the cycle results in making glyceraldehyde 3-phosphate g3p d3p consists of three carbons and so you have to have three co2 molecules enter the calvin cycle and get fixed and if it's subsequently reduced so that you can get one g3p molecule to exit the calvin cycle and we'll talk about why when you go through this fixing and reducing process you're going to need to use up so much atp and nadph and then to reform the starting materials in the cycle you're going to need even more atp to get things ready to go for the next set of carbon dioxide molecules so three steps you start with your three co2s when that is happening the carbon is being fixed it's catalyzed by this enzyme rubisco each carbon dioxide molecule is incorporated into a five-carbon molecule using that enzyme rabisco is considered to be probably the enzyme that is present in the greatest amounts in cells the six carbon molecule that is formed when the carbon dioxide molecules are fixed to that five carbon intermediate is very unstable and so when it does that if it made three six carbon molecules they actually split into six three carbon molecules and those are known as 3gp we'll talk about those when we get to the reduction process eventually well actually let's take it back so it's going to split into two molecules of 3-phosphoglycerate 3pg when we go through reduction that's where we're going to use our adp and that's where we're going to use our nadphs each of those 3pg molecules is going to use an atp to add on a phosphate group and it turns into another intermediate 1 3 bis phosphoglycerate you don't need to know that specific name that intermediate then gets a pair of electrons from nadph and makes what we need to be able to form our sugars glyceraldehyde 3-phosphate g3p that one you do need to know because it does it to all of the three gp molecules you use up six atps and six nadphs through this reduction process only one of them leaves why does only one of them leave the other five are going to be needed to regenerate your starting materials the rubp which is what accepted the carbon dioxide in the first place and so the five g3p molecules that have been formed are going to go through a series of rearrangements and as they do that we need three five carbon molecules to be formed so we need 15 carbons total well we had 15 carbons from our five three gp g3ps that were left over and so after the rearrangements occur we're ready to go to fix three more molecules of carbon dioxide but for these rearrangements to take place atp is needed to facilitate that process you need an additional three atp molecules to make that occur so this is looking at it more step by step all the gray circles are your carbons and you can kind of see where you've added the phosphate group from 3-phosphoglycerate to 1-3-bis phosphoglycerate then you can see when you've added the nadph and used that to provide a source of electrons you've pulled off that extra phosphate group that you've created a more stable compound the glyceraldehyde 3-phosphate so carbon fixation is using rubisco and rubp to get the carbon into the cycle reduction we're able to use atp and nadph to create a three carbon molecule or generate a three carbon molecule that can then be used to make glucose or other organic materials as a starting material you put two g3ps together and you get glucose and then we take the remaining five g3p molecules along with atp and use them after being rearranged to make three of those rubp molecules the ribulose based phosphate so that the cycle can continue so the final thing we're going to talk about is how we're able to fix carbon in environments that are not as conducive to having ready amounts of co2 to be taken in or the ability to let go of oxygen plants when they have their stomata open don't just let go of oxygen and take in carbon dioxide they can also give up water and they need water to survive and so if they are in an environment where it is super dry super arid and they need to hold on to their water they may close their stomata and that doesn't allow the oxygen to leave and it also doesn't allow them to take in carbon dioxide when this happens a process called photorespiration occurs which is not the best use of the resources that the plant cells have if you are in an environment where you can keep the plants can keep their stomata open we typically are dealing with c3 plants and those are the ones that fix the co2 the way we just described it via rubisco and make 3pg if you're in an environment where the stomata are not able to stay open and the oxygen is staying in the plant cells or the plant leaves the rubisco will pro add on oxygen rather than co2 to rubp it has a preference for oxygen over carbon dioxide and when it does that it makes a two carbon molecule it uses up carbon sources from the rubp it takes up oxygen and it doesn't make atp and it doesn't make a sugar so not the best use of the resources that are already available it's thought potentially that this is left over from when there was not as much oxygen in the atmosphere and that rubisco has evolved to be able to take on incorporate co2 it does help to limit light reactions that are making products so that they won't build up because otherwise you just have all this oxygen building up in your plant cells but it uses up so many resources that the plants need to be able to make their food materials that it is definitely not the way to go so a couple of types of plants um over 80 percent do see three we're talking about plants that are going to be in more arid environments more dry environments that are going to need to come up with or have evolved to come up with ways to handle photo respiration c4 plants use spatial separation they take in co2 when the stomata are open and they fix the carbon dioxide into four carbon molecules um in the mesophyll cells using a different enzyme pep carboxylase carboxylase does not have a preference for oxygen it actually has a much higher affinity for co2 so it doesn't really matter the levels of carbon dioxide pep carboxylites is going to want to choose to fix co2 over oxygen the four carbon molecules now that you don't have the co2 just running around are going to move into the bundle sheath cells which surround the mesophyll cells and then as needed they can release the carbon dioxide so that it can be used in the calvin cycle and be able to go through and make the sugars the other type of adaption that we've seen plants make are is seen in cam plants and this is a temporal separation these plants will only have their stomata typically open at night crassulation acid metabolism and so when they have their stomata open at night they take in carbon dioxide and they fix it into organic acids and basically store it in vacuoles when the stomata are closed during the day they release that carbon dioxide from the organic acids and then they can use that carbon dioxide can then be used to go into the calvin cycle and make your sugar so both of these adaptations are ways to keep the carbon dioxide levels um where you need them to be in the cells so that the calvin cycle can occur effectively with carbon dioxide as opposed to taking on oxygen so overall photosynthesis we are getting our energy from sunlight we're using that energy to make our organic compounds the organic compounds that are made are able to serve as a framework to make more complex organic molecules in your cells plants are able to store sugars so they can use them when they need to in things like roots tubers seeds and fruits and just like with respiration a side product of respiration was that water got made here in photosynthesis a side product is oxygen gets made but it's a very valuable side product because oxygen is going to be used by very many organisms in our atmosphere or in our biosphere and so just overall there you see what's kind of happening with your light reactions as well as your calvin cycle