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Current time:0:00Total duration:9:05

Conceptual overview of light dependent reactions

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Video transcript

we've seen in previous videos that photosynthesis can be broken down into the light dependent reactions and the Calvin cycle and the light dependent reactions is where we take light as an input along with water and we'll see the water is actually a source of electrons and we can use that to store energy in the form of ATP and NADPH and as a by-product we produce molecular oxygen which is very important for us to breathe and then that ATP and that NADPH can be used in the calvin cycle along with carbon dioxide to actually synthesize sugar what we're going to focus on in this video are the light dependent reactions how does this process right here work and to help us think about this we are going to zoom in onto a thylakoid membrane so this is a thylakoid right over here sitting inside of the chloroplast and if we zoom in on its membrane we see it's a phospholipid bilayer like many membranes that we see in biology and at first glance that might this might seem like a very complex diagram and that's because it is a complex diagram and you will often see things like this in your biology textbooks and it can be very intimidating these these proteins and molecules and complexes have very complicated sounding names but the general idea of what's going on is you'll hopefully find a pretty straightforward you have the energy from light photons from light are going to either directly or indirectly excite electrons those excited electrons they're in a high energy state at they're going to be transferred from one molecule to another and they're going to go to lower energy states that's what allows those transfers to be spontaneous for them to actually occur they are going from a higher energy state to a lower energy state the electrons are getting more and more and more comfortable and some of that energy that's released as the electron goes from a higher energy state to a lower energy state is used to pump hydrogen ions hydrogen ions across the membrane from the outside of the membrane in the stroma to the inside of the membrane to within the thylakoid lumen so you are building so you're building a a hydrogen ion concentration gradient concentration concentration gradient where you have a higher concentration inside then you have outside and this by itself this concentration gradient as we'll see can be used to fuel the production of ATP by ATP synthase that those hydrogen ions want to get back out they want to go down their concentration gradient and as they go back out through the ATP synthase it essentially turns that motor that can jam the phosphate group onto ADP to produce ATP so one way to think about it this is producing a hydrogen ion gradient so we could do it this way we could say H+ gradient which is then being used to produce the actual ATP now the electrons going from a high energy state to a lower energy State in this part of the light-dependent reactions that by itself isn't the only thing that is contributing to the high hydrogen ion concentration gradient once that electron gets donated you might say well how does it get replaced well the thing that's doing the donating the thing that eventually gets excited and donates that electron it's a chlorophyll a variation called p680 p680 is referring to the P stands for pigment 680 stands for 680 nanometers the wavelength of light that it absorbs best and so when it gets excited it becomes you'll see the notation often of p680 star that's when it has an excited electron and then after it gives away its electron it becomes P it becomes P 680 p680 with a positive charge and this P 680 we could call it P six eighty plus right over here or maybe a P six eighty ion this is actually a very strong oxidizing agent it one of the strongest if not the strongest do we know in biological systems and so it really likes to grab electrons from other things and the thing that is around that it can grab electrons from is actually water and so this is such a strong oxidizing agent that it can essentially oxidize the oxygen in water and oxygen is itself I mean oxidizing is named after oxygen because oxygen is such a strong it's so electronegative it's such a strong it's it's the thing that's normally doing the oxidizing so anyway it grabs its electrons once it gets this p680 + grabs an electron from water and then the water essentially falls apart so you're left just with the oxygen and then the hydrogen ions and so those hydrogen ions also contribute those hydrogen ions also contribute to the increased hydrogen ion concentration on the inside and this is where we get this is where we get the oxygen byproduct right over here here we have 1/2 of an o2 so if you do this twice you're going to have a molecular oxygen so so far I've we've talked about how the oxygen gets produced we've talked about how the ATP gets produced what about the NADPH well we started our process in photosystem 2 you might say why is it called photosystem 2 if that's where we start well it's actually that's because that's the second photosystem to be discovered you might say what is a photosystem well these photosystems and complexes they're combinations of proteins and molecules and photosystem in particular has chlorophyll and variations of chlorophyll and pigment molecules that are responsive to light that are very easy that have electrons that can get excited by light and they can transfer that energy back down to the p680 chlorophyll a pair which then can have its excited it's electron excited and then it can give that to an acceptor molecule and then it can go to lower lower energy state and to end pump that those hydrogen ions out but that's not the entire light-dependent reactions that electron can eventually make its way over to photosystem one and why is it called photosystem one well is because the first one that one that was discovered in photosystem one there's another chlorophyll a pair called p700 and that's because it optimally absorbs what light of a wavelength of 700 nanometers and you have something similar that happens that light can either directly or indirectly excite its electron and then that electron as it goes to a lower energy level it goes from one molecule to another it can be used to reduce nad plus into NADPH and so that's where the nad pH comes from and then once again once the p700 has given its electron it wants an electron and well it can get that from the electron that went that's been going from lower to lower and lower energy states that's essentially been making its way from you can conceptualize it as the electron that's been making its way from photosystem two and so that's why you'll often see these diagrams lights come in electron gets energy it gets excited it goes to lower and lower energy states as it's doing that it's being transferred from one molecule to another being facilitated by enzymes that that energy part of that energy is being used to transfer hydrogen ions into the thylakoid lumen into the interior then it then in photosystem one you have another excitation event and what that thing that got excited can grab that electron that went to lower lower energy states and it's excited electron can once again be transferred from one molecule to another in order to fuel or enter or provide the energy for nadp+ being converted into NADPH and once again the whole idea of the hydrogen ion concentration increasing here can fuel ATP synthase which allows us to jam a phosphate onto ADP to produce ATP so that is where we actually get all of these things and the byproduct of course is our oxygen and if you wanted to see that same idea but kind of just thinking from an energetic point of view without all the complexity of seeing the the physical components involved you see it right over here where you have light energy comes excites the electrons when it when once the p680 has given that electron away it wants an electron really badly it gets it from the water and then as that electron goes to lower and lower lower energy states it can eventually be grabbed by p700 that has given away it's an electron and then it and then that electron that that was excited at p700 by once again more light energy that can be transferred from one molecule to another to fuel the creation of NADPH and this part right over here this part this phase right over here as the energy goes from a higher energy state tool as as the electron goes from a high energy eight to a lower energy state fuels the pumping of hydrogen protons into into the actual thylakoid
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