welcome biologist to this session where we're going to take a look at the light dependent stage of photosynthesis so here i have a diagram of the feather core membrane which is inside the chloroplast now don't forget the thylakoid membrane can be folded into status called grana and it's within these membranes we have certain proteins that enable this light dependent stage of photosynthesis so on this side of the thylakoid membrane i have the stroma and on the other side i have what's known as the intermembrane space so the intermembrane space is here and then on the other side over here somewhere i'd have the other side of the thylakoid membrane because it kind of wraps around in loops so within this thyroid membrane i have a photosystem ii a photosystem one and atp synthase so the first stage of the light dependent reaction is photolysis of water so this is where water is broken up into hydrogen ions oxygen and electrons now the oxygen is a by-product and is released um from the plant and through diffusion through the stomata and however the electrons are used into photosystem too so what happens here is the photosystem the pigments inside the photosystem two will absorb the energy from the the light the photons of light and they'll pass on this energy to the electron and this will excite the electron to a higher energy level this electron then passes down a series of electron carriers and through a process of various redox reactions which is where they gain and lose electrons energy is released which actively transports hydrogen ions into the intermembrane space so what then happens is my electron passes into photosystem 1 where again photosynthetic pigments absorb energy from the photon of light and pass that onto the electron which is uh which is excited to a higher energy level and so i'm just going to leave the electron there for now and come back to it shortly but what i've got now is a buildup of hydrogen ions in the intermembrane space and this is also known as a proton gradient so this proton gradient that has been established will mean that the hydrogen ions can flow through this atep synthase which is an enzyme and they flow through atp synthase in a process called chemiosmosis now chemo osmosis is the process where atp is made using atp synthase within a membrane like this thylakoid membrane so what the atp synthase does is it adds an inorganic phosphate to adp to form atp and the hydrogen ion which has come through the atp synthase will then be added onto nadp along with the electron to form nadph so this is reduced nadp and nadp is a coenzyme which can be used to transport the hydrogen electron now into the light independent stage so that process that we've just gone through there is called um non-cyclic photophosphorylation now we need to know about non-cyclic photophosphorylation but also cyclic photophosphorylation so in non-cyclic photophosphorylation that we just looked at as you can see the end the the electron passes through photosystem ii along the electron carriers then into photosystem one which is where after photosystem on it then binds to nadp but in cyclic photophosphorylation what we have here is photosystem 2 is not used at all so the electron passes down a series of electron carriers and through a process of redox reactions produces releases energy which actively transports the hydrogen ions into the into membrane space this electron will then be excited to a higher energy level from the energy absorbed from the photon of light in photosystem 1 and then the electron will re-enter the electron transport chain then pass down the electron carriers through that through the redox reactions releasing energy again now the reason why a plant would undergo cyclic photophosphorylation is usually if there is a limitation on the water availability because in this particular example of cyclic photophosphorylation the electron is constantly recycled now don't forget in the previous example of non-cyclic photophosphorylation my electron here came from the photolysis of water so if i don't have a lot of water available it means that the electron can't be replenished so therefore the plant will switch to cyclic photophosphorylation and what this allows the plant to still do is still to create atp through atp synthase because by still passing the electrons through the electron carriers it means that hydrogen ions are still going to be actually transported into the into membrane space i'm still going to get that proton gradient and therefore chemosmosis can occur so here i have a table just summarizing what's going on there you do need to know the differences and similarities between those two guys good luck with your exams all the best and use as much scientific terminal