hi guys in this video we'll be looking at an overview of the light dependent stage the photolysis of water and then we'll finish with a summary so the light dependence stage is the very first stage of photosynthesis and it occurs in a particular region which is on the thylakoid membranes of the grana inside the chloroplasts so here's a diagram just to recap the structure of a chloroplast we have two regions inside the chloroplast we have these stacks of membranes where each stack is known as a thylakoid membrane and several stats together is known as a granum where several of these are grana and so it's on these membranes where we have the first step of photosynthesis this light dependent stage can be divided into three steps or three phases first of all we have the photolysis of water so in this process we have a water molecule which is h2o so two hydrogens and an oxygen and it gets broken down into its component parts using sunlight energy so the energy from the sun comes in and breaks apart the water molecule into its components and these components include the oxygen electrons and hydrogen ions then we have production of atp in the presence of light and this process is called photophosphorylation so we start off with a molecule of adp which only has two phosphate groups and we add this to a third phosphate group and in order to add these together we have to use added sunlight and in doing so we form atp which now has three phosphates we call this photophosphorylation because the photo part responds to the light and the phosphorylation is because we've added a phosphate group to adp to form atp so this is phosphorylation and then the third stage of the light dependent stage of photosynthesis is the formation of a molecule called reduced nadp so there's a particular coenzyme which is known as nadp and it can be reduced to its other form which now has hydrogen atoms added to it so it's now called reduced nadp so forming this molecule is the last stage of this reaction so first let's talk about the photolysis or photolysis of water the photolysis of water is the first main step in the light dependence stage of photosynthesis so remember this is happening on the thylakoid membranes in these grana and here we have the first step of the light dependence stage which is the photolysis of water and remember we're breaking h2o down into oxygen and electrons and hydrogen ions in photolysis and this uses energy from the sun so to go through a bit more detail in photolysis a water molecule is split into protons electrons and oxygen in the presence of light so we start with our water molecule as a reactant and remember this is taken up at the roots of the plant it's h2o so we have two h's and one single o atom and then we have sunlight added into this reaction to provide the right energy the water gets broken down into the oxygen atom of which there is obviously only one of them and the hydrogen atoms are very simple a hydrogen atom only consists of one proton surrounded by one electron so when each of these gets broken up they form an electron and a hydrogen ion a hydrogen ion is just a proton so we get two of these because obviously there were two hydrogens and so we get two electrons and two hydrogen ions the photolysis reaction is catalyzed by an enzyme and this enzyme is found in the photosystem 2 or ps2 so here's the photosystem 2 which is this funnel structure on the thylakoid membrane remember it's a funnel shape so that we've got all of the pigments funneling light energy downwards and somewhere in here are the enzymes which are able to capture the light energy from the sun and allow this photolysis reaction to occur the oxygen that gets produced in photolysis is used for respiration by the plant so remember in breaking down the water we formed not only the electrons and protons but the leftover oxygen too and oxygen is an important reactant in respiration the plant has to do respiration to provide energy and atp for particular processes like growth or development and so the oxygen doesn't go to waste it can actually be used in respiration if the rate of photosynthesis is greater than the rate of respiration then any oxygen which isn't needed or excess oxygen diffuses out of the plant leaves through the stomata so if there's more photosynthesis than respiration then basically the plant is making more and more oxygen than it needs in respiration so the oxygen that's building up in the leaves simply diffuses down its gradient out into the environment and remember the pores of the leaves are known as stomata so the oxygen can go one of two ways but the protons in the electrons from photolysis will be important in the next step of the light dependent reaction which is photophosphorylation so we had our water to begin with we went through a process of photolysis using sunlight in the process we made oxygen two electrons and two protons and it's these products which will go forward into step two which is photophosphorylation hey guys i hope you enjoyed the video if you are looking for an amazing a level biology resource join me today in my series of engaging bite size video tutorials just click the snap revise smiley face and together let's make a level biology a walk in the park