hi everyone and welcome to learn a double biology for free with mr. ik in this video I'm going to be going through the first stage of photosynthesis which is the light dependent reactions so first of all the light dependent reactions is one of two stages in photosynthesis because it begins with the light dependent reaction and then you have the light independent reaction and these two reactions whilst they both occur in the chloroplast they occur in different locations and you do need to know those for the exam it could be a short one mark question so the light dependent reactions occur in the thylakoid membranes or you can refer to those as the grana for plural or Granum for singular so the thylakoid membranes are the outer parts the membrane and they form these stacks of membranes which are called the Granum so it's the first stage and as the name suggests it's the stage that requires light and what that light is doing is is providing energy which is needed to split the water as well as some other stages which we'll go through to create ATP and reduced nad P and these two products are needed in the light independent stage so the light dependent reactions are split into four key reactions which will go through photo ionization of chlorophyll photolysis of water and we'll look at how those result in ATP and reduced nad P production and chemiosmosis so first of all photolysis of water and the way to remember this is perhaps pronounce it differently photo lysis photo in biology whenever you see this term refers to light and lysis means to break apart or to split something so photolysis or photolysis of water means light energy is splitting water and the word equation or symbol equation for this we can see below so the water molecule is split into oxygen electrons and protons and the protons which are hydrogen ions are picked up by nad P which is a coenzyme and that is how we get the reduced nad p or NADPH being formed and that is needed in the light independent reaction the electrons are passed along a chain of protein carriers and they release energy which is needed in this stage the oxygen is not used in photosynthesis so it is technically a waste product of photosynthesis however it could be used in respiration or it might simply diffuse out of the leaf through the stomata now photo ionization so again we've got that term photo so we know it's to do with light energy and ionization is the loss of an electron so light energy is causing electrons to be lost from chlorophyll so that's what's happening in this stage the light energy is absorbed by the chlorophyll and that light energy is picked up by the electrons and because they gain energy they then become excited that's the phrase we give this States they've become excited they raise up an energy level and this causes them to break free and release themselves from the chlorophyll molecule so that is why the chlorophyll has been ionized it's lost an electron and it's due to the energy gained from absorbing the light energy and that energy from the electrons that have been released goes towards making ATP and also reduced nad P in chemiosmosis so that's the final step where we can actually fully see how NADPH which is reduced nad p and ATP are created so it's a series of steps step one the electrons that were gained from the photo ionization of chlorophyll are picked up by proteins which are embedded in the brain in the chloroplasts and that's what you can see here this electron which would have come from the chlorophyll is picked up by this protein and then it passes along what we call an electron transfer chain and those are proteins embedded in the membrane now as those electrons move to each protein they release energy at each movement and that energy is used to actively transport or pump the protons across the membrane so protons that are within the stroma of the chloroplast are then using the energy from those electrons to be actively transported across that membrane into the thylakoid lumen or inter membrane space now that results in a high concentration of protons in this inter membrane space or the thylakoid lumen because we're also getting protons from photolysis of water which we can see here so we end up with lots of protons on this side of the membrane and very few in the stroma on the other side of the membrane and we describe this as an electro chemical gradient because it is an ion it has a charge that's where the electrode part comes in so because we've now got lots and lots of protons on one side of the membrane facilitated diffusion occurs but the only protein that protons or the hydrogen ions can diffuse through is this protein here which happens to be ATP synthase and this is an enzyme which catalyzes the reaction of ADP and an inorganic phosphate joining together to make ATP and as protons are diffusing through ATP synthase that actually causes a change in shape of the protein at the top here and it's that change in shape which provides the energy to catalyze this reaction so that's how the ATP is produced the final step is looking at how the reduced nad P is created and that is the protons which are then all moving back down their electrochemical gradient into the stroma some of those protons will be recycled and pumped back around against the process can continue but some of the protons are picked up by the coenzyme nad P and when nad p picks up the protons it becomes reduced and so it picks up the protons but also it picks up the electrons at the end of the electron transfer chain so that's why we describe as being reduced because picked up the electron as well as the proton and that's why sometimes you'll see it written as NADPH because a proton plus an electron makes a hydrogen atom or a QA tend to just phrase it as reduced in ADP because in picking up that proton and the electron gaining the electron means it is reduced so that is the stage of chemiosmosis this idea of creating an electrochemical gradient so then that the protons can move down that gradient and that enables the production of ATP so just to summarize then the light dependent reactions we said it involves chlorophyll absorbing light energy that leads to the photo ionization of chlorophyll some of the energy from the electrons that are released in photo ionization is required for the production of ATP and reduced nad P in chemiosmosis production of ATP involves the electrons moving down the electron transfer chain and as they do move down it releases energy to enable the protons to be pumped from the stroma into the inter membrane space that creates this electrochemical gradient so the protons then move down their concentration gradient back into the stroma by facilitated diffusion and the protein may pass through is ATP synthase and that's the step which is chemiosmosis we also have photolysis or photolysis of water which produces protons electrons and oxygen so that is it for the light tenant reactions [Music]