Testing a leaf for starch for IGCSE Biology

hi today what we're doing is we are testing leaf for starch we're testing variegated leaf and that is one which has chlorophyll in some bits and not in other bits I'll just put it there so you can see in order to do this we're going to need some hot water we're going to need tripod heating equipment and we're gonna need some iodine solution and some ethanol and some safety specs will dawn the safety specs now right let's get this water fully boiling the lighting on safety flame putting on two full roaring blue flame and let's get that heated up now what we're going to do is we're going to boil this leaf this belief in this poured draggled prior to which maybe it seemed kind of days we're going to boil this in here for about 20 seconds or so and what that will do is it's going to denature the enzymes going to kill the leaf and that will stop photosynthesis what we're trying to do is see where the leaf is now so we're going to stop all the living processes going on inside the sleep by denaturing its enzymes great our water is boiling now we're going to turn that off here it's hot enough don't need to keep it boiling just get the leaf under the water there we go and we're just counting for about twenty seconds that's all that's needed is going to soften the leaf you're going to soften the cellulose fibers in there as well as well as do you know join the enzymes so think that will do now importantly we've switched off our flame because now what we want to do is submerge our leaf in boiling ethanol ethanol of course is extremely flammable and so we don't want any naked flames near it and we'll boil the ethanol by putting it into what about here so how do you come mr. variegated leaf pop you in there and we're going to leave that's in there boiling for about two minutes what this is going to do is the chlorophyll doesn't dissolve in water you know that when you boil broccoli at home you don't get white broccoli at the end of it because the chlorophyll stays in there it's not soluble in water it is soluble in ethanol and we are now going to remove the chlorophyll that's not because we're expecting photosynthesis to carry on it's because we're going to be doing color test on this at the end of it and we don't want any green pigments from the chlorophyll in place to see to interfere with our test well reckon was boiling very nicely now we're just going to hold this under our camera here you can see it boiling really really well we're going to leave that for two minutes and come back to it yeah that is pretty much there it's done very well we've done two minutes we've dissolved the chlorophyll and I'm going to hold it under the camera here so you can see that it's got little bit of hot in the pupil so I'm going to use my tongs and you can see there we've dissolved the chlorophyll that's gone off into the ethanol and you can already see the leaf is white underneath it okay we're going to pour off the ethanol we're going to put that actually fat straight back into here and also gonna put the leaf into there as well the leaf well I'll show you first before pour it in it's gonna come out all kind of crispy it's rather peculiar now that it comes that's all kind of crispy it's not floppy at all indeed if can probably make you hear it well I'm not sure here we go so we're gonna drop it into here that'll soften the leaf up because we need it soft we're going to need to spread it out onto our white tile in order to look for results so on to our leaf down here now will remind us as why we're doing this we're looking to see where the starch is in this leaf that's because the starch is indicative of photosynthesis having happened for the synthesis makes glucose glucose is polymerized and stored as starch so the regions of the leaf where there is starch that suggests that that's where photosynthesis if it has been occurring okay let's go and we can put our iodine solution in it and we're going to drizzle our iodine solution onto it then we can we let it just this will let the stay in take for just few seconds and that's very beautiful we can see that really nice little just zoom into it there we go and you can see here the pattern of the stars that's the black areas rather than the unstained orange areas the black areas are where the starch is and also they correspond exactly to where the chlorophyll was and therefore this is evidence it's not proof but it's evidence that the chlorophyll is required to make glucose to make start the chlorophyll is required for photosynthesis thank you you