[Music] now there are some really key core practicals and experiments that you should know about in this topic you want to know about how you can show that starch is produced in photosynthesis which is comes from the glucose and you need to know how or then be able to demonstrate how oxygen is given off by a water plant in photosynthesis so you need to be able to know some experiments that you could adapt to show these two things happening and also to be able to show how light and chlorophyll are required for photosynthesis and carbon dioxide for that to work so plants make glucose during photosynthesis we know that and they can store it in their cells as starch now you can test a leaf for starch and prove that starch is there using iodine iodine is a chemical which turns color it goes from yellow brown to blue black if starch is present so what you've got to do is you take a leaf from a plant and you boil it in some water for about 30 seconds that kills the leaf then you need to boil it in ethanol but at this point it's very very important that you switch off the bunsen burner there's quite often a mark available for that in the mark scheme on a question for this because that's a safety issue ethanol is very flammable turn off the bunsen burner pop a boiling tube of ethanol into the beaker which already has the hot water in and pop the leaf into that boiling tube and the ethanol will boil anyway because it has a lower boiling point than the water what will happen is all the chlorophyll will come out of the the leaf in the boiling ethanol and so the ethanol starts to go green and so the reason you do that is because you there's no point adding iodine and looking for a color change if the leaf is all green so after a minute take the leaf out with some forceps rinse it under some cold water and spread it out nice and thinly on a white tile drop some iodine on it and you should see as you can see with these results that the leaf will go blue black as starch is present there's another leaf here which hasn't gone blue black it's stayed brown yellow with the iodine now that's because that leaf has been de-starched what that means is that leaf has come from a plant which has been kept in the dark for a few days now if the plant is in the dark it can't photosynthesize if it can't photosynthesize it needs to get another source of glucose to dawn to do respiration and it gets it out of its starch that is stored so it uses up all the starch reserves and therefore when you test it for starch there's none there you can show this is where you can adapt this experiment you can prove that chlorophyll is required for photosynthesis because you can get special leaves called variegated leaves from some plants that have areas of white on them where there's no chlorophyll if you test that for starch using the starch leaf test you'll get some kind of results like this where you can see the blue black where the chlorophyll was and the brown yellow where the chlorophyll wasn't and therefore no starch has been produced you can also prove using this apparatus and the starch leaf test that co2 is needed for photosynthesis if you put some soda lime in this in this sealed jar with the plant the soda lime absorbs co2 so that makes a very low co2 atmosphere and because there's no co2 there'll be no fluid synthesis no starch is produced and that leaf will not give you a positive result when you do the starch leaf test on it so the other experiment to know about is how oxygen is produced or how you can see oxygen being produced via pond weed now you could oxygen is being produced by all the plants for synthetizing but you can't just see it coming off the plant unfortunately but if the plant is under water and it's a water-based plant you can actually see little bubbles of oxygen being produced usually from the cut stem you might use something called elodea or you might use something called cabamba for this what you can do is you can measure the number of bubbles produced in a minute count the bubbles produced in a minute and that gives you an idea of how fast photosynthesis is occurring in the plant so measure the time for a minute count the number of bubbles record it repeat the experiment repeating experiment always makes it more reliable i always remember that as r equals r reliability slew with repeats repeats as reliability and then you could change the distance of the lamp maybe so you could alter the the light intensity repeat it again three times at each distance but you should end up being able to generate a graph to show the effect of light density on the rate of photosynthesis you could also change the amount of co2 in the water for the plant by dissolving more or less sodium hydrogen carbonate into the water which will change the co2 levels so you could investigate that as well remember only change one variable at a time either the light the temperature or the co2 that is what we call your independent variable all the other variables investigation must stay controlled and there'll be one variable which you measure which will be your result which is what we call the dependent variable which in this case will be the number of bubbles per minute you could make the measurements more accurate get the more accurate results by instead of just doing the number of bubbles but you can actually collect the volume of gas i'm going to give you more accurate results and get closer to the true value rather than just counting bubbles you