hi friends welcome to IGCSE study buddy where you can revise biology topics from the Cambridge IGCSE syllabus this video summarizes part two of topic 6 plant nutrition in the previous video we left off at the factors that are important for photosynthesis chlorophyll is needed because it helps absorb light light is important because it provides the energy to drive the reaction and carbon dioxide is important because it is converted into sugars such as glucose let's look at some investigations that will prove the importance of these factors for photosynthesis basically what we need to do is get a functioning plant and deprive it of each of these factors individually and prove that the plant stops photosynthesizing when these factors are absent starch is stored in chloroplasts where photosynthesis occurs so testing a leaf for starch is a reliable indicator of which parts of the leaf are photosynthesizing although glucose is produced in photosynthesis it is not reliable to test for it because the glucose is quickly used converted into other substances and transported or stored as touch there are two things that has to be removed from the leaves before the investigations they are starch and chlorophyll first of all we are going to destouch all our plants by leaving the plants in the dark for 48 hours during this period the plants will be unable to photosynthesize and therefore use up all its starch for respiration this means that in the beginning of our experiments all plants test and control will have absolutely no starch in them so after the experiment if we do a starch test with iodine and we find starches present it would indicate that photosynthesis has occurred so as you can see leaving the plants in a dark place will help remove all the starch from the plant next we need to remove the chlorophyll to decolorize the leaf so that the color change from iodine is easier to see as shown in the illustration the color change is easier to determine when the chlorophyll is removed from the leaf also we can't just add iodine onto a fresh leaf and expect results first of all we need to break the leaf so that the iodine can seep in to begin with these are the steps to remove chlorophyll boil the leaf in water and make it permeable boil the leaf in ethanol to remove the chlorophyll from the leaf and to decolor it rinse the leaf in water to soften it the leaf is now ready to be tested for starch with iodine solution the first investigation we'll look at is the need for chlorophyll in photosynthesis to prove that chlorophyll is needed for photosynthesis a variegated Leaf may be used since it's a type of leaf that is partially green and partially white that is only parts of the leaf has chlorophyll first the leaf is put in boiling water to kill the cells and break down the cell membranes this makes it permeable next the leaf is put for 5 to 10 minutes in hot ethanol in a boiling tube this removes the chlorophyll so color changes from iodine can be seen more clearly then the leaf is dipped in boiling water to soften it after that the leaf is spread out on a white tile an iodine solution is added to it the white areas of the leaf contain no chlorophyll and when the leaf is tested only the areas that contained chlorophyll will stain blue black the other areas that had no chlorophyll will not turn blue black since no photosynthesis occurs here and so no starch is stored next let's investigate the plant's need for light in order to photosynthesize just as with the previous experiment the plant must be first detached by leaving it in a dark place for 48 hours then partially cover a leaf of the plant with aluminum foil and place the plant under sunlight for a day the leaf can then be removed and tested for starch however we must first do steps three to five like in the previous experiment and prepare the leaf before the iodine test these steps are carried out to make the leaf permeable to remove the chlorophyll from the leaf and to soften it the leaf is placed on a white tile and tested for starch using iodine solution the covered areas will be deprived of light whereas the rest will be exposed the area of the leaf that was exposed to sunlight will turn blue black while the area that was covered with aluminum foil will not turn blue black as it did not receive any sunlight and could not photosynthesize therefore starch is not present in that area this proves that light is necessary for photosynthesis and the production of starch in the next investigation we will determine if carbon dioxide is needed for photosynthesis destarch two plants by placing them in the dark for a long time as discussed previously this ensures that no starch is present in the plant at the beginning of the experiment Place one plant in a bell jar which contains a beaker of sodium hydroxide which will absorb carbon dioxide from the surrounding air place the other plant in a bell jar which contains a beaker of water this is the control experiment which will not absorb carbon dioxide from the surrounding air Place both plants in bright light for several hours prepare the leaves as shown in steps three to five and test both plants for starch using iodine the leaf from the plant placed near water will turn blue black as it has all necessary requirements for photosynthesis however the leaf from the plant near the sodium hydroxide will not turn blue black it will remain orange brown the color of iodine solution as it could not photosynthesize due to the lack of carbon dioxide now that we've established that plants need chlorophyll light and carbon dioxide for photosynthesis let's investigate the factors that affect the rate of photosynthesis they are light intensity carbon dioxide concentration and temperature to investigate how fast photosynthesis is happening we may use an aquatic plant that photosynthesizes in water and count the number of oxygen bubbles it releases in a minute as photosynthesis occurs oxygen gas is made and released since the plant is in water the oxygen released can be seen as bubbles leaving the plant the number of bubbles produced over a minute can be counted to record the rate the more bubbles produced per minute the faster the rate of photosynthesis so for investigating the effect of changing light intensity on the rate of photosynthesis the setup may look like this a photosynthesizing aquatic plant is placed in a beaker of water with sodium hydrogen carbonate an inverted funnel is placed on the plant and an inverted boiling tube is placed on the end of the funnel a thermometer is used to monitor the temperature of the solution a lamp is placed near the beaker and with the help of a ruler it should be placed at different distances from the beaker and the rate of photosynthesis must be calculated the resulting data will produce a graph like this so initially increasing light intensity increases the rate of photosynthesis until some other Factor becomes limiting after which the rate becomes constant so without enough light a plant cannot photosynthesize very quickly even if there's plenty of water and carbon dioxide increasing the light intensity will boost the rate of photosynthesis however at one point the rate of photosynthesis stops increasing this means that even with a stronger light intensity there may be some other Factor limiting the rate of photosynthesis this limiting factor may be either not enough carbon dioxide to make the rate of photosynthesis faster or temperature not being high enough next let's investigate the effect of changing carbon dioxide concentration on the rate of photosynthesis the experiment is set up similar to the previous one where a photosynthesizing aquatic plant is placed in a beaker filled with sodium hydrogen carbonate solution an inverted funnel is placed on the plant and an inverted boiling tube is placed on the end of the funnel a thermometer is used to monitor the temperature of the solution a lamp is placed near the beaker and the distance of it from the beaker must be kept constant to change the carbon dioxide concentration different amounts of sodium hydrogen carbonate may be dissolved in the water in the beaker and the rate of photosynthesis must be calculated remember that care must be taken when investigating a condition to keep all other variables constant in order to ensure a fair test the resulting data will produce a graph like this so initially increasing carbon dioxide concentration increases the rate of photosynthesis until some other Factor becomes limiting after which the rate becomes constant therefore even if there is plenty of light a plant cannot photosynthesize if there is insufficient carbon dioxide the factors which could be limiting the rate when the line on the graph is horizontal include temperature not being high enough or not enough light now let's investigate the effect of changing temperature on the rate of photosynthesis once again the experiment is set up similar to the previous one where a photosynthesizing aquatic plant is placed in a beaker filled with sodium hydrogen carbonate solution an inverted funnel is placed on the plant and an inverted boiling tube is placed on the end of the funnel a thermometer is used to monitor the temperature of the solution a lamp is placed near the beaker and the distance of it from the beaker must be kept constant to change the temperature a hot plate may be used to heat up the solution in the beaker and the rate of photosynthesis must be observed and calculated at different temperatures the resulting data will produce a graph like this the rate of photosynthesis initially increases with temperature as the number of collisions between enzymes and substrates increases until the optimum temperature is reached after which the graph reverses and eventually drops down to zero this is because high temperatures will denature enzymes that are required for photosynthesis if it's too cold also the rate of photosynthesis will decrease so that concludes part two of topic six Plant Nutrition hope you found this video useful thank you for watching and please don't forget to subscribe to IGCSE study buddy for more biology revision videos bye bye