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An investigation of the effects of the quantity of light on the rate of photosynthesis in Elodea

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Biology GCSE Coursework "An investigation of the effects of the quantity of light on the rate of photosynthesis in Elodea" Planning I am going to investigate the effect of the quantity of light on the rate at which a piece of Elodea photosynthesises. The term photosynthesis means making food in the light. It is the basis of all food chains and webs, because green plants are able to use sunlight as an energy source. Green plants are called primary producers, and the food they make from simple inorganic substances is passed through the eco system. Plants take in two raw materials, water from the soil, and CO2 from the air, and along with minerals, produce all the organic molecules they require. Plants are therefore said to be autotrophic (self-feeding), where as animals are heterotrophic (they eat food). Photosynthesis can be summarised by a simple equation: 6CO2(g) + 6H2O(l) ? C6H12O6(l) + 6O2 The glucose is soluble and turned to insoluble starch for storage, hence we test for starch to show photosynthesis has taken place. From the above equation, we can see the factors that affect photosynthesis: * Light - quantity and quality (colour) * Temperature - needs to be at an optimum for the plant enzymes * CO2 - the amount of (not usually limiting, but in excess) ...read more.


My preliminary work showed that distances of 5, 15, 25, 35, 45, 55, and 65cm from the elodea, gave a good spread of results (distances further away from the elodea were not worth experimenting on, as the elodea photosynthesised at a very slow rate, or not at all. I know this from preliminary series of experiments). Temperature: I will not be heating the water in which I will place the elodea, as this will keep the temperature (fairly) constant. My preliminary experiments showed that using this method, the temperature only varied by 2 or 3?C at the most - and this small amount does not impact the results. CO2: I will be placing the elodea in a test tube filled with a solution of 0.1% Na+HCO3- (and the test tube will be placed in a water bath at a constant temperature). The Na+HCO3- ions will release CO2 for photosynthesis. The CO2 will always be in excess, and my preliminary work showed that this solution worked well, and will not affect the photosynthesising of the elodea. It is by this method that I will control the amount of CO2, keep it constant, and always in excess. Chlorophyll: I will control the amount of chlorophyll, by using the same elodea all the way through my experiment. ...read more.


This test tube will be placed in a beaker, in a water bath. The water will not be heated, so that the temperature will be constant throughout the experiment at room temperature. Room temperature (22-25?C) is also the optimum temperature for the plant enzymes - meaning they will be working at the optimum rate, and will not become denatured, as the temperature will never rise more than 2 or 3?C above the optimum temperature. The test tube will then be fixed in place by a clamp and stand. I will then leave the elodea for a five-minute "settling-in" period, where the elodea will adapt to the conditions. Then, I will place the lamp 5cm away from the elodea, and begin timing. I will be recording the number of O2 bubbles given off by the elodea every minute - this is the rate of photosynthesis. For every distance, I will record the number of O2 given off every minute, and repeat the procedure 3 times. I will then take the average of the 3 readings. This is to be able to accommodate any anomalous results that may crop up. The entire procedure will be repeated at distances of 15, 25, 35, 45, 55, and 65cm. These distances should provide me with sufficient readings for a suitable graph in my analysis. ...read more.

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