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# Photosynthesis. I have been given the task of analysing and evaluating an experiment carried out by year 10 students to find the optimum distance for a lamp to be placed to enable aquatic plants to photosynthesise

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Introduction

Photosynthesis Coursework Thomas Hansen I have been given the task of analysing and evaluating an experiment carried out by year 10 students to find the optimum distance for a lamp to be placed to enable aquatic plants to photosynthesise. Here is a table of results showing what five different groups recorded. Number of Bubbles Recorded in Five minutes Distance Group 1 Group 2 Group 3 Group 4 Group 5 Average (1.d.p) 0cm 32 48 36 96 21 46.6 10cm 39 45 36 110 27 51.4 20cm 26 52 29 58 32 39.4 30cm 10 27 11 38 45 26.2 40cm 3 7 1 15 7 6.6 50cm 2 2 1 5 0 2 60cm 1 2 1 4 1 1.8 I have drawn a graph to illustrate the average values of these groups. As you can see, there are three clear sections to the graph, labelled A, B, and C. The shape of the graph shows a steady increase in the amount of bubbles given off through the experiment, showing a clear optimum distance for the light, and levelling out where the lamp is so far away it adds an almost negligible amount to the background light. The optimum distance according to these results is 10cm, where the average amount of bubbles is 51.4. This can be seen as the peak on the graph. This is the maximum amount of bubbles that can be given off by the pondweed during photosynthesis, indicating the ideal amount of light that the pondweed needs to be most productive. ...read more.

Middle

This can be seen as true in section B on the graph, as the closer the lamp got to the pondweed, the more bubbles were given off in proportion to it. The results show that this is not true though in sections A and C, but this is not because of the lamp, but of external factors, in A heat becomes a factor that affects the rate of photosynthesis, and in C, the lamp is so far away that backlight from the room affects the rate of photosynthesis more than the lamp. The second prediction was that the rate of photosynthesis would increase at a constant rate as the distance between the lamp and plant was reduced. This is also partly correct, as in section B, the rate of photosynthesis seems directly proportional to the distance of the lamp from the plant. But in A and C, again, this is not correct, as again other independent variables are introduced which affect the rate of photosynthesis. Both predictions would have been right, had the light from the lamp been the only factor being changed to affect the rate of photosynthesis, we would have seen a straight line on the graph. But because the experiment became susceptible to factors apart from the light from the lamp, the predictions were only partly right. Evaluation This experiment did collect results that supported the predictions made to some extent, and so it could be called a success. But because the results recorded indicated that other factors had affected the experiment apart from the light intensity from the lamp, this experiment failed to get accurate results that fully supported the predictions made. ...read more.

Conclusion

This experiment could have been improved by the use of light sensors, which could have been used to make sure each piece of pondweed got the same amount of light. Heat from the lamp could have affected the rate of photosynthesis, to prevent this happening, a protective screen made of glass could have been put in between the lamp and pondweed to prevent heat from the lamp getting to it. The amount of CO2 in the water would have been depleted at the end of the experiment, decreasing the rate of photosynthesis. To avoid this, the water in the boiling tubes could have been replaced for each separate light intensity, so preventing the CO2 from becoming an unwanted variable. The way in which bubbles were counted was not specified, they could have been counted as they formed on the plant, or as they reached the surface of the water, etc. Without a specific way of counting the bubbles, we cannot assume that all bubbles were counted accurately; some may have been counted twice if they were counted both when on the plant, and as they left it. To prevent this, bubbles should only be counted when they leave the plant's surface. Future experiments could include investigating the other factors affecting the rate of photosynthesis, such as temperature, CO2, or mineral concentration. The effect of temperature could be measured by heating the tube in which the pondweed is, and seeing how that affects the rate of photosynthesis by again counting bubbles. Distilled water could be used compared with tap water to see how CO2 and mineral concentration affect the rate of photosynthesis by counting how many bubbles were produced for each. ...read more.

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