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# Analysing Evidence and Drawing Conclusions - Photosynthesis

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Introduction

Skill Area O: Obtaining Evidence Distance from light (cm) Light intensity (lux) Rate of Photosynthesis (number of bubbles/minute) Average bubbles/min Temperature (0C) 1st 2nd 3rd (if needed) 70 147 12 7 7 7+7/2=7, 7 20 60 190 14 15 - 14+15/2=14.5, 15 20 50 240 21 20 - 21+20/2=20.5, 21 20 40 355 26 26 - 26+26/2, 26 20 30 609 28 29 - 28+29/2=28.5, 29 20 20 1359 36 36 - 36+36/2=36, 36 20 10 4780 44 40 42 40+42/2=41, 41 20 Secondary Data Distance from light (cm) Light intensity (lux) Rate of Photosynthesis (number of bubbles/minute) Average bubbles/min Temperature (0C) 1st 2nd 3rd (if needed) 70 20 0 0 - 0+0/2=0, 0 20 60 25 3 5 - 3+5/2=4, 4 20 50 35 8 12 - 8+12/2=10, 10 20 40 55 19 21 - 19+21/2=20, 20 20 30 90 32 32 - 32+32/2=32, 32 20 20 180 48 47 - 48+47/2=47.5, 48 20 10 440 50 49 - 50+49/2=49.5, 50 20 5 920 49 51 - 49+51/2=50, 50 20 Skill Area A: Analysing Evidence and Drawing Conclusions (Continued from graphs) Trend/Pattern The graphs show me that from A - B as the light intensity increases so does the rate of photosynthesis. However from B - C the rate of photosynthesis starts to level off. I think that if I'd done another measurement at 5cm the rate of photosynthesis would have levelled off completely. ...read more.

Middle

Skill Area E: Evaluating Evidence On my lux graph I have noticed that I do have one slightly anomalous result. However it is unclear which result it is. This is because if you look at this graph you can tell that either the fifth or sixth result must be an anomalous result, but the graph would make a smoother line if either of them was taken out. However I believe that the fifth result is probably the anomalous one as if it wasn't there or if it had had a quicker rate of photosynthesis the graph would have increased more steadily for longer and then it would have probably flattened off more than it has, which I believe would have been more accurate. I think that this anomalous result (the fifth result) may have occurred due to the fact that it might have needed to have a bit more time to equilibrate than the other results. This would explain why the rate of photosynthesis is a bit lower for this result than you might expect. One thing that you must also realise about my conclusion is that it is only valid over the range of values that I investigated (i.e. the distance between the light and the pondweed being from 70cm away to 10cm away) and for the particular organisms (elodea, Canadian pondweed) and materials used in this experiment. In my experiment I have been able to identify a few sources of error. ...read more.

Conclusion

However if you were to take three readings one after another, you would be more likely to spot the possibility that the pondweed hasn't fully equilibrated, for the simple reason that it takes longer to do. I think that this might have helped with my anomalous result. On the same topic of 'equilibration', to improve the reliability and accuracy of the results I would also give the pondweed more time to equilibrate e.g. give it two minutes instead of one. This would also lessen the chance of getting similar results before the pondweed has finished equilibrating. There is also some further work that I think I could do to find out more about the effect of light intensity on the rate of photosynthesis. The first thing that I think I could do would be to repeat the whole experiment again, with a different piece of pondweed, and taking measurements three times every 5cm instead as I suggested earlier; I would also make sure that my lamp was the only light source in the room and I would use a proper electric water bath. This would just double check my results again as I did with my friends results earlier on, although it would be more accurate. This is because the measurements would be taken more regularly, I would be sure that the only light in the room was coming from my lamp and the temperature would definitely be the same throughout the experiment. ...read more.

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