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# An experiment to investigate the effect of Light intensity on the rate of photosynthesis.

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Introduction

AN EXPERIMENT TO INVESTIGATE THE EFFECT OF LIGHT INTENSITY ON THE RATE OF PHOTOSYNTHESIS. Prediction As the light intensity increases, so the rate of photosynthesis will increase. The rate of photosynthesis should be directly proportional to the intensity of light directed, given that the other possible variables remain constant. There should be an optimum temperature above and below which the rate of photosynthesis will be lower. I expect that this temperature will be 30oC and therefore, this is the temperature at which the experiment will be conducted. I chose this temperature because the 25oC used in the trial did not produce the optimum conditions for production of oxygen. The oxygen is formed by the combining of carbon dioxide and water to make glucose and oxygen. The glucose is needed by the plant as a source of energy and the oxygen is just a waste product of this process. Carbon Dioxide + Water Glucose + Oxygen 6C02 + 6H20 C6H12O6 + 602 (Nuffield co-ordinated Sciences Biology) This equation relies on the unlikely occurrence of twelve different molecules all reacting in the same place at the same time and in the right order. There are in fact, two different stages of photosynthesis, one light stage and one dark stage. This means that it is much more likely that the reaction will take place as it is broken down into simpler stages. All of the molecules do not, therefore, have to be in the same place at the same time. ...read more.

Middle

Check that the temperature has not changed from 30oC and add hot or cold water to the beaker if necessary to adjust. 6. Measure the volume of gas produced in 2 minutes. 7. Repeat twice more to ensure an accurate result and to eliminate errors, then take the mean readings of the three. 8. Move the light to a distance of 148cm and repeat steps 1 - 7 above. 9. Move the light to distances of 191cm, 230cm, and 330cm and repeat steps 1 - 7 above for each distance. Table of Results Distance of light from plant (cm) Length of bubble of gas produced (mm) Average Length of bubble (mm) 1 2 3 135 33 35 39 36 148 27 31 30 29 165 23 24 21 23 191 12 10 12 11 230 10 8 11 10 330 8 8 6 7 To find out what the light intensity was at each of the distances I used the formulae shown above the table. This is the factor I compared the total distance of the oxygen bubble to. Distance from plant (cm) Rate of Photosynthesis-length of O2 bubble (mm/s) Light 1 intensity d2 (lux) Light intensity scaled up by factor a of 100,000 (lux) 135 36 0.000055 5.5 148 29 0.000046 4.6 165 23 0.000038 3.8 191 11 0.000027 2.7 230 10 0.000019 1.9 330 7 0.0000092 0.92 Conclusion The graph shows that there is a direct correlation between the light intensity and the amount of oxygen produced through photosynthesis in a plant. ...read more.

Conclusion

To solve this, the apparatus could be placed in a thermostatically controlled water bath. This would control the ambient temperature around the apparatus and therefore the temperature of the oxygen bubble To extend the experiment, I could use different plants to see if there is a correlation between the light intensity and the rate of photosynthesis for other plants including land plants. I could also aerate the water around the plant so there is a constant supply of carbon dioxide in the water for the plant to use during photosynthesis. Aerating the water would also prevent any oxygen being lost by dissolving into the water. To keep the same amount of minerals available for each test, I would also have to change the water each time. The plant needs the minerals to grow and to produce the chloroplasts needed for photosynthesis. The main mineral needed for this is magnesium so this would have to be very carefully controlled in order to keep all of the variables constant. I could also measure the amount of gas produced more accurately by measuring the mass of the gas produced. To do this, I would have to measure the mass of the apparatus before the experiment and after the experiment. The water inside the apparatus could not be changed between weighing, as this would change the results. The mass would have to be measured to a very high accuracy because the mass of a gas is so small. This would have to be used on a land plant, as it would be very difficult to measure the correct mass of a water plant. Water is constantly moving through the plant and so the mass would be constantly changing. ...read more.

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