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How does light intensity affect the rate of photosynthesis?

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Emma Lazarus 10Y1 10/PLR Poorly Pondweed Questions 1) How does light intensity affect the rate of photosynthesis? 2) How does temperature affect the rate of photosynthesis? sunlight carbon dioxide + water -->-->-->-->-->--> glucose + oxygen chlorophyll sunlight 6CO2 + 6H2O -->-->-->-->-->--> C6H12O6 + 602 chlorophyll Prediction I predict that the more intense the light, the higher the rate of photosynthesis. To photosynthesise, plants need light. It provides the energy for the process to happen. Chlorophyll is an enzyme and it speeds up the reaction. If a plant does not get enough of either of these things, photosynthesis will not happen as quickly, if at all. Therefore, I predict that when the light is not very intense we will not see so many bubbles being produced. This is because the plant will not have so much energy (derived from light) to activate photosynthesis. All reactions require a certain activation energy, and if this is not reached the reaction will occur more slowly. I think that as we move the lamp away (and therefore reduce the light intensity) from the elodea pondweed the number of bubbles produced will decrease steadily. For instance, say at 10cm distance 50 bubbles are counted, it is likely that at 20cm distance 25 bubbles will be counted, as the lamp is twice the distance away. This means the rate of photosynthesis is halved. I think that if we move the lamp any further away than 50cm no bubbles at all will be produced because there will simply not be enough light for photosynthesis to work. I predict that for temperature, it will not be a case of an increase in x = an increase in y. ...read more.


The other problem may be that the plant has used up all the carbon dioxide form the sodium hydrogen carbonate solution. In this case you need to add two or three 1ml pipettes of the solution to the water the pondweed is in. For the temperature investigation... Set up the apparatus as shown in the diagram below. Ensure that the black paper is attached to the water bath so that only a small area at the front of the beaker is exposed to the light. Using the previous investigation, find the distance in which the pondweed produced the greatest amount of bubbles. This is the distance you will use throughout the whole temperature investigation, because then we will have more reliable results. There is no need to move the lamp at any time during the experiment. Put some ice in the water bath, along with the thermometer. When the temperature has reached between 0-10�C, leave the pondweed for 2 minutes to adjust to its new environment. Otherwise, the plant will not produce bubbles at a steady rate. Then, start the stopwatch and count the bubbles produced. After one minute, stop the stopwatch and note down how many bubbles were counted (it does not matter if no bubbles were produced, just write down 0). Repeat this paragraph 3 times. Take some of the ice out of the water bath, but do not remove the thermometer. When the temperature is between 11-20�C, leave the pondweed for 2 minutes to adjust to its new environment. Then, start the stopwatch and start counting the oxygen bubbles. After one minute note down how many bubbles have been produced. ...read more.


This is nowhere near as high as I expected. However, at least the results were not completely anomalous and I was able to draw reasonable conclusions from them. There is not really any way in which we could improve this procedure, because living things are so unreliable. However, we could take results over a period of several weeks for more accuracy. This way we would catch the pondweed at times when it was working at a high level and at times when it was working at a low level. Another thing that was not very suitable was the way of measuring the rate of reaction. Counting the bubbles got very boring and I could easily have miscounted, leading to anomalous results. A better way would be to find some way of collecting the oxygen bubbles in a capillary tube and measuring how far along the oxygen goes. We would have to fill the capillary tube with water and seal the top, and attach the tube to the pondweed. Although fiddly this would be more reliable. There were no anomalous results, but if there were there are many reasons why it could be. For a start, I could have miscounted the bubbles. Also, the pondweed might not have been bubbling properly due to an air lock that I did not notice. Similarly, there might not have been enough sodium hydrogen carbonate solution, which provides carbon dioxide for the plant. Also, there is the possibility that the piece of pondweed used was damaged - for example perhaps it had been heated up too much and the chlorophyll had become denatured. Because there were no anomalies, this supports my prediction in that they were reliable. 1 ...read more.

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