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Investigation to ascertain the extent to which light intensity is a limiting factor to photosynthesis.

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Investigation to ascertain the extent to which light intensity is a limiting factor to photosynthesis. Gina Collier Abstract The aim of this investigation is to ascertain the extent to which light intensity is a limiting factor on the rate of photosynthesis of elodea canadensis. The hypothesis was that light intensity will have a significant effect on the rate of photosynthesis. As light intensity increases so too will the rate of photosynthesis. The length of gas bubble produced in five minutes was taken at differing light intensities and temperatures. The results showed that light intensity does have a significant effect on the rate of photosynthesis. As light intensity increased so too did rate of photosynthesis until it was limited by another factor such as temperature. When the temperature was raised, the rate of photosynthesis continued to rise. Introduction Null hypothesis: Light intensity will not have any effect on the rate of photosynthesis in elodea canadensis. Experimental hypothesis: Light intensity will have a significant effect on the rate of photosynthesis in elodea canadensis. As light intensity increases so too will the rate of photosynthesis Biological knowledge to support hypotheses: In a freshwater environment PFD (wavelengths of sunlight used for photosynthesis) is low for submerged leaves, because light penetration is reduced when passed through the water. At the surface there is unobstructed full sun for a photosynthetic organ floating. An emergent canopy may intercept high PFD, which may be harmful to the plant. The concentration of carbon dioxide and oxygen dissolved in water is low. Minerals and nutrients are scarce or dilute within the water medium, when compared with drier soil. Elodea canadensis is a leafy submerged aquatic hydrophyte originating from North America. It is commonly found in still or slow flowing waters in various locations around the state. Elodea thrives in temperate climatic zones and grows prolifically during summer once water temperatures exceed 15�C Elodea canadensis is also known by several other common names such as Canadian waterweed, common elodea, or anacharis. ...read more.


From this a graph will be plotted, light intensity against rate. The results from both temperatures will be on this graph. Light intensity will be calculated by 1/d^2 with d being distance of the lamp from the plant. Rate is calculated by volume of bubble in mm divided by time in seconds. The volume of the bubble is calculated by lpr^2. L is the length of the bubble in mm and r is the radius of the capillary tube. Separate graphs will be drawn up for each temperature with error bars, these graphs will be length of bubble produced in three hundred seconds against light intensity using the mean result of bubble length. Error bars are calculated by calculating the standard deviation of the raw results, then creating minimum and maximum deviation bars from the mean by adding and subtracting the standard deviation from the mean respectively. The results will be analysed for correlation using the Spearman's rank statistical test. This test was chosen because the data is non parametric i.e. not normally distributed and the data is quantitative. Methods Method Using a ten cm piece of elodea, the apparatus was set up as in the diagram so that the oxygen given off by the plant accumulated in the tubing to create a single bubble that was measured against a scale when it was drawn up through a capillary tube. This meant placing the pondweed in an inverted glass funnel in a glass beaker to collect all the oxygen produced and funnel it into the rubber tubing connecting to the capillary tube. A syringe was connected to the capillary tube by rubber tubing to enable the bubble to be drawn up through the apparatus into the capillary tube for measurement. The water level was higher than that of the funnel to keep the apparatus airtight. This was then placed into a water bath held at 20�C with a 5% concentration of sodium hydrogen carbonate. ...read more.


A way of overcoming this problem would have been to add a little detergent to the solution. This would have made it easier for the bubbles to escape into the capillary tube and merge into a single bubble. For the experiment at 20�C, the results obtained showed much lower variability making conclusions drawn much more reliable. The lower variability was due to the fact that the rate of photosynthesis was much lower and hence a smaller volume of gas was evolved making it easier to measure the length of the bubble in the capillary tube. The average standard deviation was much lower compared with at 30�C. Individual standard deviation for each light intensity showed quite low variability showing reliability in the method. The method used for both experiments was the same however; the results obtained from the two conditions vary greatly in their reliability throwing doubt over the design. It would seem that this method was a very good way of obtaining reliable results when the rate of photosynthesis was lower, as the rate increased it became much more difficult to obtain the same standard of reliability and validity of the results. A solution to this problem would be to take more readings of the bubble length at the higher temperature. Instead of taking one single reading at three hundred seconds, take one reading at one hundred and fifty seconds and another at three hundred seconds adding them together to gain the final result. This method would provide smaller volumes of gas to measure which would be more manageable than one very large bubble. A limitation of this experiment was that the range of light intensities was not large enough. A range of five is very small compared with for example fifteen. A range of that size would show more accurately the relationship between light intensity and rate of photosynthesis. To obtain much more reliable and valid results from this experiment it would be necessary to undertake many more replicates therefore making the mean result much more reliable and possibly ironing out any anomalous results which would greatly affect the mean. ...read more.

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