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# Investigatingthe affect of light on a plant's rate of photosynthesis

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Introduction

Investigating the affect of light on a plant's rate of photosynthesis Aim The aim of this investigation is to find out how varying the amount of light a plant receives affects its rate of photosynthesis. The input variable in this investigation is the distance between the plant and the light source. Prediction I predict that moving the plant closer to the lamp will increase the rate of photosynthesis at a proportional rate where LI is inversely proportional to 1/d2 when LI= light intensity and d= distance (from light source to plant). I think this will be true to a certain point until another factor is limiting the rate of photosynthesis. The greater the light intensity, the more light energy that can be transferred and harnessed to fuel reaction in photosynthesis. Light intensity is inversely proportional to the distance squared because the light energy spreads out as it travels further and further from its source. Light energy travels along the circumference of an expanding circle. When light energy is released from a point, the energy is dispersed equally along the circumference. But since the circle is expanding, the circumference increases and the same light energy is distributed along a greater surface. Therefore I expect the graph to look like this: Distance (cm) For a plant to photosynthesise it needs certain substances. These substances are shown in this equation: Light CO2 + H2O C6H12O6 + O2 Chlorophyll Light Carbon Dioxide + Water Glucose + Oxygen Chlorophyll Light is needed to drive this chemical reaction. ...read more.

Middle

The only way to control this variable is to use the same plant throughout the experiment. Limiting Factors- Light, carbon dioxide, temperature, and chlorophyll are all limiting factors, meaning that even when there is surplus of every other variable, the rate of photosynthesis will be limited by the limiting factor until there is enough of the limiting factor to increase the rate of photosynthesis further. Otherwise, the rate of photosynthesis can no longer increase. Results Distance (cm) Number of Bubbles (per min) Average Rate Temp 1st time 2nd time 3rd time 4th time 20 20 21 20 22 20.75 34.58 21�c 40 6 6 8 7 6.75 11.25 21�c 60 3 8 7 8 6.5 10.83 21�c 80 2 7 4 5 4.5 7.5 21�c 100 1 4 3 4 3 5 21�c The rate of photosynthesis used in the previous table is worked out using the following formula: Rate = average number of bubbles � 100 60 Conclusion From the results that I have gathered I can state that an increase in light intensity certainly does increase the rate of photosynthesis. As was also expected in my prediction, the relationship between light intensity and the rate of photosynthesis was non-linear. From both graphs there is a best-fit curved line. This means that the rate of photosynthesis increases at an exponential rate. However, my prediction that light intensity is inversely proportional to the distance squared did not fit into my results perfectly. ...read more.

Conclusion

If the measuring volume option was to be chosen, volume should be measured for a smaller time frame to reduce the overall time to complete the experiment. With low light intensity, the pond weed receives some light energy from background light such as sunlight seeping through curtains or the light from the lamp of another student's experiment. To eliminate most or all of the background light, the experiment must be performed in a completely dark room. This could be achieved by using a photographic dark room. The experiment could also have been improved by using pond water at about 10�c to mimic the natural habitat of a pond weed. To take this experiment to a higher level I think it would be interesting to investigate the effect of different wavelengths of light on the plant. This is relatively simple to do as it involves very little change to the original experiment. This could be achieved by: ?Using the same set-up as in our experiment ?Inserting a different colour filter between the lamp and the plant ?Measuring the amount of oxygen produced at each distance and repeating the experiment at least three times so as to provide reliable and usable results ?Graphing wavelength against volume of oxygen produced. However, in this experiment green light CANNOT be used as the plant would reflect the wavelength and not receive any light to fuel the reaction. I would predict the plant would react best to the wavelengths at the two extremes of the spectrum (red and violet). ...read more.

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