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Light Intensity and Photosynthesis.

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Light Intensity and Photosynthesis Biology Coursework Aim The aim of my experiment was to determine whether or not the intensity of light would affect the rate of photosynthesis in a plant. To do this, I placed a piece of Canadian pondweed in varying light intensities, and observed the amount of oxygen being given off. I used Canadian pondweed because of its unusual quality of giving off bubbles of gas from a cut end, when placed in water. Introduction Photosynthesis occurs only in the presence of light, and takes place in the chloroplasts of green plant cells. Photosynthesis can be defined as the production of simple sugars from carbon dioxide and water causing the release of sugar and oxygen. The chemical equation for photosynthesis can be expressed as: (light) 6CO2 + 6H2O � C6H12O6 + 6O2 (in the presence of chlorophyll) The fact that all plants need light in order to photosynthesise has been proven many times in experiments, and so it is possible to say that without light, the plant would die. The reason that light intensity does affect the rate of photosynthesis is because as light, and therefore energy, falls on the chloroplasts in a leaf, it is trapped by the chlorophyll, which then makes the energy available for chemical reactions in the plant. Thus, as the amount of sunlight, or in this case light from a bulb, falls on the plant, more energy is absorbed, so more energy is available for the chemical reactions, and so more photosynthesis takes place in a given time. There are many factors, which affect the rate of photosynthesis, including light intensity, temperature and carbon dioxide concentration. The maximum rate of photosynthesis will be constrained by a limiting factor. ...read more.


The light intensity was measured in the same way as described in the preliminary experiment, and assumed to be the same at any point at any particular distance. When bubbles are being produced at a steady rate, clear any previous bubbles from the tubing by moving the syringe. Start the stopwatch, and wait for 1 minute. Move the bubbles, which have been collected at the bend in the tubing to the part of the tube with a scale. Find the length of the bubble collected. Repeat for all other readings, and then repeat all readings a second time to get an average result for each distance. Audus apparatus Using the described method, I found the following results: Results for main experiment Distance Light intensity length 1 length 2 average length (cm) (lux) (mm) (mm) (mm) 5 1015 3.5 3.5 3.5 10 945 3.5 3.5 3.5 12 770 4 3 3.5 14 639 3.5 3.5 3.5 16 500 3 3.5 3.25 18 395 3 3 3 20 310 2 3.5 2.75 25 208 1.5 2.5 1.75 30 149 1.5 1.5 1.5 35 110 1 1 1 40 80 0.5 1 0.75 45 55 0 0.5 0.25 Although, because I was using light intensity as my variable, I did not need to record the distances as well, I did, simply to use them as a marker for each result, so that I only had to record the light intensity once at the beginning and from then I just had to align the lamp at the correct distance each time. Analysis My graph was in the form of a best-fit curve. I drew it as a curve rather than a straight line because of the clear pattern of the points. ...read more.


Again if I were to carry out the experiment over a longer time period, it would have been necessary to add sodium hydrogen carbonate to the water to increase the carbon dioxide concentrations. The last inaccuracy, though a small one, was in the time keeping. The main problem here was in when to begin the minute. If for one reading, the minute was started just after one bubble had been produced, and in another reading it was just before, this could have had a negative effect on the accuracy of my results. I therefore ensured that in each case I started the stopwatch just after a bubble had been produced, thus heightening the accuracy. Overall, I felt that due to the small volumes of oxygen involved, my experiment was not as accurate as it could have been, however I believe it was accurate enough to support and justify my hypotheses. Improvements could have been made as I have stated, mainly by simply increasing the time taken. However, due to practical time constraints in taking the readings for my investigation, and some consequential problems relating to time extension, I could not in fact make these adjustments. The other obvious way of increasing the reliability of my results would be to take many repeat readings and find an average. To extend my enquiries into the rate of photosynthesis, I could perhaps try to link in some of the other limiting factors to the same experiment, as well as investigating them in their own right. It could also be interesting to explore the effects of coloured lights on the rate of photosynthesis, which could lead to the question of whether or not other types of light, such as fluorescent lights or halogen lights, would have a different effect on the rate of photosynthesis. ...read more.

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Here's what a teacher thought of this essay

4 star(s)

Overall, this is a very competent piece of work which investigates the effect of light intensity on the rate of photosynthesis. The writer has produced a concise and well-written account of the investigation with most aspects of a GCSE practical report covered in sufficient detail to gain the higher grades. Preliminary work was effectively used to establish the best method, and the results collected were analysed in some detail.

The report would benefit, however, from the following improvements:

[1] Reference in the introduction to previous work carried out by other students. There is a great deal of this online. This would have enabled the student to make detailed predictions.

[2] A risk assessment was missing and is now a vital part of every report.

[3] A more detailed quantitative analysis of the results was needed to enable the reader to fully understand the relationship between the IV and DV. Comparisons of the data collected here with that collected by other students would also prove useful in measuring the reliability of this work.

Overall, however, a good effort which, with small improvements, would have gained 5 stars.

4 stars.

Marked by teacher Ross Robertson 24/06/2013

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