Light intensity =
In order to keep the method safe, I will wear safety goggles. This is because I am using water, and a powerful 250 watt lamp. The lamp will get very hot, and if hot glass comes into contact with water, it can not only crack, but if it is hot enough, it can explode. By wearing goggles, I can protect my eyes from shattering glass if the worst happens.
There are a few variables in this experiment. They include water temperature, carbon dioxide in the water, distance the apparatus is from the lamp, and the piece of elodea. I must constantly check the temperature of the water in the boiling tube, as it is an important variable and must be kept the same throughout the experiment. This is why I am going to use a thermometer. If the water temperature in the boiling tube drops or rises significantly, the test will be unfair. This is why I am going fill the beaker with water, as it acts like an insulator. I found this out in my preliminary experiment. The water in the boiling tube does not suffer from great heat loss, or gain, because the insulating water around it, in the beaker, prevents this. This way I can control the water temperature and keep this variable the same. When I do the experiment, I will do it all in one go, and so I shall be using the same piece of elodea plant and water throughout the experiment. This means these variables will stay the same. As for the CO2 level in the water, this will stay roughly the same throughout the experiment as the elodea will use very little of it. So little I will not need to replace the water, or add CO2 to the water I started with. The distance the beaker is from the 250 watt lamp is the variable I will be changing in the experiment.
After completing the experiment, I predict that the elodea will have photosynthesised a lot more while it was in a greater light intensity position (closer to the lamp). Here is a sketched graph to show my prediction.
Volume
of
oxygen
collected
Light intensity
I believe that the elodea will photosynthesise more when the light intensity is greater, because of the chlorophyll. Chlorophyll is found in chloroplasts, and is what makes most plants green. Chlorophyll absorbs energy from the sun, in this experiment a powerful lamp, and uses it to combine CO2 and H2O to produce glucose. (a plants ‘food’). If I increase the light intensity, the chlorophyll in the elodea will absorb a lot more light energy at any point, thus photosynthesising at a greater rate and producing more oxygen. This describes the rise of the line in my predicted graph. The reason I have levelled the line out, is because the elodea will eventually stop photosynthesising at faster rates, even though the light intensity may be increasing. This is because the CO2 and water temperature are staying the same. (the CO2 level will probably decrease a tiny fraction throughout the experiment) If I increase the light intensity to such a great point the elodea stops photosynthesising, it means I will need to increase the temperature and CO2 level. This is because the CO2 and temperature are limiting factors in my experiment. As I must keep them the same, the elodea will stop photosynthesising more rapidly and stay at the same rate.
In order to help me make a prediction, I read pages 12, 13 and 14 of the CGP revision booklet for biology. I also used a science prompt sheet, provided to me by my biology teacher, to help me structure my plan. Included with my plan on separate sheets, is my preliminary experiment method, results and observations.
