Method
In my pilot experiment no bubbles were produced. This meant I had to change my method. In my investigation experiment I gave the Elodea more time to bubble and increased the temperature as well changing the piece of elodea if it wasn’t bubbling to ensure that the quality of weed wasn’t influencing my results too much.
The independent variable is the temperature of the water. It was changed and kept at the desired temperature using icy cold water and also by adding hot boiled water when necessary. I had 9 different temperatures, starting at 0 and increasing in steps of 10oC.
The dependant variable is the number of bubbles the elodea produces in a minute. I measured this because it shows directly the rate of photosynthesis, the more bubbles produced the quicker the elodea is photosynthesising. I measured as accurately as the eye can, by counting the bubbles as accurately as possible. I kept these results as reliable as possible by using the same stopwatch, counting the bubbles myself every time and keeping the same apparatus.
The factors I kept constant (control variables) were; the piece of Elodea, the light intensity and the concentration of Carbon Dioxide. I kept the piece of Elodea constant by not changing it or its position. I kept the light intensity the same by not moving the lamp and having the same amount of pondweed in direct contact with the light throughout the experiment. I kept the concentration of Carbon Dioxide by keeping the amount of sodium hydrogen carbonate solution constant.
Risk Assessment
I wore goggles throughout the experiment as there were some substances that were harmful to the eyes.
I was careful not to spill water on any electrical appliances in order to prevent electrocution.
I was careful to tuck in any loose articles of clothing or hair when using the Bunsen and to wear goggles.
I stood up throughout the experiment due to dealing with warm and sometimes boiling water.
Apparatus
Results
Conclusion
The graph obtained supports the prediction I made. It shows that the number of bubbles produced and therefore the rate of photosynthesis increased rapidly until around the 30oC where it drops steadily and stops producing bubbles completely in nearly all cases by 80oC. The anomalous result, shown in italics, is anomalous because it is so contrasting to all other results of the same temperature and doesn’t fit into its own line at all. I conclude that the temperatures effect on photosynthesis is important and that the optimum temperature for elodea photosynthesis is around 30 oC. At the lower and higher temperatures the plant was unable to produce many bubbles, if any.
Chlorophyll is used to make photosynthesis happen in plants. It is an enzyme, and all enzymes need some heat to activate them. This is why at the lower temperatures the rate of photosynthesis was so low. As the temperature increased, so did the rate of photosynthesis. At around 30°C was a peak. This was when the rate of photosynthesis was the highest. After that, it decreased. You might expect the trend to be that the higher the temperature, the higher the rate of photosynthesis, but this is not the case. Enzymes work rather like a lock and key. They are specifically designed for one purpose, in the case of chlorophyll it is to activate photosynthesis. If a key is heated up, it will melt and become denatured and can never be restored to its original shape. This is how enzymes work. When the temperature is over 80°C, the enzymes are damaged and no longer fit the “lock” they were designed for. This is what causes the peak. The graph illustrates this peak perfectly.
The results fully support my prediction. The peak of every line comes at around 30°C. The shape of the graph I predicted is similar to the graph I obtained as well. The anomalous results undermine my prediction but these are to be expected. The wide range in results was caused by the different quality of elodea, for example at 30°C in set F and set A there were 107 and 100 bubbles whereas in set C and E there were 15 and 12 bubbles. These results seem odd when compared to each other but when compared to the other results obtained from that piece of elodea they are not anomalous.
Evaluation
I think that the procedure used was quite suitable, as I managed to achieve fairly reliable results. However, when dealing with living organisms such as elodea results can never be that accurate, because they do not work at a constant rate. So we cannot know if the plant was working at a high or a low rate or somewhere in between when we did the experiment. The rate of photosynthesis varied a lot as shown above. 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 I 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.
There were no anomalous results, but if there were there are many reasons why it could be. I could have miscounted the bubbles. Also, the pondweed might not have been bubbling properly due to a deficiency in quality 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.
The experiment worked but the wide range in rate of reaction shows how the experiment was highly unreliable. To stop this unreliability experiments would have to be carried out in the same spot at the same time of day and with the same apparatus every time. The experiment does show, however, that the rate of photosynthesis is directly affected by temperature.
