Therefore when I count the numbers of bubbles when there is no light it will be less than if there was light due to the fact that there is no light allowing the photosynthesis to increase. Vice versa for when there is light, because photosynthesis produces glucose from photosynthesis.
Plan:
- Set up the apparatus as shown in the diagram below but leaving out the pondweed.
2. Fill the beaker with 500mls of water and NaHCO3.
3. Select 1 or 2 pieces of pondweed each roughly 5-10 cm long and cut off the stems.
4. Place the pondweed in the beaker and secure the funnel upside down over (on top of) the pondweed.
5. Place a water-filled test tube upside down and over the funnel (see diagram).
6. Place the ruler so that the "0" measurement is aligned with the side of the beaker. (Distance measured from side of beaker to edge of light bulb)
7.) Place the lamp directly in front of the plant so that it is 0 cm away from the beaker.
8.) Then start measuring with no light at all but with a black piece of material covering it, without the light shining on the plant, with the lamp on but 20cm away then 40cm then 60cm. Record the number of bubbles emitted in a 1 minute duration. Switch off the lamp and wait for another minute before taking another reading so the test is fair.
9.) Take 3 readings at the current distance and move the lamp further away from the plant.
10.) Repeat steps 8 and 9 until 3 readings have for all the values in my table
11.) Proceed to the data analysis stage.
Table of Results:
Conclusion: From the results I have gathered I can state that an increase in light intensity without a doubt does increase the rate of photosynthesis. Which was also expected in my prediction, the relationship between light intensity and the rate of photosynthesis was non-linear. From the graph there is a best-fit curved line apart from one result.
The gradual decrease in the rate of photosynthesis (the swallowing of the curve) can be attributed to the other factors limiting the rate of photosynthesis. As light intensity increases, the photosynthetic rate is being limited by certain factors such as carbon dioxide and temperature. These factors do not immediately limit the rate of photosynthesis, but rather gradually.
Overall, my graph and my results supported my predictions fully. My idea that the rate of photosynthesis would increase with light intensity was comprehensively backed up by my results. This is because a higher light intensity involves a greater level of light energy, which can then be transferred to a special protein environment designed to convert the energy. Here, the energy of a photon is used to transfer electrons from one chlorophyll pigment to the next.
Evaluation: Overall, I would state the experiment as a success since my predictions were supported by my results. This photosynthesis investigation was probably not performed as accurately as it could have been due to some controllable and uncontrollable conditions.
While performing the experiment, the piece of pondweed did not photosynthesise at a steady rate, even when the distance from the plant to the light source was kept a constant. The second reading at 40cm was far greater than the first reading at 40cm. While the number of oxygen bubbles was being recorded, the rate at which the plant was photosynthesising had increased several times. This may be due to the poor circulation of sodium hydrogen carbonate at the beginning of the experiment.
The negative effects from this problem may be inaccurate data for some readings. These would show up on my graph. However, there seemed to be few anomalies apart from when the lamp was 20cm away. To improve the accuracy of the results, the readings would have to be taken several more times. The entire experiment could have been performed again, and the new results could be combined if the same plant is used. But the photosynthetic rate of the same piece of pondweed would eventually decrease over time anyway. Repetitions would however, improve the overall reliability of the results.
There are quite a few factors that could affect the results of my experiment. Some of these are variables that were mentioned earlier and could not be controlled, or they were variables that were not initially considered.
While performing the experiment, some of the oxygen produced from photosynthesis may have dissolved into the water. Microorganisms living on the pondweed may have even used some oxygen. As the rate of photosynthesis decreased due to a decrease in light intensity, the size of the bubbles produced also became smaller. This change in bubble size was not accounted for when the results were analysed. For a more accurate analysis of the collected data, volume should have been measured instead of bubble quantity since the size of bubbles can vary. Using a capillary tube in place of the test tube so that the volume of each bubble could have been measured could have done this.
During the high intensities I had experienced counting difficulties of the bubbles being produced. There are also factors affecting accuracy at low light intensities. With low light intensity, the pondweed 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 all background light, the experiment must be performed in a completely dark room. Even then, some of the light from the lamp in my experiment would reflect of the table and reach the plant though this amount of light is probably insignificant in affecting the rate of photosynthesis.
Temperature was also another factor that was controlled by the lamp being used. So I should have used a heat shield. The method of the experiment could probably also be improved to obtain more reliable results. Due to the high rates of photosynthesis of the pondweed, readings should be taken within shorter time periods. I had originally chosen to count the number of bubbles in two minute but these produced miscounts in the readings. If during a repeated experiment, counting bubbles is still used, there is a smaller chance for human error when counting within a smaller time frame. If the capillary tube option was to be chosen, volume should be measured for a smaller time frame to reduce the overall time to complete the experiment.
Due to the nature and convenience of the experiment, it could be easily modified to investigate another variable of photosynthesis. Since sodium hydrogen carbonate (NaHCO3) is used to provide the pondweed with carbon dioxide. Performing the experiment with different volumes of NaHCO3 could vary the amount of CO2. The plant would be kept at a constant distance from the lamp and a constant volume of water would be added to the sodium hydrogen carbonate. Although I feel that my experiment was sound overall, I thought there were many points at which the accuracy was not perfect. As I have already stated, mostly due to the fact that I was relying on all the bubbles being the same size, which they clearly weren’t, however many of the smaller inaccuracies also apply to my main experiment.
Written By L Itam
