I can also see from my results that after a certain light intensity, the graph begins to curve and level off, which shows that the plant can only absorb so much light energy and then any increase after this point does not affect the reaction. Therefore light stops being a limiting factor to the rate of photosynthesis. This is because for water and carbon dioxide to react and for photosynthesis to occur, chlorophyll is needed. The light travels through the transparent epidermal cells to a palisade cell to Chloroplast. Chloroplast contains chlorphyll, which absorbs the light. The light and enzymes in the Chloroplast allow water and carbon dioxide to react. However chlorophyll can only absorb so much light so after a while it has no effect.
Evaluation
Although I was happy with my results, looking back at my experiment there are a number of factors that could have made my results inaccurate or unreliable.
The pondweed’s rate of photosynthesis was not always at a steady rate. This was shown when I repeated each experiment three times and each time was slightly different. However I tried to overcome this problem by taking the average from the three results which meant that my results were more reliable.
Another factor that could have effected my results was human error. This could have occurred from miscounting the number of bubbles. To improve the accuracy of my results I repeated each three times and found the average. If I were to repeat this experiment I could also take the readings from a shorter time period, where there would be a smaller chance of error.
The plant could have also been effected by natural sunlight in the room. If I were to repeat the experiment again, I could eliminate this light by shutting blinds, dark room etc. Heat given off from the lamp could have changed the temperature of the water. This could have been stopped by using a glass block in between the plant and lamp which would of allowed light through but not heat.
Microorganisms living in the weed may have used some of the oxygen. Some oxygen could have been dissolved into the water. However as only the number of bubbles were counted and not the volume, it shouldn’t have effected my results too much. I feel that too measure the volume would be too difficult and impractical. However I noticed that as the rate of photosynthesis decreased due to a decrease in light intensity, the bubbles also got smaller.
This investigation could have been modified to investigate another variable of photosynthesis. Heat could have been added by using different temperatures of water. 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. 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.
I think that overall the evidence is reliable and that the results show what I predicted. It could have been more accurate but I think it proves that as light intensity is increased photosynthesis speeds up.