The more light energy that is captured by the chloroplasts, the more glucose can be created. We already know that Water + Light = Chemical energy. Therefore, the more water and light that gets to the plant, the more chemical energy can be created. We also know that Chemical energy + Carbon Dioxide = Sugar. Glucose is a simple form of sugar. As chemical energy is made from water and light, the more of these there are then the more chemical energy can be created. Glucose also needs carbon dioxide to be made. Therefore, the more of this there is as well, the more glucose can be produced.
BPM
Method
- We will position the four lamps around the beaker, which contain the water and the pondweed.
- Then we will switch all of the lamps on and wait for 30 seconds for the light to acclimatize with the plant.
- After 30 seconds, two people will count the number of bubbles of oxygen being given off by the plant. For every reading, we must repeat it three times to make sure our readings are accurate. Then, we will calculate the average BPM using our three results.
- Then we will repeat the process in the same way. However, this time we will only have three lamps on.
- Once we have repeated the experiment with just 1 lamp on, we will finish by counting the number of bubbles of oxygen given off when there is no light being absorbed by the leaves, without any of the lamps on.
What must we keep the same during this Experiment?
There are a number of things that must stay constant during this experiment, but just one thing that must change which is known as the input variable. This will be the amount of light radiated by the lamps, which will be measured in watts. The output variable, or the thing that is going to change as a result of the input variable, will be the number of bubbles of oxygen given off by the plant. As the number of bulbs that are on decreases, the number of bubbles of oxygen given off by the plant should also decrease. We must also keep the amount of pondweed and the volume of water the same throughout the experiment. In addition to this, we must keep the distance that the lamps are away from the beaker the same. The lamps will be 5cm away from the beaker.
Results
Conclusion
During our experiment we found out a lot about the factors that limit the rate in which green plants photosynthesise. We found out that as the light intensity increased, so did the amount of oxygen given off by the plant. This proves that my prediction was correct. If we increase the amount of light then the chloroplasts can absorb more of it, producing more oxygen and glucose. We calculated the amount of oxygen given off by the plant by counting the number of bubbles given off by the plant every minute. This is because the more light energy that is supplied to the plant; the more glucose can be reacted with the raw materials to produce starch. The raw materials are the light energy, which the sun supplies and the water that is found in the soil when it rains. My graph shows that generally, as the amount of light energy increases, so does the BPM. However, as it should, the light energy does not reach the point in which it stops limiting the rate plants photosynthesise. This could be because we did not supply the plant with enough light for the light energy to no longer make a difference on the rate of photosynthesis. There was not enough light shone on the green plants for the graph to level off.
The error bars that I have drawn on my graph show the range and accuracy of the readings that I took. The longer these bars are, the less accurate my readings were. My error bars are smaller at the start but as the amount of light energy increases, my error bars begin to get longer. This may be because as the amount of light energy increases, the more bubbles of oxygen were given off and so it was harder to count them as there were more of them.
When I compared my results with other people, I found that my results were fairly accurate. However, many of their results seemed to be starting to level off after they had drawn their graph.
Evaluation
I was generally very pleased with our method and the way in which we carried out our experiment. However, as we were collecting together our results and counting the number of bubbles, a number of things were brought to our attention that could have affected the fairness of our experiment. Firstly, sunlight from the outside could have entered through the windows and warmed up the water in which the pondweed was standing. Also, the heat from the light bulbs may have warmed up the water and made the plant photosynthesise a lot quicker. We know that temperature also limits the rate in which photosynthesis is carried out. If there was more heat within the beaker warming up the pondweed, then it may have photosynthesised a lot more. Therefore, the change in the BPM may have been because of the temperature rather than the amount of light energy.
I believe our results were fairly accurate in comparison to other peoples'. There was one result that was anomalous, which was the last reading we took. We took three readings for every amount of power given off by the lamps. The error bars show the range in these three readings. For 160 watts, with every lamp on, our readings have a range of 6. This means our results here were not as accurate as the rest we took.
If I were to carry out the same experiment again, there would be a number of things I would change and do differently. I would have more lamps supplying the pondweed with more light, so that hopefully the graph would level off and the light would stop being a limiting factor. Also, we could take more than three readings to make our results even more accurate. Although three readings is enough to produce a graph, we might be able to draw a better conclusion. We would also be able to reinforce the fact that light intensity does have an effect on photosynthesis. As heat from the outside and from the lamps was heating up the water surrounding the pondweed, if we did the experiment again we would have to think of a way to get rid of this excess heat. We could try using a troff filled with cold water and put this into the beaker where the pondweed stands. This would then hopefully absorb all of the heat inside the beaker and stop temperature from tampering with our results.
