Ben Holston
Leaf Disc Lab
IB Biology-Pd. 4
In this lab we examined the effects of various colors of light on the rate of photosynthesis occurring in a leaf. To do this we put ten leaf discs in a clear glass of water and then measured how many were floating after every minute for fifteen minutes. These results are shown below.
Number of Leaf Discs Floating Overtime when Exposed to Light
Time (in minutes) / White (in # of discs) / Green(in # of discs) / Purple(in # of discs) / Orange(in # of discs) / White Corrected Value / Green Corrected Value / Purple Corrected Value / Orange Corrected Value0 / 2 / 2 / 6 / 5 / 0 / 0 / 0 / 0
1 / 3 / 3 / 7 / 8 / 1 / 1 / 1 / 3
2 / 5 / 6 / 7 / 9 / 3 / 4 / 1 / 4
3 / 6 / 7 / 8 / 9 / 4 / 5 / 2 / 4
4 / 7 / 8 / 8 / 7 / 5 / 6 / 2 / 2
5 / 8 / 8 / 8 / 10 / 6 / 6 / 2 / 5
6 / 8 / 9 / 8 / 10 / 6 / 7 / 2 / 5
7 / 8 / 10 / 9 / 10 / 6 / 8 / 3 / 5
8 / 9 / 10 / 9 / 10 / 7 / 8 / 3 / 5
9 / 9 / 10 / 9 / 10 / 7 / 8 / 3 / 5
10 / 10 / 10 / 9 / 10 / 8 / 8 / 3 / 5
11 / 10 / 10 / 9 / 10 / 8 / 8 / 3 / 5
12 / 10 / 10 / 9 / 10 / 8 / 8 / 3 / 5
13 / 10 / 10 / 10 / 10 / 8 / 8 / 4 / 5
14 / 10 / 10 / 10 / 10 / 8 / 8 / 4 / 5
15 / 10 / 10 / 10 / 10 / 8 / 8 / 4 / 5
In an ideal experiment we would have been able to begin with zero leaf discs floating at the beginning of the experiment. However, this was not an ideal experiment and we had several leaf discs already floating by the start of the experiment. In order to compensate for this I made corrected values for each point in order to account for how many leaf discs really floated to the top of the water during the experiment. Here is a graph of the corrected values of the number of floating leaf discs over time.
Once we had finished with this first part of the experiment we covered the glasses with aluminum foil to block out light, and once again measured how many leaf discs were floating after each minute, results are below.
Number of Leaf Discs Floating Overtime when Not Exposed to Light
Time (in minutes) / White(in # of discs) / Green(in # of discs) / Purple(in # of discs) / Orange(in # of discs) / White Corrected Value / Green Corrected Value / Purple Corrected Value / Orange Corrected Value0 / 10 / 10 / 10 / 10 / 8 / 8 / 4 / 5
1 / 10 / 10 / 10 / 10 / 8 / 8 / 4 / 5
2 / 9 / 10 / 9 / 10 / 7 / 8 / 3 / 5
3 / 9 / 10 / 9 / 10 / 7 / 8 / 3 / 5
4 / 9 / 10 / 9 / 9 / 7 / 8 / 3 / 4
5 / 9 / 10 / 9 / 9 / 7 / 8 / 3 / 4
6 / 9 / 10 / 8 / 9 / 7 / 8 / 2 / 4
7 / 9 / 10 / 8 / 9 / 7 / 8 / 2 / 4
8 / 9 / 10 / 9 / 9 / 7 / 8 / 3 / 4
9 / 9 / 9 / 10 / 9 / 7 / 7 / 4 / 4
Corrected values were continued here in order to stay consistent with the data points already recorded. The graph with corrected values is below.
Errors:
My group made plenty of errors throughout this experiment. First, when we first set up our light generator we measured two inches out, but our generator was bumped, instead of remeasuring we just guessed where it should be, which could have led to some problems in the lab. Also, we struggled defining exactly what the definition of “floating” was. Since each of us was recording information for only our color it could have been possible that since different standards were used to define “floating” there were errors in these calculations. Finally, we didn’t do a very good job of covering up the glass beaker at the end with aluminum foil. This easily could have let in outside light which could have continued the process of photosynthesis in the leaf discs, even though this process was supposed to be ending.
Conclusion and Evaluation:
As photosynthesis is performed oxygen is created. When this oxygen is created it should cause leaves to float to the top of the water. This was backed up by our data. As photosynthesis progressed while the leaf was exposed to light, more leaves floated to the surface. Our trend lines show an upward trend in all light colors, showing that it was photosynthesis that caused the leaf discs to float. The question now becomes which color of light provided for the most efficient photosynthetic production. According to our data the green lit leaf discs all reached the surface at a faster rate then any others. This can be shown by how steep the line is that accompanies the green lit leaf discs. According to the trend lines green lit discs are most efficient followed by white, then purple and finally orange.
When we covered the leaves we saw a fairly similar decline in the number of leaves floating. Once again when looking at the trend lines for this graph we can see that they are all fairly similar and consistent in showing a decrease in the number of floating leaves. This is because each of the leaf discs were subjected to the same circumstances. At this time these discs started to perform cellular respiration. This is what happens in the absence of light. As photosynthesis stopped they started using oxygen to perform cellular respiration, which led to the leaves sinking over time.
Weaknesses in this experiment were plentiful. For example it was impossible to block out all ambient light in the experiment. This meant that while the leaves were exposed to their individual colors of light they were still performing photosynthesis due to interaction with ambient light as well. Also, the method for cutting leaf discs proved to often be harmful to the discs, and wound up making it very difficult for them to not float at the beginning of the experiment.
We also had to deal with limitations in the experiment. For example there wasn’t enough room in the classroom for all of the groups to comfortably work. We were put right next to another group, and due to the close proximity of our two groups there was constant bumping and jostling of the glass jars containing the leaves, which could have caused irregularities in our results. The biggest limitation was time. While we were able to record the leaves exposed to light for the full fifteen minutes we were not able to work for thirty minutes when not exposed to light as the lab suggested we do. In fact, we were only able to work for nine minutes while the leaves were sinking, which may not have been sufficient time to draw a correct conclusion from the data.