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Photosynthesis/Leaf Disks Lab
Period 6
Photosynthesis Lab
DATA COLLECTION AND PROCESSING
Aspect 1: Raw Data
Turned in separately
Aspect 2: Processed Raw Data
Beaker 1 (Green Light)
Time
(Minutes)
0
1
2
3
4
5
6
7
8
9
10
11
12
Raw
2
2
2
3
3
3
3
3
3
3
3
3
3
Corrected
0
0
0
1
1
1
1
1
1
1
1
1
1
Number of Leaf Disks
Floating Due to
Photosynthesis Over
Time With Light
Number of Leaf Disks
Floating
Beaker 2 (White Light)
Raw
6
7
7
7
7
7
7
7
7
7
7
7
7
Corrected
0
1
1
1
1
1
1
1
1
1
1
1
1
Beaker 3 (Red Light)
Raw
2
2
2
2
2
2
2
2
2
3
3
4
5
Corrected
0
0
0
0
0
0
0
0
0
1
1
2
3
Beaker 4 (Blue Light)
Raw
5
5
5
5
6
6
6
6
6
7
7
7
7
Corrected
0
0
0
0
1
1
1
1
1
2
2
2
2
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Photosynthesis/Leaf Disks Lab
Period 6
Beaker 1 (Green Light)
Raw
3
3
4
6
6
6
6
6
6
2
2
Corrected
1
1
2
4
4
4
4
4
4
0
0
Raw
7
7
7
3
3
3
3
3
3
3
3
Corrected
1
1
1
-3
-3
-3
-3
-3
-3
-3
-3
Beaker 3 (Red Light)
Raw
5
5
5
3
3
3
3
3
3
2
2
Corrected
3
3
3
1
1
1
1
1
1
0
0
Beaker 4 (Blue Light)
Raw
7
7
7
7
7
6
6
5
5
5
5
Corrected
2
2
2
2
2
1
1
0
0
0
0
Number of Leaf Disks Floating Due to Photosynthesis Due to
Photosynthesis Over Time With Light
3.5
Corrected Number of Leaf Disks Floating
(Number Floating Minus Number
Floating at Start of Experiment)
Time
(Minutes)
0
1
2
3
4
5
6
7
8
9
10
Number of Leaf Disks
Floating Over Time
Without Light
Number of Leaf Disks
Floating
Beaker 2 (White Light)
3
Beaker 1 (Green Light)
2.5
Beaker 2 (W hite Light)
2
Beaker 3 (Red Light)
1.5
Beaker 4 (Blue Light)
1
0.5
0
0
5
10
-0.5
-1
Time (Minutes)
15
Linear (Beaker
Light))
Linear (Beaker
Light))
Linear (Beaker
Light))
Linear (Beaker
Light))
2 (W hite
4 (Blue
3 (Red
1 (Green
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Photosynthesis/Leaf Disks Lab
Period 6
Number of Leaf Disks Floating Over Time Without Light
Number of Leaf Disks Floating (Corrected
Based off Corrected Values With Light)
5
4
Beaker 1 (Green Light)
3
Beaker 2 (White Light)
2
Beaker 3 (Red Light)
1
Beaker 4 (Blue Light)
0
0
5
10
-1
-2
-3
-4
15
Linear (Beaker
Light))
Linear (Beaker
Light))
Linear (Beaker
Light))
Linear (Beaker
Light))
1 (Green
2 (White
3 (Red
4 (Blue
-5
Time (Minutes)
Aspect 3: Presenting Processed Data
First, in looking at the data, it is important to note the meaning of the “corrected”
data points. In the first part of the experiment in which the oxygen-extracted leaf disks
were left exposed to different colors of light, several leaf disks still had oxygen in them
post-extraction, causing them to float to the top initially without the creation of oxygen
due to photosynthesis. The corrected value subtracts the number of leaf disks initially
floating from the number of disks observed floating in the experiment, thus accounting
for oxygen-extraction errors and ensuring that the experiment only measures number of
leaf disks floating due to oxygen produced by photosynthesis. The corrected value in the
part of the experiment without light starts the number of leaf disks at the corrected value
at which the first part of the experiment ended. This accounts for the number of disks
that should have been floating had the experiment been ideal and all leaf disks were not
floating initially. It therefore most accurately measures the cellular respiration that
occurred in the leaf disks that had previously undergone photosynthesis (as they used up
oxygen, they fell).
One error noticed in the course of the experiment was that Beaker 2, that in front
of white light, was a fair amount closer to its light source than the other beakers. This did
not, however, seem to have an impact on the data of the experiment. A much more
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Photosynthesis/Leaf Disks Lab
Period 6
significant error was the fact that not all of the oxygen in the leaf disks was extracted,
resulting in several initially floating disks, as explained in the previous paragraph—2
disks were initially floating in Beaker 1, 6 in Beaker 2, 2 in Beaker 3, and 5 in Beaker 4.
This was corrected for by using the corrected values as explained previously, but it is still
quite possible that it skewed the experiment, as some leaf disks perhaps had levels of
oxygen low enough at the beginning of the experiment that they were not floating
entirely, but high enough that they were faster to float once small amounts of oxygen
were produced during photosynthesis. Due to certain peculiar results of the experiment
(discussed below and in the conclusion), it is very possible that there were other errors
made in the experiment, although these odd results are more likely products of certain
limitations and weaknesses in the experiment (also discussed in conclusion and
evaluation).
For the most part, there is relatively little uncertainty in the data—almost all of it
followed the trend of floating in the first part due to the production of oxygen in
photosynthesis and sinking in the second part due to the use of oxygen in cellular
respiration. There is, however, one massive uncertainty found in the behavior of the leaf
disks in Beaker 1 (Green Light) in the second part of the experiment. Rather than the leaf
disks falling as one would expect, or as they did in Beakers 2 through 4, in darkness more
leaf disks floated in Beaker 1 from 1 minute to 8 minutes. The number of floating disks
increased by 3 before abruptly decreasing by 4 to yield a net decrease of 1 (the net
decrease of 1 was to be expected given that there was an increase of 1 in the first part of
the experiment, but the increase was not at all expected). The increase in number of
floating disks in darkness, when the leaf disks were not performing photosynthesis, the
process that produces oxygen, makes no sense whatsoever, and seems rather inexplicable.
Especially the fact that more leaf disks floated in darkness than floated in light makes this
result even more peculiar. There is no plausible scientific explanation for this result
given the processes these disks should have been, and were capable of, performing in
darkness. It therefore must be the result of some sort of error. One possibility is that
some disks that should have been floating were perhaps caught on the sides of the beaker,
and that, in displacing the beaker’s tin foil cover during readings, the beaker was jostled,
thus loosing the leaf disks and allowing them to float. It seems odd that this error would
only occur for one student at his or her beaker, but it is quite possible due to the
possibility of a single person not adequately paying attention to the contents of his or her
beaker. It is also very possible that this beaker was closer to the window or had a not
entirely enveloping tin foil cover, allowing light into it and therefore more photosynthesis
to occur. The only other apparent uncertainty lies in Beaker 2 of the non-light part of the
experiment. In all the other beakers, the number of leaf disks that sank was ultimately the
same number as those that originally floated, indicating that the oxygen produced by
photosynthesis with light was consumed by cellular respiration in the absence of the light.
In Beaker 2 (White Light), however, 3 more disks sank than originally floated, indicating
that the leaf disks in Beaker 2 evidently underwent more cellular respiration. This is also
somewhat inexplicable, and does not seem to root from any apparent error in the
experiment, except that perhaps the student observing this beaker counted some leaf disks
as floating that were not entirely floating with light that were counted as not floating
when they sank more without light.
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Photosynthesis/Leaf Disks Lab
Period 6
CONCLUSION AND EVALUATION
Aspect 1: Concluding
This experiment ultimately sought to reach two conclusions—first, the more
broad conclusion regarding the processes performed by a plant to obtain energy in the
presence of light and without light, and second, the more specific conclusion regarding
which colors or wavelengths of light are optimal for photosynthesis (at least in spinach
leaves). The first conclusion is proven quite well by this experiment, and is quite simple.
Oxygen was extracted from the leaf disks in this lab (or was attempted to be extracted) so
that the leaf disks would have no oxygen within them and would sink in the beakers. If
those leaf disks that did so—which were the only ones accounted for in the corrected
values—floated, then it could be reasonably concluded that some process occurred within
those plants that created oxygen. That process was clearly photosynthesis, which
produces oxygen via the light reactions when enzymes split water using energy garnered
from photons of light. This conclusion is proven wholly by the data, which in every
instance indicated that the leaf disks underwent photosynthesis when exposed to light, as
in each beaker leaf disks (ranging from a total of 1 to a total of 3) floated. The second
aspect is the process occurring once the beakers were covered in the second segment of
the experiment. In darkness, plants no longer perform photosynthesis, but rather will
perform the Calvin Cycle, which uses CO2, fixing it to ultimately produce sugars, or
cellular respiration, which uses oxygen as an electron acceptor to ultimately produce
sugars. In this experiment, it was quite clear that cellular respiration occurred, as, for the
most part, leaf disks fell, indicating the use of the oxygen that initially kept them floating.
There was one peculiar exception in Beaker 1, which is explained in Aspect 3 of
Presenting Processed Data, but ultimately the data indicates that the disks in each and
every beaker performed cellular respiration seeing as they sank. Additionally, for each
beaker except Beaker 2, the leaf disks that ultimately floated and then sank were the same
number (the ending corrected value is 0), indicating that the oxygen produced by
photosynthesis in part 1 was used up in cellular respiration in part 2. This uncertainty in
data is also discussed in Aspect 3 of Presenting Processed Data, and still affirms the
conclusion that the disks were undergoing cellular respiration.
The less simple and perhaps more interesting conclusion has to do with the
wavelengths of light that best fell within the leaf disks’ action spectrums, or the
wavelengths of light that were best absorbed by the leaf disks’ pigments thus yielding the
most photosynthesis. Based on the data, it can be concluded that, for spinach leafs and
other plants with similar or identical pigments, red light yields the most photosynthesis,
followed by blue light and then white and green light. This is supported by the data, as
the most leaf disks floated in the beaker by the red light, indicating the most
photosynthetic behavior (3 disks, corrected value), followed by blue light (2 disks) and
then white and green light (1 disk). This conclusion makes sense given the action spectra
of typical green-leafed plants like spinach. Red would likely be the most optimal area of
wavelengths for photosynthesis, as chlorophyll a, the most prevalent chlorophyll, absorbs
red wavelengths the most; one can see for reference the P680 and P700 chlorophylls in
Photosystems II and I, as 680 and 700 nanometers of wavelength fall within a red color
range. It would also make sense that blue be the next best color (out of these 4) for
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Photosynthesis/Leaf Disks Lab
Period 6
photosynthesis, as it is relatively high when looking at an action spectrum, and very few
leafs, or at the very least none of the ones in the this experiment, have blue color,
indicating that blue colored light is absorbed. It also makes sense that green was the
lowest, as green light is reflected in the leafs and not absorbed, and it is low on action
spectra. In fact, it would make more sense for green to have not yielded any
photosynthesis, but because of the weaknesses in the experiment (explained below) in
terms of many other sources of light in the room, this discrepancy is easily accounted for.
Lastly, it makes sense that white light be below red and blue light in terms of optimizing
photosynthesis, as it contains lower concentrations of ideal wavelengths, but it makes
slightly less sense that it was as low as green in terms of photosynthesis, as it does
contain wavelengths of light that are absorbed much more than the wavelengths in green
light. This perhaps would have manifested itself had more time been available (see
Aspect 2 below). Overall, however, the conclusion is completely supported by the data
and generally makes a lot of sense considering the ability of plant pigments to start
photosynthesis in different wavelengths of light.
Aspect 2: Evaluating Procedure
There were some considerable weaknesses and limitations in the experiment that
likely had bearing on the data. The first and probably most significant weakness was the
sources of light. For the data to fully reflect what it intended—the photosynthesis of the
leaf disks in certain lights and then the respiration of the leaf disks in complete
darkness—there should have only been 4 sources of light, shining exclusively on their
respective beakers, and then total darkness in the second part of the experiment. An
enormous weakness in the experiment was that this was simply not true. There were
many other sources of light: mass amounts of light came into the classroom from the
hallway, there were about 6 other groups emitting colored light from their stations, and,
while the blinds were closed, a fairly large amount of light leaked through the windows.
Our station, in fact, was no less than one or two feet from a window, and therefore a lot
of light leaked through to our beakers. This meant that more photosynthesis occurred in
the leaf disks than would have otherwise, and that the photosynthesis that occurred was a
result of far more than one source of light, skewing the results. This likely explains the
photosynthesis that occurred in Beaker 1 (Green Light) that probably should not have, as
green light is reflected by pigments. This weakness also probably created a disparity in
results, as some beakers were closer to the window, probably the largest external source
of light. We should have made note of which beakers were closer to the window as those
beakers were in different light conditions than the other ones, but unfortunately we did
not. Regardless, this light sources constituted a notable weakness. The amount of light
also affected the second part of the experiment in which the purity of only one source of
light was not a necessity, but rather the absence of light altogether. Despite the tin foil
covers, much light definitely affected the beakers (through slits in the covers, underneath
them, and when they were removed during data recording), quite possibly skewing the
results as photosynthetic processes could still have very likely been occurring rather than
a total switch to cellular respiration.
The major limitation of this experiment was clearly time constraints. The first
part of the experiment measuring photosynthesis could only be conducted for 12 minutes,
and the second part measuring respiration only lasted 10 minutes. This definitely does
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Photosynthesis/Leaf Disks Lab
Period 6
not allow for much time for the leaf disks to photosynthesize and respirate, meaning clear
differences between the photosynthesis affinity of the different colors of light could not
as clearly be observed. More time would also have allowed for much more respiration to
occur, which would iron out odd peculiarities in the data such as in Beaker 1, as
described. Perhaps most importantly, more time for all disks to ultimately
photosynthesize and then respire would allow for a whole new conclusion to be reached
regarding the comparative rates of respiration for different wavelengths of light (e.g. how
long it takes all the disks to rise in each beaker), rather than the short term, immediate
effects of the light.
Aspect 3: Improving the Investigation
It is quite obvious, given the weaknesses and limitations of the experiment, what
needs to be improved, but the way to make those improvements might be somewhat more
difficult. The first improvement is simple, which is giving much more time for the
experiment. Ideally, one would perform the first part of the experiment over the course
of 30 minutes, and the second part of the experiment over the course of 30 minutes. This,
as mentioned in Aspect 3, would allow for much more clear observation of the
differences between the beakers, and would allow for a whole new conclusion regarding
the rate of photosynthesis for the different colors of light.
The other major improvement is in regards to light. The best improvement to the
experiment would be for it to be conducted in complete darkness (with the exception of
the 4 light sources being used), in a room with no windows, the doors shut, and all lights
off. The four different colored light sources and their respective beakers would ideally be
separated enough—even by partitions of poster boards, or some other easily accessible
material—so that only the intended light source reaches the beaker it is supposed to
reach. One must, of course, ensure that the light source is bright enough that
photosynthesis can easily occur. In the second part of the experiment when these light
sources are turned off, it is again important that there is total darkness, and when readings
are taken, they would be ideally taken quickly, using only a small amount of light
(perhaps from a flashlight or cell phone) to see enough to take the readings and write
them down.
These two improvement would help the experiment as a whole immensely.