date
HL Biology Year 2
sample
Introduction
Photosynthesis is the process by which light energy is converted into chemical energy that
can be described by the following formula:
6CO2 + 6H2O → C6H12O6 + 6O2
Plants and other photosynthetic organisms take in carbon dioxide and water, which reacts
with light to be converted into its products: sugar and oxygen. Photosynthesis is necessary
for plants to survive, as it is their means of receiving nutrients. A photon is a particle of light
from the sun that hits the chloroplast (site of photosynthesis) and starts the process of
photosynthesis. Sunlight is composed of a particular range of wavelengths known on the
electromagnetic spectrum as the visible light spectrum. The range of these wavelengths is
around 400 to 700 nanometers with the shorter wavelengths consisting of the violet color on
one end and the longer wavelengths consisting of the red color on the other end.
Figure 1: Electromagnetic Spectrum
Please see works cited for the citation of this digital image under Visible Light Spectrum.
The photosynthetic pigments such as chlorophyll that absorb the photons absorb certain
wavelengths of colors better than other wavelengths of colors. Chlorophyll a and b, the main
photosynthetic pigment in plants absorbs blue and red light much better than green light.
Most of the green light is reflected, thus plants usually are of a green color.
The elodea plant is commonly used in experiments to measure the rate of photosynthesis by
the process of counting the number of bubbles that it produces under the desired conditions.
In the process of photosynthesis, photolysis happens in the light dependent reaction where the
photon splits the water molecule into oxygen, hydrogen and electrons. These molecules of
oxygen would accumulate and rise up in water if the stem of the elodea was cut. Thus, it is
possible to measure the rate of photosynthesis of elodea plants by counting the number of
bubbles that rise per minute. In this experiment, elodea was used to examine the relationship
between the color of light and the rate of photosynthesis. The research question under
investigation was:
How does isolating the elodea’s exposure blue, red and green light affect its rate of
photosynthesis?
It was hypothesized that blue and red light will yield the highest rates of photosynthesis since
plants absorb these two wavelengths the best while green light will yield a lower rate because
1
date
HL Biology Year 2
sample
it is mostly reflected. In order to answer the research question, bubbles produced by the
elodea plants were counted in different lighting conditions in order to determine how the color
of the wavelength affects the rate of photosynthesis. Clear, red, blue and green cellophane
were used to cover a lamp in order to isolate only that color light shining on the test tube of
elodea. The data was then collected and analyzed in order to observe the correlations.
Variables
The independent variable in this experiment is the color of light that was exposed to the
elodea. In the first condition with the lamp shining on the elodea without the cellophane
covering, the number of bubbles produced by elodea in a minute was recorded. The process
was repeated for five other conditions: a lamp with clear cellophane, red cellophane, blue
cellophane and green cellophane. The dependent variable in this lab is the number of bubbles
that rose per minute in each trial. The number of bubbles produced per minute is a measure
of the rate of photosynthesis, which is assumed being affected by the independent variable of
the lab. The cut elodea will produce oxygen that accumulate and float up in bubbles, an
indication of photosynthesis. The larger the amount of oxygen bubbles counted per minute,
the higher the rate of photosynthesis. The controls were the trials with a lamp without the
cellophane covering and the lamp with the clear cellophane covering. The results of these
controls will be used as a basis for analyzing the results gathered from the other conditions.
Additionally, other variables were kept constant in order to control the experiment and
increase the accuracy of the results. Photosynthesis is mainly affected by three factors:
temperature, concentration of carbon dioxide and light intensity. It is important that these
three factors are kept constant throughout the trials because it will increase the validity of the
data collected.
Firstly, the same 10 pieces of elodea that were each cut in the same way were used for each of
the conditions to provide more consistent and reliable data. The rate of photosynthesis
specific to each plant is taken into account by reusing the same plants. Secondly, the light
intensity of each of the different colored lights conditions were kept the same at 550 lux. This
is to ensure that light intensity is not the factor influencing the rate of photosynthesis and
instead the rate is dependent on the wavelength of light emitted. Thirdly, the placements of
the apparatus, such as the ring-stand holding the test tube, were kept in the same positions
throughout the experiment. The experiment, though conducted over a period of 3 days, was
conducted approximately during the same period of time only ranging from early afternoon to
late afternoon (1.00 pm to 3.30 pm) and in the same exact area (in the Biology classroom).
Further, since the temperature of the water could fluctuate with the light shining on the test
tube, a thermometer was placed in the test tube before and after the experiment in order to
control for the degree of temperature change. If a trial yielded a 3 or more degree difference
in temperature change then it was discarded. Elodea was placed in a sodium bicarbonate
solution in order to provide an environment with more carbon dioxide, stimulating the rate of
photosynthesis. However, the molarity sodium bicarbonate solution was kept constant at 0.3
moles to eliminate this as a confounding variable.
Procedure
2
date
HL Biology Year 2
sample
Set up the experiment by placing a ring-stand next to a lamp and preparing the sodium
bicarbonate solution. Using a scale, 25.2 grams of sodium bicarbonate was measured in a
paper cup (placed upon the scale first in order to take into account its weight) then mixed
into a liter of water to create a 0.3 mole of sodium bicarbonate solution. Ten pieces of elodea
were selected and the tip was cut to shorten all of the elodea pieces to 10 cm long. Then, a
slanted cut was made at the bottom of each strand of elodea, which is where the oxygen will
escape the stem, and the bubbles will rise. Place these pieces of elodea into 10 test tubes of
water set on a test tube stand for easy access to the elodea when switching between the trials.
Secure an empty test tube to the clamp on the ring-stand and pour 50 ml of the sodium
bicarbonate solution in. Check again that the light from the lap shines directly on the test tube
and use Logger Pro’s light intensity meter to measure the light intensity before each trial to
ensure a more controlled experiment. With the light turned on, place a thermometer into the
test tube and measure the temperature of the sodium bicarbonate solution. Place the elodea
upside down (the cut side upwards) in the test tube, wait 1 minute before using a stopwatch
to time 1 minute of counting the bubbles produced by the piece of elodea. After the data is
recorded, measure for the temperature once again (discard the trial if there is more than a 3
degree fluctuation), place the elodea back into its original test tube (on the test tube rack),
empty the test tube, pour another 50 ml of sodium bicarbonate into the test tube, secure it to
the clamp on the ring-stand and take a new piece of elodea to test. Repeat the above
procedures for the 9 other pieces of elodea before changing the condition tested by covering
the lamp with the clear cellophane. Following the clear cellophane condition, cover the lamp
with red, blue then green cellophane.
Figure 2: Experimental Set up
Figure 2 shows the set up of the
experiment, the lamp is placed to the
side of the test tube with elodea and
the sodium bicarbonate solution.
The picture is of the “normal light”
set up, for the other conditions, the
cellophane in the background was
wrapped around the lamp. The light
intensity meter used to control the
amount of light emitted is not
shown.
Data collection
3
date
HL Biology Year 2
sample
Data Collection
Table 1: Initial, Final and Net Change of Temperature (in Celsius) For Each Trial
Normal Light
Clear
Blue
Green
Red cellophane
cellophane
cellophane
cellophane
±1
±1
±1
±1
±1
1
2
3
4
5
6
7
8
9
10
1
2
Δ
1
2
Δ
1
2
Δ
1
2
Δ
1
2
Δ
24
25
24
24
24
24
26
25
24
24
25
26
24
24
26
25
26
25
26
26
1
1
0
0
2
1
0
0
2
2
24
24
24
25
23
23
26
24
24
25
25
25
24
25
23
24
27
25
25
25
1
1
0
0
0
1
1
1
1
0
24
24
24
24
24
26
25
24
24
23
25
24
24
26
25
27
27
24
24
24
0
0
0
2
1
1
2
0
0
1
25
24
25
24
26
24
24
25
24
26
25
25
25
24
26
24
25
26
26
27
0
1
0
0
0
0
1
1
2
1
26
24
23
24
24
24
25
24
23
24
27
24
24
24
25
25
25
24
25
25
1
0
1
0
1
1
0
0
2
1
Bubbles produced per minute
The table above shows the initial, final and net changes in temperature of the 0.3 mole sodium bicarbonate
solution for each trial. In each condition, “1” represents the temperature before data was recorded, “2”
represents the temperature after the data was recorded and “Δ” represents the change in temperature from
beginning to the end. It is evident that the temperature changes ranged from 0-2° C, any trial with a change
greater than 2° C was discarded and redone.
Table 2: Number of Bubbles Produced by Elodea Per Minute In Different Colored Light
Settings
Trial
Normal Light Clear
Blue
Green
Red
cellophane
cellophane
cellophane
cellophane
±1
±1
±1
±1
±1
1
31
32
20
17
18
2
30
28
17
13
19
3
27
33
25
17
23
4
44
40
38
25
34
5
36
38
22
9
16
6
29
27
31
14
16
7
39
39
24
10
21
8
32
30
19
16
17
9
35
29
27
25
25
10
42
40
20
17
15
The table above shows the number of bubbles produced per minute produced by the elodea plant in normal light
(which meant a lamp without any cellophane), clear cellophane, blue cellophane, red cellophane and green
cellophane conditions. Ten trials were conducted with the same 10 pieces of elodea for each trial number.
Associated qualitative data:
 The bubbles that floated up were not all of the same sizes, sometimes the bubbles
would accumulate to make a bigger bubble before floating up
 There was no covering for the experiment so the ambient room light and sunlight from
the window could have affected the experiment
 The clamp blocked some parts of the elodea from receiving light from the lamp
 For several pieces of elodea, the bottom had to be cut again because when it was
placed in the water no bubbles floated up (though the new cut was cut in the same way
4
date

HL Biology Year 2
sample
to the old one, it still creates uncertainty in the data because the results could have
been affected)
Some pieces of elodea kept floating up and were pushed down with a finger in an
attempt to secure its place
Data Processing
Sample Calculations
Finding the Minimum Uncertainty of the Data
Ex) When counting bubbles, the minimum uncertainty of the data is ± 1 for each data
point collected because bubbles are counted in whole numbers
Calculating the temperature change of the sodium bicarbonate solution, before and after each
trial (as seen in figure >@)#@)
1. Final temperature – Initial temperature = Temperature Change
Ex) *As measured by the thermometer
25 − 24 = 1
Change in temperature is 1°
Calculating the Average
Ex) Trial 1 + Trial 2 + … + Trial 10
31 + 30 + 27 + 44 + 36 + 29 + 39 + 32 + 35 + 42 = 345
Ex) Sum/# of trials = Average
345/10 = 34.5
Calculating the Uncertainty of the Averages
Ex) Subtract the value recorded for trial 3 (smallest value) from trial 4
Trial 4 – Trial 3 44 – 27 = 17
Ex) Difference/2
17/2 = 8.5
Uncertainty is ± 7.5
Calculating the Standard Deviation and using an unpaired T-test to find the P-value
When selecting the “calculate a t-test” option, the
standard deviation of the data set is also given (seen at
the bottom by the numbers given in the row entitled
“SD”)
A paired t-test was used to in the process of
calculating the standard deviation of the data set. All
data points were copy and pasted onto graphpad.com.
Calculating a paired T-test
5
date
HL Biology Year 2
sample
1. Follow the same instructions as above, once the result page loads, the page will list the t
value, the corresponding p- value and the degree of significance
Ex)
In the process of calculating the
standard deviation, the t-test for
statistical significance has also been
completed for the two sets of data.
Results
Table 3: Average Number of Bubbles Counted Per Minute From Elodea In Different
Colored Light Settings and their respective Standard Deviations
Normal
Clear
Blue
Green
Red
Light
cellophane
cellophane
cellophane
cellophane
Average
34.5
33.6
24.3
16.30
20.40
Amount of ± 8.5
± 6.5
± 10.5
±8
± 9.5
Bubbles
Produced
Standard
5.72
5.19
6.36
5.40
5.78
Deviation
The table above shows the average amount of bubbles produced in each colored light setting and the uncertainty
for each average calculated by the difference between the highest value and the lowest value collected in the
data set divided by 2. The standard deviation for each condition was also calculated in order to analyze how
spread out the data is around the mean number.
Table 4: T and P-values to Test the Significance of the Data from a Paired T-test
Normal light
Normal light
Normal light
Normal light
and Clear
and Blue
and Green
and Red
cellophane
cellophane
cellophane
cellophane
T value
0.3685
4.6578
8.5313
7.0025
P-value
0.4174
0.0012
0.0001
0.0001
Degree of Significance Not
Very
Extremely
Extremely
statistically
statistically
statistically
statistically
significant
significant
significant
significant
Clear cellophane and
Clear cellophane
Clear cellophane
6
date
HL Biology Year 2
Blue cellophane
T value
P-value
Degree of Significance
T value
P-value
Degree of Significance
3.9853
0.0032
Very statistically
significant
Blue cellophane and
Green cellophane
4.4263
0.0017
Very statistically
significant
sample
and Green
cellophane
7.1283
0.0001
Extremely
statistically
significant
and Red cellophane
Blue cellophane and
Red cellophane
2.7662
0.0219
Statistically
significant
Green cellophane
and Red cellophane
3.0290
0.0143
Statistically
significant
6.1875
0.0002
Extremely
statistically
significant
Above shows the results of the t-test for various combinations of the different light settings. The t test measures
whether the data collected in two sets can be statistically distinguished from each other. The p-value can be
calculated from the t test to find the chance that the data correlates as a result of luck. The t value, p-value and
degree of significance are all according to graphpad.com.
Figure 3: Average Number of Bubbles Counted Per Minute From Elodea In Different Colored
Light Settings
Normal light
Clear cellophane Blue cellophane Green cellophane
Red cellophane
The figure above is a bar graph to demonstrate the average number of bubbles produced per minute by elodea in
different colored light settings. Error bars showing the standard deviation of each data set were placed for each
category. The normal light condition where the elodea was exposed to light from a lamp without any cellophane
wrapping yielded the highest average number of bubbles because it encompassed the whole visible light
spectrum while the green cellophane condition yielded the lowest average number of bubbles.
Conclusion
7
date
HL Biology Year 2
sample
The purpose of this experiment was to investigate the relations between different colored light
settings and its effect on the rate of photosynthesis of an elodea plant. In this case, the rate of
photosynthesis is in terms of the number of bubbles produced by elodea per minute.
In comparing the normal light condition and the clear cellophane condition, both yielded
similar results and thus can be considered the two control groups for the experiment. The
control group acts as a baseline for the other results, since white light encompasses all of the
wavelengths in the visible light spectrum it makes logical sense that the rate of photosynthesis
would be highest when exposed to all the different colors of light combined. The p-value
resulting from the t-test between the normal light condition and the clear cellophane condition
is 0.4147, indicating that there is a 41% chance that the data correlates by chance. This
implies that the difference between the two means is not statistically significant and supports
that there is only a minor difference in the rate of photosynthesis in the two controlled
conditions. However, when comparing the two controlled conditions to the three
experimental conditions (Blue, green and red cellophane settings) the results of the t-test
showed extremely low p-values that were usually of 0.001 indicating that the data points are
statistically distinguished from each other. However, there is an outlier in trial 6 of the blue
cellophane condition because the number of bubbles counted for this round exceeded the
number of bubbles counted for the control when theoretically the controls should yield higher
rates because it is composed of all the colors in the visible light spectrum. This outlier
accounts for why the p-value is relatively higher for the condition with the controls and blue
cellophane as opposed to the values of the other settings.
As predicted in the hypothesis, out of all the experimental groups, the blue cellophane
condition provided the fastest rate of photosynthesis with an average of 24.3 bubbles per
minute while the green cellophane condition provided the slowest rate of photosynthesis with
16.3 bubbles per minute. The elodea in the red cellophane condition had an average rate of
photosynthesis of 20.4 bubbles per minute, this rate yielding slower results than the blue
cellophane condition but faster than the green cellophane condition. The standard deviation
of the controls and experimental groups ranged from 5.19 to 6.36, suggesting that the range of
the data spread around the mean was generally the same for all five settings. The blue
cellophane has the highest standard deviation of 6.36, suggesting that the relatively larger
deviations of data points indicates a less reliable result when compared to the average of the
other sets of data. The opposite applies for the clear cellophane condition, which had the
lowest standard deviation of 5.19. When comparing the results of one setting to another by
means of a t-test, it is clear that the relationships as expressed in the results are hold statistical
significance. The p-values of the blue versus green, blue versus red and green versus red
conditions all report to have less than 5% chance of the data correlating due to luck. This
strengthens the results in supporting that the data collected for each condition do demonstrate
a correlation between the color of light and the rate of photosynthesis.
The results support the information presented in the absorption and action spectrum that
plants absorb blue wavelengths of light the best and reflect the green wavelengths, which is
why plants are usually the color green (“Light Absorption”). The rate of photosynthesis,
diagramed by the action spectrum, shows the rate being the highest in blue wavelengths, high
in red wavelengths and lowest in green wavelengths (Simmon). One difference to note is that
the action spectrum illustrates how plants absorbs different wavelengths of light within its
exposure to visible light, but in this experiment, each color is isolated and the plant is exposed
8
date
HL Biology Year 2
sample
to only that color. Nonetheless, the results still support previous research and the hypothesis
in that elodea absorbs blue and red wavelengths better than green wavelengths.
Please see works cited for the citation of
this digital image under Absportiton and
action spectrum.
Figure 4: Absorption and Action
Spectrum
Evaluation
Several improvements could be made
to the lab in order to increase the
accuracy of the results. Firstly, there
was no covering for the experimental
set up so the room lights and sunlight
coming from the window could have affected the rate of photosynthesis recorded. Although
this margin of error was kept constant throughout the whole experiment, the possibility of
other light affecting the experiment questions the validity of the results in answering the
research question because the research question refers to the rate of photosynthesis in an
isolated colored light setting. In order to control this confounding variable, a box with the
inside painted black and a side cut out for the experimenter to observe the bubbles could be
placed on top of the set up. Another option is to carry out the same experiment but in a dark
room and make sure that the window light is not shining directly onto the experiment.
However, this alternative option may not be viable because when many students are
conducting their experiments simultaneously thus access giving one student access to the
whole room by himself or herself may be difficult. Nonetheless, by using a black colored
covering, the black absorbs the light that hits the box preventing it from bouncing back to the
test tube. This will help isolate the light source of the elodea plant to only the lamp, which
will produce results more fitting to answer the research question.
Secondly, as mentioned previously, one of the major factors affecting photosynthesis is the
temperature. Although there was an attempt to control the temperature by making sure the
initial and the final temperature did not fluctuate by more than 2 degrees, fluctuations—no
matter how minor—can still affect the data collected. An increase in the temperature
increases the rate of photosynthesis, so it is possible that a trial with a larger fluctuation could
have a higher rate of photosynthesis than a trial with a smaller fluctuation though both may be
testing the plant in the same condition. To better control the temperature in the future, various
methods could be tested out. One idea is to place the test tube in a larger bucket of water
because one biological property of water is that in large amounts, a lot of energy is needed to
9
date
HL Biology Year 2
sample
in order to actually increase the temperature of water. This would decrease the temperature
fluctuations for each trial, producing more reliable results.
Thirdly, a problem mentioned in the associated qualitative data section was that the bubbles
that rose were of different sizes. This became a problem because for some trials, small
bubbles would rise very quickly but for other trials the bubbles would accumulate to become
a larger bubble before rising. If the oxygen bubbles accumulated before rising, then only 1
bubble would be counted as opposed to if the oxygen bubbles rose individually thus
questioning the validity of the data collected. Thus, alternative ways to measure the rate of
photosynthesis for elodea plants must be explored. One option would be to place the elodea
in a graduated cylinder filled with sodium bicarbonate solution, cover the top with a finger
and submerge it into a larger beaker so the graduated cylinder lies upside down. The rate of
photosynthesis would then be measured by the decrease in the water level in the graduated
cylinder filled with sodium bicarbonate solution. Another option would be to use a gas
pressure sensor to measure the increase in oxygen in the test tube. The procedure of this
process would remain the same as before, differing in inserting a stopper probe and
electronically calculating the amount of oxygen in the test tube instead of counting the
bubbles produced. However, using a pressure probe would be an indirect way of measuring
the rate of photosynthesis so experimenters should take note of the difference between taking
a more direct method versus a more indirect method. By testing out other ways to collect data,
a more accurate procedure could be created for future experiments.
Additionally, a spectrometer could be used in future experiments to pinpoint the wavelengths
of the light that is exposed to the elodea. By using a spectrometer, the photons that hit the
elodea would not be classified by colors, which is a more ambiguous way to classify, but
rather by the length of the wavelength, which is more accurate. In the future, various shades
of the same color could be tested in order to determine which wavelength induces the fastest
rate of photosynthesis. The spectrometer would help to distinguish between various shades of
color, placing actual units on the independent variable instead of having to rely on terms that
can embody different values for different people, such as colors.
If these changes were implemented in future studies, the procedure would yield more accurate
and reliable results because variables are being controlled in an even more stringent manner.
A more valid data set not only translates to a conclusion that is better fit to answer the
research question, but can also better indicate the corresponding areas of the research needed
to better understand the topic.
Works Cited
10
date
HL Biology Year 2
sample
Absorption and Action Spectrum. Digital image. Dr. Simmon's Cell and Cellular Processes.
Web. 12 Oct. 2011. <http://kentsimmons.uwinnipeg.ca/cm1504/photosynthesis.htm>.
"Light Absorption for Photosynthesis." Biology. Web. 12 Oct. 2011.
<http://hyperphysics.phy-astr.gsu.edu/hbase/biology/ligabs.html>.
Simmons, Kent. "Photosynthesis." Dr. Simmon's Cell and Cellular Processes. Web. 12 Oct.
2011. <http://kentsimmons.uwinnipeg.ca/cm1504/photosynthesis.htm>.
Visible Light Spectrum. Digital image. Dr. Simmon's Cell and Cellular Processes. Web. 13
Oct. 2011. <http://kentsimmons.uwinnipeg.ca/cm1504/photosynthesis.htm>.
11