Biology 171L FA15: Effect or Light color on photosynthesis
Biology 171L FA15: Effect or Light color on photosynthesis
Biol 171L – FA15 Effect of Light Color on Photosynthesis 7-2
Biology 171L FA15
Lab 7: Effect of Light Color on Photosynthesis
Student Learning Outcomes
1. Use an O2 Gas Sensor to measure the amount of oxygen gas consumed or produced by a
plant during respiration and photosynthesis.
2. Determine the rate of photosynthesis of a plant, as measured by oxygen production.
• Campbell Biology, Chapter 10, especially pp. 187-197
• A Short Guide to Writing about Biology, Chapter 9
Life on earth could not exist without photosynthesis. Photosynthesis enables plants,
algae, and some bacteria (autotrophs) to harness the energy in light and convert it to a usable
form of energy. It is the only process that is able to convert light energy to chemical energy. That
energy can then be used to create the macromolecules that serve as fuel for cellular respiration in
all other organisms (heterotrophs). Photosynthesis can only happen in the presence of light
energy and chlorophyll.
We often think of
sunlight as a single
“white” wavelength, but
in reality it is a continuum
of wavelengths – each
wavelength representing a
different color of light.
(Figure 1). Light travels in
waves and these waves
vary in wavelength, which
is defined as the distance
from the top of one wave
to the top of the next
wavelengths, like X-rays,
contain more energy,
while longer wavelengths,
like radio waves, contain
organisms contain special
Figure 1. The visible light spectrum is a small portion of the
electromagnetic spectrum. Energy is inversely proportional to wavelength,
so that long wavelengths, such as radio waves carry less energy than short
wavelengths such as gamma rays.
Biol 171L – FA15 Effect of Light Color on Photosynthesis 7-3
pigments that absorb light energy for use in photosynthesis. These special pigments are
stored in oval-shaped organelles called chloroplasts. Chloroplasts are numerous; between 20
and 40 can be found moving around within a single cell of a green plant. Pigments absorb light
energy in a narrow band we know as the visible light range (400-700 nm). The human eye sees
the wavelengths being reflected by the pigment. A pigment that appears red to the human eye
does so because it is absorbing blue and green light and reflecting orange and red. There are
many different pigments and each is distinctive in the wavelengths it absorbs and reflects (Figure
2). The three major classes of photosynthetic pigments are: chlorophylls, carotenoids, and
Chlorophyll molecules have a polar head (a porphyrin head) and a non-polar tail (Figure
3). The presence of the non-polar tail helps the chlorophyll molecule embed into the thylakoid
membrane. There are multiple chlorophyll pigments. Chlorophylls a and b are utilized by higher
plants and green algae. Diatoms and brown algae possess chlorophylls a and c, and red algae
uses chlorophyll a and phycobilins. Chlorophyll absorbs light in the red and violet wavelengths,
and uses this energy for photosynthesis. Chlorophyll absorbs very little of the green wavelengths,
which it reflects. Consequently, green is the color generally associated with plants.
The carotenoids (Figure 4) form a family of pigments that absorb light in the blue range
giving this class of pigments color which ranges from pale yellow to orange to red. Carotenoids
are categorized into two classes, xanthophylls, which contain oxygen atoms, and carotenes,
which are mostly hydrocarbon. Carotenoids are the dominant pigments in fruits and flowers
giving them their bright vivid colors. They are also present in leaves of plants, but are masked by
the chlorophyll present in mature leaves. The colors of carotenoids are apparent in young leaves,
ripening fruit, and dying plants, where the levels of chlorophyll have dropped off. Carotenoids
are important accessory pigments. They are also important in photo-protection, that is, they
protect plants from harmful UV radiation.
Figure 3. Structure of chlorophyll a
Figure 4. Structure of several carotenoid
The last class of photosynthetic pigments, phycobilins, is not present in land plants or
green algae. Instead, phycobilins are found in red algae and cyanobacteria. Phycobilins absorb in
Biol 171L – FA15 Effect of Light Color on Photosynthesis 7-4
the green and orange portion of the spectrum and are important for allowing marine and aquatic
plants to harvest the wavelengths not filtered out by the water.
PHOTOSYNTHESIS AND RESPIRATION
The process of photosynthesis involves the use of light energy to convert carbon dioxide
and water into sugar, oxygen, and other organic compounds. This process is often summarized
by the following reaction:
6 H2O + 6 CO2 + light energy ⟶C6H12O6 + 6 O2
Cellular respiration refers to the process of converting the chemical energy of organic molecules
into a form immediately usable by organisms. Glucose may be oxidized completely if sufficient
oxygen is available, illustrated by the following equation:
C6H12O6 + 6 O2 ⟶6 H2O + 6 CO2 + energy
The rate of photosynthesis is affected by numerous factors including the amount
and the wavelength of light available. In today’s experiment, you will examine how
photosynthesis is affected by the wavelength of light available to Elodea leaves by passing
light through different colored bulbs. If the wavelength available to the plant affects
photosynthesis, then we would expect to see a difference in the photosynthetic rates
produced by the different colors of light, and therefore a measurable difference in the
amount of O2 produced during photosynthesis.
Total or GROSS photosynthesis, is the total amount of carbon fixed or oxygen
released by a plant in the light. However, because plants are always undergoing respiration,
even in the dark, we must subtract the value of respiration from the total, or gross, amount
of photosynthesis. In order to compare between different plants and different treatments, we
would like to compare the amount of photosynthesis that is “left over” after the costs of
respiration are included. Therefore, the GROSS photosynthetic rate is determined from the
photosynthetic rate in the light (photosynthesis + respiration) plus the RESPIRATION (rate
in the dark). To determine this, you will measure the concentration of O2 present during a
closed reaction. First you will measure the amount of O2 consumed during respiration (in
the dark); then you will measure the amount of O2 produced by photosynthesis (in the
light). You will record the rate of O2 produced or consumed as the slope of O2 in ppm/s.
You will use the absolute values of your slopes to calculate your GROSS photosynthetic
����� Photosynthetic Rate = O! ppm s!! in ����� + O! ppm s!! in ����
You will also do an absorption spectrum of the photosynthetic pigments extracted
from Elodea and determine if there is a correlation between the rates of photosynthesis
under the different colored bulbs, the wavelengths of light that those filters transmit, and
the optimum wavelengths of light absorbed by the plant.
Biol 171L – FA15 Effect of Light Color on Photosynthesis 7-5
You will work in groups, as directed by your TA, and each group will take one part of the
experiment. You will either measure the respiration/ photosynthesis rate of Elodea under two of
five colors of lights (blue, green, orange, red, and white) (Parts A & B); or, you will record the
absorption spectrum for Elodea (Parts C & D), and measure the respiration/ photosynthesis rate
of Elodea under one of five colors of lights (blue, green, orange, red, and white) (Parts A & B).
Each color of light (blue, green, orange, red, and white) represents one treatment. You
will share your results with the other member in your class to form an initial data set.
Additionally, your TA will collect your data and submit it to the Lab Coordinator to be pooled
with data from other sections. You will analyze the entire class dataset, giving you
approximately 10-20 samples per treatment.
Your goal is to determine which color of light is the best driver of photosynthesis. (Think
about the different photosystems and the wavelengths they utilize).
Part A: Respiration Rate in the Dark
1. Turn on the computer. Connect the Vernier Lab Pro interface to the computer. If the Lab Pro
power cord is not plugged in do so now. The Lab Pro will beep when the power is
connected. Connect the O
2 Gas Sensor to the Lab Pro interface.
2. Prepare the computer for data collection. Open Logger Pro 3 by clicking on the Logger
Pro icon in the computer desktop dock.
3. Go to File in the menu and click on Open.
4. The Logger Pro window for measuring O2 production will open. Under ‘Experiment>Data
Collection…’ set the Duration to 15 minutes and set the Sampling Rate to 20
samples/minute. Make sure that units are in ppm (go to ‘Experiment>Change Units, if
5. Find the respiration chamber and wrap the entire respiration chamber in aluminum foil so
that no light can enter.
6. Obtain a piece of Elodea from one of the aquaria at the back of the room. Measure and cut a
piece 10 cm long. Place the remaining tissue back into the aquarium.
7. Carefully place the Elodea into the respiration chamber. Try to spread the plant out as much
as possible in the chamber.
8. Move the lamp so that the bottom of the bulb or the rim of the shade, whichever is lowest, is
7 cm from the table. Do not let the lamp touch the respiration chamber. Make sure that
the light is as close to 90° to the respiration chamber as possible. Place the O2 Gas Sensor
into the bottle. Place the sensor/chamber on its side under the fluorescent light on the table.
Even though this is the “dark” control, the light should be turned on (why?). Wait 5 minutes
for the chamber to acclimate before proceeding.
9. To begin measuring the O2 concentration in the chamber, click the green button in the
Biol 171L – FA15 Effect of Light Color on Photosynthesis 7-6
Logger Pro menu bar. Data will be collected for 15 minutes.
10. When data collection has finished, you will determine the rate of respiration.
a. Autoscale the data by clicking the Autoscale button on the toolbar.
b. Move the cursor to the point where the data values begin to increase. Hold down the left
mouse button and drag the cursor to select the region of decreasing oxygen gas
c. Click on the Linear Fit button, , to perform a linear regression. A box will appear with
the formula for a best fit line.
d. Record the slope of the line, m, as the rate of respiration in Table 1.
e. Close the linear regression box.
11. Store your data by choosing Store Latest Run from the Experiment menu.
12. Remove the aluminum foil from around the respiration chamber. Do NOT remove the
2 sensor from the respiration chamber.
Part B: Photosynthesis/Respiration (Net) Rate under Colored Light
13. Position the chamber under the light. Do NOT remove the O2 sensor from the
14. If necessary, reposition the lamp so that the bottom of the bulb or the rim of the shade,
whichever is lowest, is 7 cm from the table. Do not let the lamp touch the respiration
chamber. Wait 5 minutes prior to beginning data collection.
15. After the five-minute acclimation period is up, begin data collection by repeating steps 9-
11 above. When you click on the green Collect button, you will be given a set of options,
be sure to choose Store Latest Run. This will allow for all trials to be displayed on the
16. Remove the Elodea from the respiration chamber. Clean and dry the respiration
Rate of O2
Rate of O2
(|rate in the
light| plus |rate in
Biol 171L – FA15 Effect of Light Color on Photosynthesis 7-7
17. Once you have finished on one side of the table, swap places with your tablemates and
perform a second set of measurements. This may be a second color, or the absorbance
readings, depending on your table.
Part C: Extracting the Photosynthetic Pigments from Elodea
18. Obtain a small amount of Elodea from one of the tanks at the back of the room. Take short
pieces that are floating in the tank when ever possible. Blot your piece(s) dry with a paper
towel. Weigh out approximately 4g±0.2g of Elodea tissue and place it in a 250 ml glass
19. Cover the Elodea with 95% ethanol (EtOH) (approximately 50 ml).
20. Place the beaker on a hotplate with a stir bar. Adjust the stirrer until the stir bar is just
spinning. Heat until the Elodea-EtOH solution is just boiling (approximately 10 minutes).
The photosynthetic pigments will b e extracted out of the leaves into the EtOH. Remove
from the hotplate.
21. Fold a piece of filter paper in quarters, and place it into a funnel. Moisten the filter paper
with a few drops of EtOH.
22. Put the end of the funnel into a clean beaker. Pour the pigment extract through the filter
paper into the beaker.
23. Take 0.5 ml of the filtered pigment extract and put it into a 50 mL beaker containing 5.0 ml of
24. Pipette 3.0 ml of the diluted extract into a cuvette for your sample.
25. Pipette 3.0 mL of 95% EtOH into another cuvette for you blank.
Part D: Determining the Absorption Spectrum of Elodea Photosynthetic
25. If using the Zero your spectrophotometer following the methods for the Spectronic 20D+,
listed in previous labs. Set the spectrophotometer to 380 nm. Make sure the filter is set to
340-599 nm (left side). Blank the spectrophotometer with 95% EtOH and then take the
absorbance reading of your extract.
26. Set the spec to 400 nm. Re-blank with the 95% EtOH and take an absorbance reading of
your Elodea pigment extract.
27. Continue taking readings at 20 nm intervals up to 600 nm. At 600 nm, flip the filter to the right
(600-950 nm). Be sure to re-blank the spec at each wavelength before taking the reading of
your extract. Continue taking readings at 20 nm intervals from 600 nm up to 720 nm.
28. When you are done with your readings dispose of the EtOH and the pigment extract in the
“waste ethanol” container.
Biol 171 – FAFA15 Effect of Light Color on Photosynthesis 7-8
Lab 7 Homework – Due Week of Oct 19, 2015
Part 1 – Mastering Biology (36 points):
A. Answer the questions in the assignment entitled “08. DNA” on the Mastering Biology
site. You have until the night before lab at 11:59 PM to complete these questions.
Part 2 – Science Communication & Part 3 – Data Analysis (20 points)
A. Writing a Full Lab Report (20 points)
Using Pechenik, 2013 (A Short Guide to Writing about Biology, Chapter 9, and Gillen, 2007
(Reading Primary Literature) as guides, write a full lab report for this experiment. You must
include the following sections: Introduction, Materials & Methods, Results, Discussion, and
Reference List. You must turn in two copies of this draft!! Put your secret number
on both copies.
Find at least one article from the primary literature to use in your Introduction to introduce
the experiment, and at least one article to use in your Discussion to support one of your
conclusions. Be sure to cite these articles appropriately.
Your report must be typed, 12-pt font, Times New Roman font, double-spaced with 1”
margins. Follow the guidelines in the Universal Rubric and the Lab Report Grading Template
Rubric on Laulima.
Your report must consider/ include all the colors of light that were measured, and the
absorption data of the chlorophyll extracted from the Elodea, even if you didn’t do the
actual experiment. Additionally, you must consider the following questions when you are
interpreting your results:
1. Were either the respiration or photosynthetic rate values a positive number? If so,
what is the biological significance of this?
2. Were either the respiration or photosynthetic rate values a negative number? If so,
what is the biological significance of this?
3. Do you have evidence that cellular respiration occurred in the Elodea?
4. Do you have evidence that photosynthesis occurred in Elodea?
5. Think of at least three factors that might influence the rate of carbon dioxide
production or consumption in plants. How do you think each will affect the rate?
6. What is white light? How is it different from red light, or any of the other colors you
7. Why do you think the color of light has an influence on the rates that you measured?
Biol 171L – FA15 Effect of Light Color on Photosynthesis 7-9
8. How does the absorption spectrum of the chlorophyll extracted from the Elodea
influence which colors are the best drivers of photosynthesis?
This paper will be the first of two drafts you will write before you turn in the final paper at the
end of semester. In the coming weeks, you will have the opportunity to re-write the report. Your
final lab report, which will be due in the last lab, will be graded for content. There are many
resources, both on campus and online, to help you succeed. The following websites have many
Your grade this week will be based on how hard you tried, not on the content. In other words,
you should put the same amount of effort into this report that you would, if you had only
one chance to get it right. For example, if you write the materials and methods section in
bulleted form, you will lose points. Or, if you write the introduction, but do not include a journal
reference, you will lose points.
Criteria Not Completed
finished paper on
time to allow
comment on your
Incomplete / not
finished/ or past
and in on time,
e.g., may be
to journal articles
Fully complete and
in on time
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