Download B) Alga`s photosynthetic pigments absorb photons at specific

Name __________________________
Which Wavelengths of Light Drive Photosynthesis?
Theodor Engelmann was one of the first scientists to describe chemotaxis: the movement of bacteria toward regions
where specific compounds are most concentrated. In the early 1880s, scientists already knew that plants and algae used
sunlight to produce sugars, with (O2) as a byproduct.
They also knew that
(the visible spectrum).
sunlight was a mixture of many colors
Engelmann was interested in determining which colors (or wavelengths) of light were most effective in driving
photosynthesis in green algae. He devised an experiment using aerotactic (oxygen-seeking) bacteria to answer this
question. (Aerotaxis, or movement toward oxygen, is one form of chemotaxis). A diagram of Engelmann’s apparatus is
shown below:
1) What is the role of the alga in this experiment?
A. The alga move to the wavelengths where it can photosynthesize the most.
B. Photosynthetic pigments in the algae absorb specific wavelengths of light, and the light energy is used to drive
the reactions of photosynthesis.
C. The alga photosynthesize, producing sugars that attract the bacteria.
D. The alga photosynthesize, producing CO2 that attracts the bacteria.
2) What is the role of the bacteria in this experiment?
A.
B.
C.
D.
The bacteria are attracted to specific wavelengths of light.
The bacteria are attracted to specific photosynthetic pigments in the algae.
The bacteria are attracted to sugars produced by the photosynthesis in the algae.
The bacteria are attracted to oxygen produced by photosynthesis in the alga.
3) Deduce the logical link between the production of photons by the sun
(Engelmann’s light source) and the distribution of bacteria that Engelmann
observed. Write the letter that corresponds to the appropriate label that belongs
next in the flowchart to show the relationship. Not all choices will be used.
A) Prism disperses sunlight
into individual wavelengths.
B) Alga’s photosynthetic pigments
absorb photons at specific
wavelengths.
C) Bacteria attracted to regions of
highest oxygen concentration.
Sun emits photons at
all wavelengths
E) Bacteria attracted to specific
wavelengths of light.
F) Alga gives off oxygen as it
photosynthesizes.
G) Alga’s photosynthetic pigments
absorb photons at all wavelengths.
Number of
bacteria in
different
wavelength
regions
counted
D) Absorbed photons drive
photosynthesis in alga.
Now, consider two different Cladophora cells (a type of filamentous algae), each with the same number of chloroplasts.
4) Compare the distribution of chloroplasts in these 2 cells. _______________________________________________
5) Which of the following statements correctly describes which cell(s) would be well-suited for Engelmann’s
experiment? Assume that about three cells in the algal filament would cover the width of the visible spectrum on the
microscope slide.
A. Either of the two cells could be used because both are capable of absorbing light and producing oxygen in
photosynthesis.
B. Either of the two cells could be used because they both contain the same number of chloroplasts.
C. The top cell should be used because the arrangement of chloroplasts allows for light to be absorbed and
produce oxygen across the entire length of the cell.
D. The bottom cell should be used because the higher the concentration of chloroplasts in one area will allow for a
greater absorption of all wavelengths of light.
Engelmann counted the number of bacteria that were attracted to the algal filament associated with each color of light.
The image below shows the number of aerotactic bacteria that cluster near an illuminated algal cell as time passes.
6) Write a conclusion that can be drawn from this image. _______________________________________
7) Using the image below:
a. Identify the independent variable ________________________
b. Identify the dependent variable _________________________
c. Deduce which colors (wavelengths) of light were most effective in driving photosynthesis within the alga.
Justify your answer using experimental evidence.
8) What assumption did Engelmann make in order to conclude that red and violet-blue light were more effective than
green light in driving photosynthesis?
A. The number of bacteria clustered at each wavelength (color) was approximately proportional to the amount of
oxygen being produced by that portion of the alga.
B. Every photon absorbed by the alga was used to drive oxygen production by the alga.
C. The algae absorbed the same number of photons at each wavelength (color).
D. The distribution of chloroplasts within each algal cell was not uniform.
9) Given the 2 images below, explain the relationship between the action spectrum and the absorption spectrum of
photosynthetic pigments in green plants.
Engelmann’s experiment demonstrated that red light is effective at driving photosynthesis, and thus oxygen production,
in the alga he studied. Now suppose Engelmann did an experiment in which he illuminated an algal cell with red light
for 3 minutes and counted the number of bacteria that aggregated near the cell. Next, he repeated the experiment but
doubled the intensity of the red light, i.e., he doubled the number of red photons hitting the cell.
10) Predict how the number of bacteria aggregating near the cell would change from the first experiment to the second.
11) If light intensity were increased beyond what Engelmann simulated in his controlled experiment, would this factor
continue to have the same effect that you predicted? Explain your answer.