Life: The Science of Biology, Ninth Edition
Sadava • Hillis • Heller • Berenbaum
Working with Data
Tracing the Pathway of CO2
(Textbook Figure 10.13)
Introduction
To elucidate the sequence of reactions that allow carbon
fixation, Melvin Calvin and colleagues exposed
suspensions of the green alga Chlorella to 14CO2 for 30
seconds. They then killed the cells and denatured the
enzymes by submerging them in a beaker of boiling
alcohol. The 14C-labeled compounds, which represented
carbon-containing molecules produced in the Calvin
cycle, were then separated from one another using paper
chromatography. Upon exposure to X-ray film, each
carbon-containing compound on the chromatograph
produced a dark spot. Results showed that after 30
seconds of 14CO2 exposure, many carbon-containing
compounds were produced. To determine the first
compound produced in the Calvin cycle, the researchers
limited the 14CO2 exposure to only 3 seconds. Results
from this second test identified a single compound, 3phosphoglycerate, as the initial product of carbon
fixation. Calvin and colleagues then expanded on this
result and were able to determine the exact sequence of
reactions and reaction intermediates in the Calvin cycle
by exposing the cells to 14CO2 for various periods of
time. Studies have suggested that one of the main causes
of global warming is an increase in atmospheric carbon dioxide levels. Given the role of
carbon dioxide in photosynthesis, scientists have predicted that an increase in the level of
carbon dioxide will have both positive and negative effects on crop yields. Excess carbon
dioxide is expected to have a positive physiological effect because of an increased rate of
photosynthesis, although other studies have suggested that this increase in yield may be at
the expense of crop quality. Interestingly, however, an increase in oxygen as a result of
higher rates of photosynthesis may in fact mitigate the warming from elevated carbon
dioxide levels. Certainly, climate changes due to factors other than carbon dioxide levels
will affect agricultural production in the future. However, knowledge of the Calvin cycle
and its role in photosynthesis will enable us to anticipate and adapt farming practices to
account for changes in global climate.
© 2011 Sinauer Associates, Inc.
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Original Papers
Calvin and his colleagues described their experiments in a series of 26 papers entitled
“The Path of Carbon in Photosynthesis.” Perhaps the most important was one that
showed how paper chromatography and labeled CO2 could be used as tracers:
Benson, A. A., J. A. Bassham, M. Calvin, T. C. Goodale, V. A. Haas, and W. Stepka.
1950. The path of carbon in photosynthesis. V. Paper chromatography and
radioautography of the products. Journal of the American Chemical Society 72: 1710–
1718.
http://pubs.acs.org/doi/pdf/10.1021/ja01160a080
An engaging summary of the group’s experiments is given in Calvin’s Nobel Prize
lecture:
http://nobelprize.org/nobel_prizes/chemistry/laureates/1961/calvin-lecture.pdf
Links
(For additional links on this topic, refer to the Chapter 10 Experiment Links.)
Plant Physiology Online: How the Calvin Cycle Was Elucidated
http://3e.plantphys.net/article.php?ch=t&id=77
Berkley National Laboratory Library: Nobel Laureates: Melvin Calvin
http://www.lbl.gov/LBL-PID/Nobelists/LibM_Calvin.html
Smith College: Clark Science Center: Animation of Calvin Cycle
http://www.science.smith.edu/departments/Biology/Bio231/calvin.html
Rensselaer Polytechnic Institute Calvin Cycle—Photosynthetic Carbon Reactions
http://www.rpi.edu/dept/bcbp/molbiochem/MBWeb/mb2/part1/dark.htm
The Marshall Institute: Increasing Carbon Dioxide and Global Climate Change
http://www.marshall.org/article.php?id=13
United States Department of Agriculture: Global Climate Change
http://www.ars.usda.gov/Research/docs.htm?docid=6347
© 2011 Sinauer Associates, Inc.
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Analyze the Data
The first reaction in CO2 fixation occurs in the dark. To show this, Calvin exposed
Chlorella cells to 14CO2 in bright light for 20 minutes. Then they made chromatograms,
identified the labeled compounds, and used a radioactivity detector to quantitate the label
on each compound. Finally, they turned off the light (dark conditions) and analyzed the
radioactivity periodically for 5 minutes. The data (amount of label) are shown in Table 1.
Table 1
PGA
RuBP
Sucrose
After 20
minutes light
5,500
4,900
13,000
30 seconds
dark
10,100
680
13,500
2 minutes dark
10,000
1,850
15,000
5 minutes dark
5,200
1,800
14,750
Question 1
Using the data in Table 1, plot radioactivity in PGA versus time. What does the data
show? Why did the RuBP go down after 5 minutes in the dark?
In another experiment, Calvin and colleagues exposed Chlorella to continuous 14CO2 in
bright light for 3 minutes. They measured radioactivity in PGA, hexose monophosphate,
and sucrose, with the following results (see Table 2).
Table 2
Time (sec)
15
30
45
60
90
120
180
PGA
100
180
220
250
300
300
300
Hexose
monophosphate
100
260
380
500
600
690
750
Sucrose
1.8
2.5
5.5
8.0
20
40
60
Question 2
Using the data in Table 2, plot radioactivity in PGA versus time. What does the data
show? Plot a graph to show what the data show would have looked like if the experiment
had continued for an additional 5 minutes in the dark.
© 2011 Sinauer Associates, Inc.
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