Andrew Taylor

Calvin’s “Lollipop” Experiment

Melvin Calvin was an American scientist most famed for discovering the Calvin Cycle, the series of intermediate reactions that take place in photosynthesis.

Calvin began his work in 1945 (at the University of California), [1] when much of exactly what happened in the process of photosynthesis was still unknown to science. Joseph Priestley discovered the basic principle of photosynthesis in 1772, which is that plants use energy from the sun to convert carbon dioxide and water into carbohydrates and oxygen. By the 1940s, scientists knew about the conditions required for photosynthesis and the products it produced, but the intermediate reactions we are aware of today were mostly unknown. [2]

In order to discover the processes in the light-independent reaction, Calvin and his team of biologists, chemists & physicists devised an experiment to track the path of carbon through a photosynthesising organism, from carbon dioxide to carbohydrate, by the use of radioactive carbon (14C). The single-celled algae Chlorella was chosen as the subject as it was a “very convenient green plant that had already been the subject of much photosynthetic research” and were easily reproducible. [3] It was hoped that from the reactions taking place in this algae that an accurate model of photosynthesis in all plants could be devised.

The experiments were performed in a thin, transparent vessel known as a “lollipop” (because of its shape) which

Fig 1; The apparatus used by Calvin. The “lollipop” is positioned above a flask of hot methanol to allow the samples to be easily drained off. [4]

Andrew Taylor

contained a suspension of Chlorella, which was allowed to undergo photosynthesis in the presence of normal CO2. Carbon-14 in the form of hydrogen carbonate [4] was introduced to the suspension and the whole vessel was exposed to light. The CO2 produced by the hydrogen carbonate was taken up by the Chlorella, and samples of the solution taken every 5 seconds or so for several minutes. [5] The samples were dropped into hot methanol to kill the Chlorella, to stop enzymatic reactions and to begin the extraction of chemicals for analysis. [3]

Calvin used two-way chromatography to analyse the compounds found in each of the samples. He put a drop of each sample on a piece of chromatography paper and dipped the pieces into a solvent to allow the chemicals present to separate. The radioactive compounds which Calvin was interested in were identified by placing sheets of photographic film over the chromatography paper, which were marked black by exposure to the radioactive areas. The compound that appeared first in the samples was the first one in the pathway; the second to appear was the second to be made, and so on. It took Calvin and his team around 10 years to properly identify and label the compounds which each black are represented. [3]

By this method, the intermediate reactions of the light-independent reaction, now known as the Calvin Cycle, were brought to light.

Fig 2; A chromatogram of extract from algae indication uptake of carbon-14 during 30 seconds of photosynthesis. [3]

Fig 3; The Calvin Cycle, showing the conversion of CO2 into GP, GALP, glucose & RuBP, using ATP and reduced NADP.[5]

Andrew Taylor

References

[1] Edexcel A2 Biology, Ann Fullick, published 2009.

[2] "Melvin Calvin" 20 July 2009. HowStuffWorks.com. <http://science.howstuffworks.com/dictionary/famous-scientists/chemists/melvin-calvin-info.htm> 19 September 2013.

[3] “The path of carbon in photosynthesis”, Melvin Calvin, December 1961. (Nobel Prize lecture, digitally available here -> http://www.nobelprize.org/nobel_prizes/chemistry/laureates/1961/calvin-lecture.pdf)

[4] “Secrets of Science: The lollipop experiement” Tilly Tees, June 27 2013, http://ellemedit1234.wordpress.com/2013/06/27/secrets-of-science-the-lollipop-experiment/

[5] “Advanced Biology” by Michael Kent, 2000, Oxford University Press