Calvin cycle

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The Calvin cycle.

The Calvin cycle (also known as the Calvin–Benson cycle) is the set of chemical reactions that take place in chloroplasts during photosynthesis.

The cycle is light-independent because it takes place after the energy has been captured from sunlight.

The Calvin cycle is named after Melvin C. Calvin, who won a Nobel Prize in Chemistry for finding it in 1961.[1] Calvin and his colleagues did the work at the University of California, Berkeley.[2]

Context[change | change source]

Using the radioactive carbon-14 isotope as a tracer, Calvin, Andrew Benson and their team mapped the complete route that carbon travels through a plant during photosynthesis. They traced the carbon-14 from its absorption as atmospheric carbon dioxide to its conversion into carbohydrates and other organic compounds.[3][4] The single-celled algae Chlorella was used to trace the carbon-20.[5]

The Calvin group showed that sunlight acts on the chlorophyll in a plant to fuel the manufacture of organic compounds, not directly on carbon dioxide as previously believed.

Steps[change | change source]

The steps in the cycle are as follows: [6]

1. Grab: A five-carbon carbon catcher catches one molecule of carbon dioxide and forms a six-carbon molecule.

2. Split: the enzyme RuBisCO (with the energy of ATP and NADPH molecules) breaks the six-carbon molecule into two equal parts.

3. Leave: A trio of three carbons leave and become sugar. The other trio moves on to the next step.

4. Switch: Using ATP and NADPH, the three carbon molecule is changed into a five carbon molecule.

5. The cycle starts over again.

The product[change | change source]

The carbohydrate products of the Calvin cycle are three-carbon sugar phosphate molecules, or 'triose phosphates' (G3P). Each step of the cycle has its own enzyme which speeds up the reaction.

References[change | change source]

  1. "The Nobel Prize in Chemistry 1961 Melvin Calvin". nobelprize.org. Retrieved January 14, 2011. 
  2. Bassham J, Benson A, Calvin M (1950). "The path of carbon in photosynthesis". J Biol Chem 185 (2): 781–7. PMID 14774424. http://www.jbc.org/cgi/reprint/185/2/781.pdf.
  3. Calvin M. 1956, "The photosynthetic cycle.", Bull. Soc. Chim. Biol., 38 (11), pp. 1233–44, PMID 13383309 
  4. Barker S.A. et al 1956, "Intermediates in the photosynthetic cycle.", Biochim. Biophys. Acta, 21 (2), pp. 376–7, doi:10.1016/0006-3002(56)90022-1, PMID 13363921 
  5. Calvin, Melvin 1961. "The path of carbon in photosynthesis" (PDF). p. 4. Retrieved July 11, 2011. 
  6. Sadava, David et al 2009. Life: the science of biology. Macmillan, p199–202. ISBN 1429219629.