Calvin Cycle

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•  Calvin’s experiment to elucidate the carboxylation of RuBP

The light independent reactions are also collectively known as the Calvin cycle – named after American chemist Melvin Calvin

  • Calvin mapped the complete conversion of carbon within a plant during the process of photosynthesis

Calvin’s elucidation of photosynthetic carbon compounds is commonly classed the ‘lollipop experiment’

  • This is due to the fact that the apparatus he utilised was thought to resemble an upside-down lollipop

Lollipop Experiment

  • Radioactive carbon-14 is added to a ‘lollipop’ apparatus containing green algae (Chlorella)
  • Light is shone on the apparatus to induce photosynthesis (which will incorporate the carbon-14 into organic compounds)
  • After different periods of time, the algae is killed by running it into a solution of heated alcohol (stops cell metabolism)
  • Dead algal samples are analysed using 2D chromatography, which separates out the different carbon compounds
  • Any radioactive carbon compounds on the chromatogram were then identified using autoradiography (X-ray film exposure)
  • By comparing different periods of light exposure, the order by which carbon compounds are generated was determined
  • Calvin used this information to propose a sequence of events known as the Calvin cycle (light independent reactions)  

Summary of the Lollipop Experiment

Key Events of the Calvin Cycle

The Calvin cycle outlines the events that result in the formation of organic molecules from inorganic sources (CO2)

  • Ribulose bisphosphate (RuBP) is carboxylated by carbon dioxide (CO2) to form a hexose biphosphate compound
  • The hexose biphosphate compound immediately breaks down into molecules of glycerate-3-phosphate (GP)
  • The GP is converted by ATP and NADPH into molecules of triose phosphate (TP)
  • TP can be used to form organic molecules or can be recombined by ATP to reform stocks of RuBP

Calvin Cycle Compounds

calvin table