explain why the Calvin cycle is now called the light independent stage of photosynthesis
Benson (and Edward McMillan) demonstrated that CO2 fixation was independent of light by illuminating on a sample of algae in the absence of CO2, immediately transferring the algae to a black flask containing 14CO2 and analysing the products for radioactivity. Sucrose (formed from glyceraldehyde phosphate) contained radioactive 14C was isolated at a rate approaching it’s formation in light. This experiment proved that the energy absorbed by chlorophyll was used for the production of phosphorylating (ATP) and reducing agents (NADPH) capable of driving the conversion of CO2to sugar in the dark.
Although the fixation of carbon does not require light, it does not take place in the dark! The enzymes of the Calvin cycle respond indirectly to light activation. When light energy is available to generate ATP and NADPH, the Calvin cycle proceeds. In the dark, when ATP and NADPH cannot be produced by the light dependent reaction, fixation of CO2 ceases. Therefore, the Calvin cycle is now called the light independent stage of photosynthesis.
Melvin Calvin, Andrew Benson and James Bassham determined the series of biochemical reactions in the Calvin cycle in the late 1940s and 1950s. It was completed in 1958 and earned Melvin Calvin the Nobel Prize for Chemistry in 1961. Calvin recognised the fact that the CO2 was added to a 5-carbon acceptor to form a 6-carbon intermediate that instantaneously split in half. Benson and Bassham collaborated to identify the substances.