The Calvin Cycle and Carbon Fixation

The Calvin cycle (also known as the light-independent reactions) is the second stage of photosynthesis, occurring in the stroma of the chloroplast. It uses ATP and reduced NADP from the light-dependent reactions to fix atmospheric CO₂ into organic molecules. This lesson covers every step in detail for the Edexcel A-Level Biology (9BI0) specification.

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Overview of the Calvin Cycle

The Calvin cycle was elucidated by Melvin Calvin and colleagues using radioactive carbon-14 (¹⁴C) as a tracer and the green alga Chlorella. By exposing the algae to ¹⁴CO₂ for varying lengths of time and using paper chromatography combined with autoradiography, they identified the sequence of intermediates in the cycle.

The cycle can be divided into three main stages:

1. Carbon fixation — CO₂ is incorporated into an organic molecule
2. Reduction — The fixed carbon is reduced using ATP and reduced NADP
3. Regeneration of RuBP — The CO₂ acceptor molecule is regenerated using ATP

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Stage 1: Carbon Fixation