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The Calvin Cycle (Carbon Fixation)

1950

Melvin Calvin
Andrew Benson
James Bassham

The Calvin cycle, or light-independent reactions, uses the ATP and NADPH produced during the light-dependent stage to convert inorganic carbon dioxide into organic sugar molecules. This process occurs in the stroma of the chloroplast. The key enzyme, RuBisCO, catalyzes the first step: the fixation of \(CO_2\) into an organic molecule, initiating a cycle that produces carbohydrates.

The Calvin cycle proceeds in three main stages. The first is carbon fixation, where the enzyme RuBisCO attaches a molecule of CO2 to a five-carbon sugar, ribulose-1,5-bisphosphate (RuBP). This creates an unstable six-carbon intermediate that immediately splits into two molecules of a three-carbon compound, 3-phosphoglycerate (3-PGA).

The second stage is reduction. Using energy from ATP and reducing power from NADPH (both supplied by the light reactions), the 3-PGA molecules are converted into a three-carbon sugar, glyceraldehyde-3-phosphate (G3P). This is an energy-intensive step that effectively stores the light energy in chemical bonds.

The third stage is regeneration. For every six molecules of G3P produced, one exits the cycle to be used by the plant to synthesize glucose and other organic molecules. The other five G3P molecules, along with more ATP, are used to regenerate the three molecules of RuBP that began the cycle. This ensures the process is sustainable, ready to fix more CO2.

UNESCO Nomenclature: 2402

– Biochemistry

Type

Biochemical Process

Disruption

Foundational

Usage

Widespread Use

Precursors

discovery of radioactive carbon-14 by martin kamen and sam ruben, which was used as a tracer
development of paper chromatography by archer martin and richard synge
understanding of ATP and NADPH from the light-dependent reactions
f.f. blackman’s law of limiting factors, suggesting a two-stage process