The light-independent reactions are the second main stage of photosynthesis, occurring after light energy is captured. This metabolic process uses the energy products from the first stage to build sugar molecules, which serve as the plant’s food source. The most common alternative name for this complex series of chemical reactions is the Calvin Cycle. This cycle converts inorganic carbon dioxide from the atmosphere into organic compounds, forming the foundation of most food webs.
The Preferred Terminology
The name “Calvin Cycle” honors American biochemist Melvin Calvin. He, along with colleagues Andrew Benson and James Bassham, elucidated this complex pathway in the 1940s and 1950s. Using the radioactive isotope carbon-14, the team mapped the route carbon atoms travel as they are converted from carbon dioxide into carbohydrates. Calvin earned the Nobel Prize in Chemistry in 1961 for his work tracing the path of carbon in photosynthesis.
The reactions are sometimes more formally referred to as the Calvin-Benson Cycle, or the Calvin-Benson-Bassham (CBB) Cycle, acknowledging the entire research team. Another term used is the C3 pathway. This classification is based on the chemistry of the initial carbon fixation step, where the first stable organic compound created is a three-carbon molecule.
Function and Location
The function of the Calvin Cycle is carbon fixation, converting inorganic carbon dioxide into an organic form. This step makes carbon available for constructing biomolecules necessary for life, such as glucose and cellulose. The entire cycle occurs within the stroma, the dense, enzyme-rich fluid inside the chloroplasts.
The cycle uses carbon dioxide from the air, which diffuses into the stroma. The initial reaction is catalyzed by the abundant enzyme RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase). RuBisCO attaches carbon dioxide to a five-carbon sugar, yielding a three-carbon sugar called Glyceraldehyde-3-phosphate (G3P). G3P is the precursor molecule plants use to synthesize larger sugars and other organic compounds.
Energy Dependency
The term “light-independent reactions” can be misleading. Although the process does not directly require light energy, it cannot proceed indefinitely in the dark. The cycle is entirely dependent on the energy-carrying molecules produced during the light-dependent reactions: Adenosine Triphosphate (ATP) and Nicotinamide Adenine Dinucleotide Phosphate (NADPH).
ATP supplies the chemical energy, and NADPH provides the reducing power required to convert the fixed carbon into sugar. These energy carriers are not stable and must be continuously regenerated by the light-dependent reactions. If the light source is removed, the supply of ATP and NADPH quickly depletes, causing the Calvin Cycle to slow down and stop. The enzymes involved in the Calvin Cycle are also chemically regulated to be active only when light is present.