Photosynthesis allows plants and other organisms to convert light energy into chemical energy, creating their own food. This conversion occurs in two main phases within the chloroplasts. The first phase, the light-dependent reactions, harnesses sunlight, and the second phase, which includes the Calvin Cycle, uses those products to build sugar molecules from carbon dioxide. This process fixes atmospheric carbon into organic compounds, forming a foundation for nearly all life on Earth.
The Definitive Answer: Light-Independent Reactions
The Calvin Cycle is classified as a light-independent reaction because its chemical steps do not directly require photons of light to proceed. These reactions occur in the stroma, the fluid-filled space within the chloroplast. The term “dark reaction” is now mostly avoided because it inaccurately implies the process only happens at night.
The cycle is completely dependent on the products generated during the light-dependent reactions, which cease shortly after the sun sets. Enzymes responsible for key steps are also indirectly activated by light, ensuring the process only runs when energy is available. Therefore, the Calvin Cycle is indirectly dependent on light for its continued operation, as it quickly halts without the continuous supply of energy-carrying molecules.
Powering the Cycle: Products of the Light-Dependent Reactions
The light-dependent reactions generate the fuel required to run the Calvin Cycle. This stage occurs on the thylakoid membranes, where pigments like chlorophyll capture sunlight. The captured energy powers the conversion of water into oxygen and creates two chemical products: adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH).
ATP stores chemical energy, and its conversion to adenosine diphosphate (ADP) releases energy to fuel the cycle. NADPH acts as a high-energy electron carrier, providing the reducing power necessary for building sugars. Since both molecules have a short lifespan, they must be used immediately by the Calvin Cycle, reinforcing its indirect dependence on continuous light.
Building Sugar: The Three Stages of Carbon Fixation
The construction of sugar from atmospheric carbon dioxide occurs in the stroma through the cyclic reactions of the Calvin Cycle. The process involves three distinct stages: carbon fixation, reduction, and regeneration. The cycle recycles the spent energy carriers, ADP and NADP+, back to the light-dependent reactions to be recharged.
Carbon Fixation
The cycle begins with Carbon Fixation, incorporating a molecule of carbon dioxide into an organic molecule. The enzyme Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) catalyzes the reaction between the five-carbon sugar ribulose bisphosphate (RuBP) and CO2. This combination creates an unstable six-carbon compound that quickly splits into two molecules of 3-phosphoglycerate (3-PGA).
Reduction
The second stage, Reduction, heavily utilizes the energy and reducing power from the light reactions. ATP provides the energy to convert the 3-PGA molecules into an intermediate form. Subsequently, NADPH donates electrons and a hydrogen ion, reducing the intermediate to glyceraldehyde-3-phosphate (G3P), a three-carbon sugar precursor. For every three molecules of CO2 that enter the cycle, one G3P molecule is produced, which exits to synthesize glucose and other carbohydrates.
Regeneration
The final stage is Regeneration, which keeps the cycle running. The remaining G3P molecules are rearranged to re-form the initial five-carbon acceptor molecule, RuBP. This rearrangement requires an additional input of ATP from the light reactions. By regenerating RuBP, the cycle ensures a constant supply of the molecule needed to fix more atmospheric carbon dioxide.