The Calvin cycle represents the second main stage of photosynthesis, often referred to as the light-independent reactions. This intricate series of biochemical conversions functions to transform atmospheric carbon dioxide into simple sugars, which serve as foundational energy sources for plant growth and survival. The cycle takes place within the stroma, the fluid-filled space found inside chloroplasts, the specialized organelles within plant cells responsible for photosynthesis. This process allows plants to build organic molecules from inorganic carbon, thereby forming the basis of many food webs on Earth.
Inputs and Environment of the Cycle
The Calvin cycle requires specific components to initiate its reactions, drawing directly from the preceding light-dependent reactions of photosynthesis. Carbon dioxide (CO2) from the surrounding atmosphere enters the plant’s leaves through small pores called stomata, then diffuses into the chloroplast stroma. Two energy-carrying molecules, adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH), are also necessary inputs.
These ATP and NADPH molecules are generated during the light-dependent reactions, which capture light energy and convert it into chemical energy. ATP provides the necessary energy to drive the various reactions within the cycle. NADPH contributes high-energy electrons, serving as a reducing agent for molecule synthesis.
Stage One: Carbon Fixation
The Calvin cycle commences with the process of carbon fixation, where an inorganic carbon atom from carbon dioxide is incorporated into an existing organic molecule. A molecule of atmospheric CO2 combines with ribulose-1,5-bisphosphate (RuBP), a five-carbon sugar molecule already present in the stroma. This reaction is catalyzed by the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase, commonly known as RuBisCO.
RuBisCO is widely considered the most abundant enzyme on Earth, reflecting its profound importance in global carbon cycling. The immediate product of this combination is an unstable six-carbon compound. This transient molecule quickly splits into two identical three-carbon molecules, called 3-phosphoglycerate (3-PGA).
Stage Two: Reduction
Following carbon fixation, the 3-PGA molecules undergo a series of transformations in the reduction stage, where chemical energy from the light reactions is invested. Each 3-PGA molecule receives a phosphate group from ATP, converting it into 1,3-bisphosphoglycerate. This phosphorylation step utilizes the energy stored in ATP, preparing the molecule for the next energy input.
The 1,3-bisphosphoglycerate molecules are then reduced by NADPH, which donates high-energy electrons and a hydrogen ion. This conversion transforms the molecules into glyceraldehyde-3-phosphate (G3P), a three-carbon sugar.
Stage Three: Regeneration of RuBP
For the Calvin cycle to continue uninterrupted, the initial five-carbon sugar, RuBP, must be regenerated. For every three molecules of carbon dioxide fixed, six molecules of G3P are produced in the reduction stage. Of these six G3P molecules, only one exits the cycle to be used by the plant for synthesizing carbohydrates.
The remaining five G3P molecules are rearranged through a complex series of enzymatic reactions to reform three molecules of RuBP. This regeneration process requires additional energy input, specifically utilizing ATP molecules.
Net Outputs and Their Role in the Plant
The primary output of the Calvin cycle, following three turns of the cycle, is one molecule of glyceraldehyde-3-phosphate (G3P). This three-carbon sugar molecule leaves the chloroplast and serves as a versatile building block for the plant. G3P can be used to synthesize a variety of more complex carbohydrates, including glucose, which is a simple sugar, and larger molecules like sucrose and starch.
Sucrose is transported throughout the plant to provide energy to various tissues, while starch serves as a long-term energy storage compound, particularly in roots and seeds. In addition to G3P, the cycle also produces adenosine diphosphate (ADP) and nicotinamide adenine dinucleotide phosphate (NADP+). These “spent” energy and electron carriers are then recycled back to the light-dependent reactions, where they are re-energized and converted back into ATP and NADPH, completing the cycle of energy exchange within photosynthesis.