Summary
Highlights
The dark reactions of photosynthesis are light-independent metabolic processes that are temperature-dependent due to the involvement of enzymes. These reactions occur in the stroma of chloroplasts and require ATP and NADPH from the light reactions. Oxygen, produced during photolysis, is not used in the dark reactions.
Carbon dioxide molecules are essential for the dark reactions, as they are assembled into energy-rich carbohydrates. All dark reactions are divided into three sections that together form a cycle called the Calvin cycle. The coefficients used ensure that one glucose molecule is formed per cycle.
In the fixation phase, free CO2 molecules are bound to acceptor molecules, specifically ribulose 1,5-bisphosphate (a C5 body), via enzymes. This results in the formation of C6 bodies.
The unstable C6 bodies immediately break down into two C3 bodies called 3-phosphoglycerate (PGA). The most crucial step of the reduction phase is the reduction of these PGA molecules using ATP and NADPH from the light reactions to form 3-phosphoglyceraldehyde (PGAL). Two PGAL molecules then combine to form glucose, which the plant uses for building material or energy.
The regeneration phase restores the acceptor molecule, ribulose 1,5-bisphosphate. The remaining PGAL molecules, which were not converted into glucose, are transformed back into ribulose 1,5-bisphosphate with further ATP consumption, allowing the Calvin cycle to continue.
The gross equation for the dark reactions, formulated to produce one glucose molecule, requires six CO2 molecules.