The Calvin Cycle

• The cell uses energy from the sun, converted and packaged into ATP and NADPH, as fuel to build food in carbohydrate molecules.
• Generated carbohydrate molecules contain a backbone of 5 carbon atoms.
• Carbon atoms used to build carbohydrate molecules originate from carbon dioxide.
• Reactions of photosynthesis which use energy stored by light-dependent reactions, make glucose and carbohydrate molecules within the Calvin Cycle occur.
• In plants, carbon dioxide enters the chloroplasts through stomata.
• Carbon dioxide diffuses into the stroma from the chloroplast.
• Sugar is synthesized in the stroma, the site of the Calvin cycle reactions.
• Calvin cycle reactions are organized into three stages: fixation, reduction, and regeneration.
• Within the stroma, in addition to CO2, two chemicals are present, initiating the Calvin cycle.
• RuBisCo, ribulose-bisphosphate carboxylase, is an enzyme.
• Ribulose bisphosphate (RuBP) is a molecule with five carbon atoms and a phosphate group on each end.
• RuBisCO catalyzes a reaction between CO2 and RuBP, forming a six-carbon compound.
• The six-carbon compound is immediately converted into two three-carbon compounds.
• The process of converting CO2 into organic molecules is called carbon fixation.
• ATP and NADPH use stored energy to convert 3-PGA, a three-carbon molecule, into G3P, another three-carbon compound.
• The process is called a reduction reaction because it involves the gain of electrons.
• Reducing an atom or molecule means they gain an electron.
• ADP and NAD+ molecules return to light-dependent reactions to be re-energized.
• One G3P molecules exit the Calvin cycle, contributing to the formation of carbohydrates, such as glucose (C6H12O6).
• Six turns of the Calvin cycle are required to make one carbohydrate molecule, due to the six carbon atoms within it.
• The remaining G3P molecules regenerate RuBP, allowing the system to prepare for the carbon-fixation step, using ATP.

Key Points

• Light-independent reactions in photosynthesis, named the Calvin cycle, occur in three key steps.
• Although the Calvin Cycle does not directly rely on light, it is indirectly dependent, as light-dependent reactions produce necessary energy carriers (ATP and NADPH).
• Light-independent reactions start with fixation, the first stage of the Calvin cycle.
• CO2 is fixed from an inorganic to an organic molecule.
• During reduction, ATP and NADPH reduce 3-PGA into G3P.
• ATP and NADPH convert to ADP and NADP+, respectively during reduction.
• RuBP is regenerated during regeneration.
• The system prepares for more CO2 to be fixed via regeneration.