AP Syllabus focus:
‘ATP and NADPH from light reactions power Calvin cycle reactions in the stroma, producing carbohydrates from carbon dioxide.’
The Calvin cycle is the main pathway that converts inorganic CO₂ into energy-rich organic molecules. It occurs in the chloroplast stroma and uses ATP and NADPH to build carbohydrates.
Location and purpose
Occurs in the stroma of chloroplasts (the fluid-filled interior surrounding thylakoids).
Uses chemical energy from ATP and reducing power from NADPH (made by light-dependent reactions) to convert carbon dioxide into carbohydrate.
Produces a 3-carbon sugar phosphate (G3P) that can be used to form larger carbohydrates (for example, glucose and starch) and other organic molecules.
Core idea: carbon reduction cycle
The Calvin cycle is often described in three functional phases: carbon fixation, reduction, and regeneration.
This diagram summarizes the Calvin cycle as a three-phase loop: carbon fixation (via rubisco), reduction (powered by ATP and NADPH), and regeneration of RuBP. It highlights that G3P is the net carbon product that can be diverted to build glucose and other organic compounds while the rest of the intermediates are recycled to keep the cycle running. Source
The cycle must regenerate its starting CO₂ acceptor so it can continue running.
Calvin cycle: A cyclic set of enzyme-catalysed reactions in the chloroplast stroma that fixes and, using ATP and NADPH, produces the carbohydrate precursor glyceraldehyde-3-phosphate (G3P).
Phase 1: Carbon fixation
Carbon fixation begins when CO₂ is attached to a 5-carbon sugar, RuBP (ribulose bisphosphate).
The enzyme rubisco catalyses this step.
The initial 6-carbon intermediate is unstable and immediately splits into two molecules of 3-PGA (3-phosphoglycerate), a 3-carbon compound.
For every 3 CO₂ fixed:
3 RuBP are consumed
6 molecules of 3-PGA are produced
Why fixation matters
Fixation converts carbon from an inorganic gas into an organic molecule that can be rearranged and reduced into sugars. This is the entry point of carbon into most food webs.
Phase 2: Reduction (making a sugar precursor)
ATP provides energy to phosphorylate intermediates.
NADPH provides high-energy electrons (reducing power) to reduce the carbon compound.
The 3-PGA molecules are converted into G3P (glyceraldehyde-3-phosphate).
For every 3 CO₂ fixed:
6 ATP are used (energy input)
6 NADPH are used (electrons/hydrogen input)
6 G3P are produced, but only 1 G3P is considered “net gain” because the rest are needed to regenerate RuBP.
Key roles of ATP and NADPH
ATP: drives endergonic steps by phosphorylating intermediates and supporting rearrangements.
NADPH: reduces carbon compounds, storing energy in C–H and C–C bonds that can later be harvested by the cell.
Phase 3: Regeneration of RuBP
The cycle must regenerate RuBP to keep accepting new CO₂.
The remaining 5 G3P (out of 6 produced per 3 CO₂) are rearranged through multiple enzyme-catalysed steps.
ATP is consumed to restore the high-energy phosphate configuration of RuBP.
For every 3 CO₂ fixed:
3 ATP are used to regenerate 3 RuBP
Net inputs and outputs (per G3P)
It is useful to track what must be supplied by light reactions and what is returned.
This figure emphasizes the accounting of inputs and outputs across the Calvin cycle, showing ATP and NADPH being consumed during reduction and ATP being used again during regeneration of RuBP. It also reinforces the idea that fixed carbon exits the cycle as G3P, which can be assembled into larger carbohydrates (such as glucose) after sufficient turns of the cycle. Source
= carbon source for carbohydrate skeletons
= energy input (molecules)
= reducing power input (molecules)
Because G3P is a 3-carbon product, cells typically need two G3P (from fixing 6 CO₂) to assemble one 6-carbon sugar (such as glucose) via additional biosynthetic steps.
Carbohydrate production in cells
G3P is a branching-point metabolite:
can be converted into glucose and sucrose for transport
can be polymerised into starch for storage in chloroplasts
can supply carbon skeletons for amino acids and lipids (after conversion)
FAQ
Rubisco is relatively slow and abundant enzyme molecules are needed.
Its activity can constrain the rate of CO₂ fixation when other resources are available.
Partitioning depends on metabolic demand and stromal conditions.
Regeneration is prioritised to maintain cycle continuity; export increases when carbon supply exceeds immediate regeneration needs.
$P_i$ availability can affect ATP synthesis and the exchange of triose phosphates.
Low $P_i$ can reduce the capacity to sustain high flux through carbohydrate-forming steps.
Radioactive tracer studies (e.g., $^{14}CO_2$) can label early products.
The earliest heavily labelled stable products are 3-carbon intermediates, consistent with 3-PGA formation after fixation.
Rubisco can also catalyse reactions that do not yield sugars under certain conditions.
This can divert carbon away from carbohydrate formation, especially when internal CO₂ levels are low.
Practice Questions
State two ways ATP and NADPH are used in the Calvin cycle. (2 marks)
ATP provides energy/phosphate to convert 3-PGA to a higher-energy intermediate / to drive regeneration of RuBP. (1)
NADPH provides reducing power (electrons/hydrogen) to reduce carbon compounds to form G3P. (1)
Describe how carbon dioxide is converted into a carbohydrate precursor in the Calvin cycle, including the roles of RuBP and rubisco. (5 marks)
Calvin cycle occurs in the stroma and uses ATP and NADPH to fix CO₂. (1)
CO₂ combines with RuBP (5C) to form an unstable 6C compound. (1)
Rubisco catalyses the fixation step. (1)
The 6C compound splits into two molecules of 3-PGA (3C). (1)
3-PGA is reduced (using ATP and NADPH) to form G3P, with some G3P leaving as net product. (1)
