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.
Practice Questions
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.
