13.1.2—ATP and reduced NADP in Calvin cycle

Photosynthesis links two compartmentalised stages: light-dependent reactions in the thylakoid system transfer light energy into ATP and reduced NADP, and the Calvin cycle in the stroma uses these products to build complex organic molecules from carbon dioxide.

• Light-dependent stage — thylakoids: light energy drives reactions that produce ATP by photophosphorylation and reduced NADP using hydrogen ions and electrons from water. These products carry usable energy and reducing power to the stroma.
• Transfer between stages: ATP and reduced NADP are supplied from the light-dependent stage to the light-independent stage; they are the connection between light capture and carbon fixation.
• Light-independent stage — stroma: the Calvin cycle uses the energy from ATP and hydrogen from reduced NADP to make complex organic molecules, including carbohydrates such as starch, sucrose and cellulose.
• Dependency boundary: “light-independent” means the Calvin-cycle reactions do not directly require light, not that they are independent of the light-dependent stage. Without a continuing supply of ATP and reduced NADP, they cannot continue indefinitely in darkness.

Do not treat the two stages as unrelated or say that the Calvin cycle makes carbohydrate without ATP and reduced NADP. Detailed pigment spectra, chromatography, individual photosystems, full photophosphorylation mechanisms and carbon-accounting steps belong to neighbouring cards.