Here's why:
* ATP is made during the light-dependent reactions. This is where light energy is captured by chlorophyll and used to create ATP and NADPH.
* The Calvin cycle uses ATP and NADPH. The light-independent reactions use the ATP and NADPH generated in the light-dependent reactions to convert carbon dioxide into glucose.
Think of it like this: The light-dependent reactions are like a power plant, generating energy (ATP) and reducing power (NADPH). The Calvin cycle is like a factory, using that energy and reducing power to build things (glucose).