Here's why:

* Calvin Cycle: The Calvin Cycle is the light-independent stage of photosynthesis where carbon dioxide is converted into sugar. It uses energy stored in ATP and NADPH produced during the light-dependent reactions.

* Oxygen (O2): Oxygen is a byproduct of the light-dependent reactions of photosynthesis. It is released into the atmosphere and is not used in the Calvin Cycle.

Key molecules involved in the Calvin Cycle:

* Carbon dioxide (CO2): The main reactant, it is incorporated into organic molecules.

* Ribulose bisphosphate (RuBP): A five-carbon sugar that combines with CO2.

* 3-phosphoglycerate (3-PGA): A three-carbon molecule formed from the combination of RuBP and CO2.

* Glyceraldehyde 3-phosphate (G3P): A three-carbon sugar that is the primary product of the Calvin Cycle.

* ATP and NADPH: Energy carriers produced during the light-dependent reactions.

Let me know if you have any more questions!