Why is the series of reactions in the Calvin cycle called a
"cycle”?

You can think of the Calvin cycle as being somewhat like a sugar factory within a chloroplast. It is called a cycle because, like the Krebs cycle in cellular respiration, the starting material is regenerated each time the process occurs. In this case, the starting material that gets regenerated is a compound called RuBP, a sugar with five carbons.

With each turn of the Calvin cycle, there are chemical inputs and outputs. The inputs are carbon dioxide from the air and the ATP and NADPH produced by the light reactions. The Calvin cycle uses carbon from the carbon dioxide, energy from the ATP, and high-energy electrons and hydrogen ions from the NADPH. The cycle's output is an energy-rich sugar molecule. That sugar is not yet glucose, but a smaller sugar named G3P. The plant cell uses G3P as the raw material to make glucose and other organic molecules it needs.

Explanation:

You can follow the process of the Calvin cycle in the figure.

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