Question

What reaction begins the Calvin cycle?

Answer 1

Before the Calvin cycle begins the energy is stored in bonds of ATP

Answer 2

The Calvin cycle is the set of chemical reactions that take place in the chloroplasts during photosynthesis. 
This cycle begins when a 5-carbon compound called ribulose 1-5-bisphosphate "catches" one CO2 molecule and makes a 6-compound molecule.

That's the start...immediately after that the enzyme Rubisco breaks up the 6-compound molecule into 2 equal parts (2 molecules of 3-phosphoglycerate)