Question

What makes the calvin cycle a cycle>

Answer 1

I think you can get your answer if you look at the Calvin cycle diagram
After the G3P is driven out, the remaining G3P goes to RuBP, and rubisco is added again and the cycle continues

Answer 2

the end of phase 3 (Ribulose 5-phosphate) is the reactant (minus an ATP) that starts phase 1 (ADP + Ribulose1, 5-bisphosphate) which goes to the end of phase 1.

Answer 3

good point cmkc109, lol

http://upload.wikimedia.org/wikipedia/commons/8/8e/Calvin-cycle4.svg