The Calvin Cycle takes place in the stroma of the chloroplast during photosynthesis. There are 3 phases that occur, the first one being carbon fixation. In this phase, carbon dioxide becomes a part of ribulose bisphosphate(RuBP), a 5 carbon sugar. The enzyme, rubisco, catalyzes this phase produces a 6 carbon intermediate which is then split into two molecules of 3-phosphoglycerate. In the next phase, reduction, ATP and NADPH2 convert 3-phosphoglycerate into glyceraldehyde 3-phosphate. This is the precursor to glucose and other sugars. Finally in phase three, called regeneration, ATP is used again to convert a portion of glyceraldehyde 3-phosphate back into RuBP. RuBP is the acceptor of carbon dioxide and thus the cycle is complete. An important fact to remember is that for every glyceraldehyde 3-phosphate synthesized, there are 9 molecules of ATP and 6 molecules of NADPH2 spent.