Like the citric acid cycle, the Calvin Cycle doesn’t really have a starting or ending point, but can we think of the first reaction as the fixation of carbon dioxide to Ru1,5BP. This reaction is catalyzed by the enzyme known as ribulose-1,5bisphosphate carboxylase (RUBISCO). The resulting six carbon intermediate is unstable and each Ru1,5BP is rapidly converted to 3-phosphoglycerate. As noted, if one starts with 6 molecules of Ru1,5BP and makes 12 molecules of 3-PG, the extra 6 carbons that are a part of the cycle can be shunted off as two three-carbon molecules of glyceraldehyde-3-phosphate (GA3P) to gluconeogenesis, leaving behind 10 molecules of GA3P to be reconverted into 6 molecules of Ru1,5BP. That part of the pathway requires multiple steps, but only utilizes two enzymes unique to plants - sedoheptulose-1,7bisphosphatase and phosphoribulokinase. RUBISCO is the third enzyme of the pathway that is unique to plants. All of the other enzymes of the pathway are common to plants and animals and include some found in the pentose phosphate pathway and gluconeogenesis.