The Calvin cycle begins when a molecule of CO₂ reacts with a highly reactive phosphorylated five-carbon sugar named:

Explanation

The Calvin cycle begins when carbon dioxide reacts with ribulose-1,5-bisphosphate, a five-carbon compound that serves as the starting point of the cycle. This reaction is driven by the enzyme Rubisco, which attaches carbon dioxide to the molecule, forming two molecules of 3-phosphoglycerate. This step is known as carbon fixation and marks the entry of inorganic carbon into an organic form that plants can use to build sugars. Without ribulose-1,5-bisphosphate, the Calvin cycle could not continue, as it plays a key role in capturing carbon dioxide and maintaining the cycle's flow.