The correct answer is light energy. Light energy is required to initiate the reaction between carbon dioxide (CO2) and ribulose-1,5-bisphosphate (RuBP) during the Calvin Cycle. This process, known as carbon fixation, is catalyzed by an enzyme called ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco). In the presence of light energy, rubisco helps combine CO2 and RuBP to form an unstable intermediate molecule. This molecule then undergoes a series of enzymatic reactions to produce 3-phosphoglycerate (3-PGA) and eventually glyceraldehyde 3-phosphate (G3P), which can be used to synthesize carbohydrates. Therefore, while rubisco is involved in the reaction, it is light energy that is responsible for speeding up the reaction and restarting the Calvin Cycle.
