Final answer:
RuBP (ribulose bisphosphate) is the molecule that accepts CO₂ in the first step of the light-independent reactions, also known as the Calvin cycle, in C-3 plants.
Step-by-step explanation:
The molecule that accepts CO₂ in the first step of the light-independent reactions of photosynthesis in C-3 plants is RuBP (ribulose bisphosphate). In what is often referred to as the Calvin cycle, an enzyme named RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) facilitates the reaction where CO₂ combines with RuBP. This step, known as carbon fixation, marks the start of the cycle leading to the synthesis of glucose. RuBP, by accepting the CO₂, plays a crucial role in the conversion of inorganic carbon into organic forms that the plant can use in the process of making carbohydrates.