Which reactant in photosynthesis provides the carbon atoms to form glucose?

In photosynthesis, the reactant that provides the carbon atoms to form glucose is carbon dioxide (CO₂). Plants absorb carbon dioxide from the atmosphere through small openings in their leaves called stomata. During the process of photosynthesis, carbon dioxide is fixed and incorporated into organic molecules, ultimately leading to the synthesis of glucose. This process occurs in the Calvin cycle, which takes place in the chloroplasts of plant cells.