The reactant in photosynthesis that provides the carbon atoms to form glucose is carbon dioxide (CO2).

Here's why:

* Photosynthesis Equation: The basic equation for photosynthesis is:

6CO2 + 6H2O + light energy → C6H12O6 + 6O2

* Carbon Dioxide's Role: Carbon dioxide is the source of the carbon atoms that are incorporated into glucose (C6H12O6). During the process, plants take in carbon dioxide from the atmosphere through tiny pores called stomata on their leaves.

* Light-Dependent Reactions: The light-dependent reactions of photosynthesis use light energy to split water molecules (H2O), producing oxygen (O2) and high-energy electrons.

* Calvin Cycle: These high-energy electrons are then used in the Calvin cycle (light-independent reactions). This cycle takes carbon dioxide molecules and uses the energy from the light-dependent reactions to convert them into glucose.