Here's why:

* Photosynthesis Equation: The basic equation for photosynthesis is:

* CO2 + H2O + Light Energy → C6H12O6 + O2

* This means carbon dioxide and water, in the presence of light energy, are used to produce glucose (a type of sugar) and oxygen.

* CO2 as a Reactant: Carbon dioxide is one of the main reactants in photosynthesis. Plants absorb it from the atmosphere through tiny pores called stomata on their leaves.

* CO2 Conversion: During photosynthesis, plants use the carbon from CO2 to build sugars, which are essential for their growth and energy.

* Overall Effect: As plants photosynthesize, they remove CO2 from the atmosphere and convert it into organic compounds. This process helps to balance the amount of CO2 in the environment.