Here's why:

* Photosynthesis is about building organic molecules: Plants and other photosynthetic organisms use light energy to convert inorganic molecules (CO2 and water) into organic molecules like glucose (a sugar).

* Carbon dioxide provides the carbon backbone: CO2 is the source of carbon atoms that are incorporated into the sugar molecules produced during photosynthesis.

* The process: CO2 is absorbed from the atmosphere through tiny pores on leaves called stomata. It then enters the chloroplasts, the site of photosynthesis, where it is used to create glucose.

Let me know if you'd like more details about the process of photosynthesis!