Here's why:

* Carbon dioxide is the source of carbon: Plants use carbon dioxide to build the organic molecules they need to grow, such as sugars, proteins, and fats.

* Photosynthesis is a carbon fixation process: Plants capture carbon dioxide from the atmosphere and convert it into glucose (sugar) using energy from sunlight.

While carbon dioxide is the main compound, plants also need water (H2O) and sunlight for photosynthesis to occur. Water provides the electrons and hydrogen ions needed for the chemical reactions, and sunlight provides the energy to drive the process.