CO2 is the source of the carbon in the glucose as glucose is made through the process of photosynthesis. The 6 carbon molecules come from 6 molecules of CO2. Without CO2 plants are not able to make sugar and without sugar niether plants nor animals could ever burn that suga in the process of cellular respiration to make energy.
Photosynthesis can be represented using a chemical equation.
The overall balanced equation is...6 CO+ 6 H2O ------> C6 H12 O+ 6 O2

It is an important part of the chemical process,providing the carbon needed to build carbohydrates .The overall chemical equation of the photosynthesis is 6CO2 + 6H2O ⇒ C6H12O6 + 6O2.The carbohydrates are used as food by the plant and to form the cellulose that make up the stem and leaves.