The atmosphere and the ocean surface are the largest reservoirs of easily obtained Carbon dioxide. These serve as the sources of Carbon dioxide for plants.

Volcanism and other geologic processes give off some Carbon dioxide adding to the atmospheric supply.

Plants take in Carbon dioxide from the air and ocean and using the energy from sunlight combine the Carbon dioxide and water to produce Sugars and give off Oxygen gas in a process called Photosynthesis. CO2 + H2O  Sugar + O2

The plants store some of the sugar in the form of starch The plants store some of the sugar in the form of starch. They also use some of the sugar to produce cellulose, which is a structural material for plants.

Animals eat the plants and use the sugar and starch as an energy source by reacting the sugar with Oxygen producing Carbon dioxide and water in a process called Respiration. Sugar + O2  CO2 + H2O

Plants also use some of the sugar for energy via respiration returning some of the Carbon dioxide to the environment.

Occasionally plants burn Occasionally plants burn. This will release all of the Carbon stored in Sugars, Starches and Cellulose back into the environment as Carbon dioxide.

When aquatic plants and animals die their remains settle to the bottom of the ocean forming a sediment.

Ocean dwelling animals, like clams, use some of the Carbon dioxide released in respiration to build shells made up of Calcium Carbonate (CaCO3). The remains of these animals will over time form limestone and marble deposits.

The remains of dead plants and animals may decay over thousands of years to form fossil fuels.

We use the fossil fuels for heat and energy production and for transportation returning the Carbon dioxide, that was locked away for thousands of years, to the environment.