Here's why:

* Cellular Respiration: This is the process where organisms break down glucose (a sugar) to release energy. This process consumes oxygen and produces carbon dioxide as a waste product.

* CO2 Release: The CO2 produced by cellular respiration is then released into the environment. This can happen through:

* Animals: They exhale CO2.

* Plants: They release CO2 during respiration at night.

* Decomposers: They break down dead organisms and release CO2.

The CO2 released into the atmosphere then becomes available for plants to use during photosynthesis, completing the carbon cycle.