The carbon cycle plays an important role in photosynthesis and cellular respiration.

Plants uptake CO2 from the atmosphere by diffusion via small pores in the leaves, known as stomata. It is converted to glucose during photosynthesis. A particular carbon atom may be absorbed and returned back to the atmosphere by the same plant through cellular respiration. This is the process plants and animals use to convert glucose into energy.

Alternatively, the plant may be consumed by a primary consumer which will respire the atom, or it could continue within the food chain. Eventually, it may be respired by the decomposers, such as bacteria and fungi, which break down organic matter.