In the process of photosynthesis, atmospheric carbon is absorbed by plants.
The carbon is then transferred form plants to the animals feeding on them, and then further moves up the food chain.
Respiration, digestion and metabolism, plants and animals result in some transfer of carbon back to the atmosphere.
Some carbon also moves to the lithosphere when living organisms die or when wood and leaves decay or When animals excrete. Some of these living beings were buried millions of years ago have been converted to fossil fuels.
Burning and mining of fossil fuels cause this carbon to move from the lithosphere to the atmosphere.
Some of this atmospheric carbon gets dissolved in the oceans and then completes the cycle.