Carbon is always in the atmosphere, and although you don't see it, it's always there.
The Carbon has been inside of us many times
It starts off with plants absorbing CO2, which is used to make sugars(this process is called photosynthesis). The plats helps removes CO2 from the atmosphere. Most living organisms eat plants to get energy and carbon.
An unfortunate journey for the Carbon journey underground
The cows eat the grass(plants) for carbon(this process is called consumptiom). At the same time, plants and animals release CO2 which is waste from metabolism(this process is called cellular respiration).
The Carbon atom sometimes are working in fact
Most humans do consume plants on a daily basis. Because of this, the carbon atoms end up inside of us. We also live and breathe every day which releases more CO2 into the air because of cellular respiration to break down carbon-rich foods.
When plants and animals die off, decomposers break down the carbon that was stored in them and releases them back into the ground(this process is called decomposition). Some decomposers are bacteria and fungi. Through intense heat and compression, the carbon from the once-living organisms is turned into a fuel source(aka fossilization).
Factories and humans have an effect on the carbon cycle. When things are burned such as wood or fossil fuels, it releases CO2 into the atmosphere(this process is called combustion). Sometimes this can have a negative effect on the carbon cycle and increase the levels of carbon in the atmosphere.