1. Carbon is used up by plants, in the process of photosynthesis, to make sugars.
2. Some plants are eaten by other organisms (herbivores) which in turn are eaten by other hunters (carnivores). No matter who eats who, plants, herbivores and carnivores will all die one day. Their bodies are decomposed by decomposers, and some are converted into fossil fuels, while some other are carried into the bottom of the oceans, where they form rocks. The fossils and the rocks come to the surface of the earth, where their Carbon is released either by combustion of fuels or by weathering of rocks. This carbon returns to the atmosphere.
3. All plants and animals, release carbon Dioxide into the atmosphere as a byproduct of cellular respiration. All the carbon returns to the atmosphere, after a while.