Originating point of carbon that reenters the carbon cycle; cellular respiration and combustion.

A nutrient which provides the carbon required for cellular biosynthesis.

A nutrient (such as sugar) that provides carbon "skeletons" needed in an organism's synthesis of organic molecules.

The opposite of a carbon sink. Natural systems and technologies both can be sources of CO2. For instance, a natural carbon pool that is being depleted can become a net source, such as a forest that is excessively harvested or lost altogether to development. Industrial facilities burning fossil fuels are major technological sources.