The four classes of organic compounds are carbohydrates, proteins, fats and nucleic acid.
Protein and nucleic acid contain nitrogen. Plants get their carbon from the air and their nitrogen from the soil through carbon and nitrogen cycles. Animals get their nitrogen and carbon from the plants and other animals which they consume.
In all organisms, the organic compounds are converted into various things such as tissue fats, energy, hereditary information, etc. As the organisms died and decompose they are returned to the soil by the decomposers.
