The substances like C, N, S, P, H and O enter the living body through biogeochemical cycles. The atmospheric carbon $$(CO_2)$$ is used by plants during photosynthesis to synthesize carbohydrate. When animals feed upon the plant products, carbon enters into animal's body. Phosphorus and sulphur are absorbed by plants from soil. Animals obtain it through food and water. Oxygen enters into plants and animals' body through the process of respiration. Hydrogen enters into plant body through absorbed water and into animals body through food and water.  Plants absorb nitrogen from soil as nitrates and nitrites and convert them into proteins. When these plants are eaten by and animals, nitrogen enters into animals body.