The substances like C, N, S, P, H and O enter the living body through
biogeochemical cycles. The atmospheric carbon (CO2) 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.