What is the major constituent of biomass of an organism?

Biomass is the weight or total quantity of living organisms including plants, animals and microorganisms in a given area or volume. Biochemically all these are cellulose, lignin, sugars, fats and proteins.
Chemically it is a mixture of methane, hydrogen, carbon dioxide and hydrogen sulphide. Methane is the major constituent (75%). So, the major constituent is carbon in biomass.