So far 107 elements have been discovered in the universe, out of which oxygen, silicon, aluminium, potassium, calcium, iron, sodium and magnesium are most abundant, the rest occurring in small amounts only. It is logical to think that since living matter is very much a part of the universe, it is composed of the same elements of which the earth’s crust is made. The situation is however different. Living matter is highly selective in its chemical composition and out of 20 elements essential to life, hydrogen, oxygen, nitrogen and carbon are the most abundant .These are comparatively lighter elements. Though the list of elements found in living matter is a long one, some elements, such as H, C, N, Na, Mg, P, S, Ca, K, and Cl are universally found in all organisms.
Analysis of living matter has shown that the organic compounds are largely made up of C,H,N,O,P and S.
Carbohydrates and lipids contain C, H and O, while proteins and nucleic acids contain in addition, N and S and N and P respectively. What is the logic observed by the living world in using these six elements on organic compounds ? Beside being lighter elements, they form a variety of compounds so characteristics of life. Na, K, Mg, Ca and Cl are usually found in  ionic forms whose concentration must remain within narrow limits. The presence of boron, silicon vanadium, manganese, iron, cobalt, copper,  zinc and molybdenum though in ultratrace amounts, is essential for the functioning of enzymes. These elements are used as cofactors in the catalytic function of enzymes.