Biological roles of the elements

A large fraction of the chemical elements that occur naturally on the earth's surface are essential to the structure and metabolism of living things. Four of these elements (hydrogen, carbon, nitrogen, and oxygen) are essential to every living thing and collectively make up 99% of the mass of protoplasm. Phosphorus and sulfur are also common essential elements, essential to the structure of nucleic acids and amino acids, respectively. Chlorine, potassium, magnesium, calcium and phosphorus have important roles due to their ready ionization and utility in regulating membrane activity and osmotic potential. The remaining elements found in living things are primarily metals that play a role in determining protein structure. Examples include iron, essential to hemoglobin; and magnesium, essential to chlorophyll. Some elements are essential only to certain taxonomic groups of organisms, particularly the prokaryotes. For instance, the lanthanide series rare earths are essential for methanogens. As shown in the following table, there is strong evidence that 19 of the elements are essential to all living things, and another 17 are essential to some taxonomic groups. Of these 17, most have not been extensively studied, and their biological importance may be greater than currently supposed.
The remaining elements are not known to be essential. There appear to be several causes of this.
Aluminum warrants special mention because it is the most abundant metal and the third most abundant element in the Earth's crust; despite this, it is not essential for life. With this sole exception, the eight most highly abundant elements in the earth's crust, making up over 90% of the crustal mass, are also essential for life.
The following list identifies in rank order the possible biological roles of the chemical elements, ranging from a score of 5 for elements essential to all living things, to a score of 1 for elements that have no known effects on living things. There are also letter scores for special functions of the elements. These rank scores are used to characterize each element in the following table.
The following table identifies the 94 chemical elements that occur naturally on the earth's surface, their atomic numbers, their biological rank as defined above, and their general beneficial and harmful roles in living things.