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, essenti