Of the approximately 115 elements known, only the 19 highlighted in purple in Figure 1.8.11.8.1\PageIndex{1} are absolutely required in the human diet. These elements—called essential elements—are restricted to the first four rows of the periodic table, with only two or three exceptions (molybdenum, iodine, and possibly tin in the fifth row). Some other elements are essential for specific organisms. For example, boron is required for the growth of certain plants, bromine is widely distributed in marine organisms, and tungsten is necessary for some microorganisms.