The unit of life is a cell. A human body contains approximately ~ 1014 cells, each of which is composed of chemical species grouped roughly into water, inorganic ions, sugar, amino acids, nucleotide, fatty acids, and other small molecules, macromolecules including protein and nucleic acid. The composition of the chemical species in each cell is different from organ to organ in a living body, for examples, lung, eye, skin, and so on. Chemical composition in a bacterial cell is listed in Table 1.1 as an example (Alberts et al. in Molecular biology of the cell. Garland Publishing, Inc., New York, London, 1989). Each chemical species plays its own role to maintain our life. DNA carries genetic information to be inherited from generation to generation. Protein plays many different roles to maintain the life: catalyzing chemical reactions including ATP hydrolysis, signal transduction among cells, and so on.