The science of nourishment, including the study of the nutrients that each organism must obtain from its environment to maintain life and health and to reproduce. Although each kind of organism has its distinctive needs, which can be studied separately, a far-reaching biochemical unity in nature has been discovered which gives vastly more coherence to the whole subject. Many nutrients, such as amino acids, minerals, and vitamins, needed by higher organisms may also be needed by the simplest forms of life—single-celled bacteria and protozoa. The recognition of this fact has made possible highly important developments in biochemistry.