"s used s to reveal the sequence of chemical reactions in . s followed the assimilation and turnover of key s and tagged molecules such as to track the movement and activity of s. s labeled s with radioisotopes to follow the replication and expression of s. Physicians utilized radioisotopes such as and to diagnose and detect s. Ecologists profited as well, using to trace through the living and nonliving parts of aquatic and terrestrial landscapes, giving concrete meaning to the notion of an ecosystem."