# Molecules in all organisms are composed in part of radioactive carbon-14. What best explains the presence of carbon-14 in organisms?

And green plants metabolize, or if you like $\text{fix}$, atmospheric carbon dioxide to give simple sugars. And since, SOME of the fixed carbon dioxide is ""^14CO_2, this gets incorporated into the food chain, and eventually turns up in the tissue of species that eat the plants, and in the tissues of species that eat the species that eat the plants, etc. Because matter is cycled in this way, larger concentrations of ""^14C build up in the tissues of consumers and predators. This also occurs with the cycling of common pesticides, such as DDT.