From the simplest bacteria to humans, all living things are composed of cells of one type or another, all of which have fundamentally the same chemistry. This chemistry must provide mechanisms that allow cells to interact with the external world, a means to power the cell, machinery to carry out varied processes within the cell, a structure within which everything runs, and also governance through a web of interlocking chemical reactions. Biochemistry is the study of those reactions, the molecules that are created, manipulated, and destroyed as a result of them, and the massive macromolecules (such as DNA, cytoskeletons, proteins and carbohydrates) that form the chemical machinery and structures on which these biochemical reactions take place.