ABSTRACT
Scientific observers of the natural world have been intrigued by the processes of life, as evidenced by early written records from Aristotle, for example. Early thinkers wondered about the nature of life and its “indivisible” units in much the same way that they mused about the fundamental units of matter. The advent of fluorescent protein fusions makes it possible to assess the relative quantities of a given protein of interest on the basis of the fluorescence of its fusion partner. One of the most intriguing implications of our census of the molecular parts of a bacterium is the extent to which the cellular interior is crowded. The spatial scales associated with biological structures run from the nanometer scale of individual molecules all the way to the scale of the Earth itself. All living organisms are based on cells as the indivisible unit of biological organization.
