Figure 1.

The range of applications of key technologies that are used for imaging objects, from molecules to humans. The figure shows a schematic comparison on a single size scale of the relative sizes of various objects that are of interest in biology. At one end of the spectrum are small entities such as lipids, proteins and whole viruses, which are generally described in terms of their molecular weight. For larger entities, such as whole cells and organelles, it is conventional to describe their size in terms of linear dimensions such as length and width, which are typically expressed in micron units. For very large entities, such as whole organisms, sizes are frequently measured by the weight of a single item and are expressed in units of mass. In the unified scale shown here, the sizes of all objects are described in units of ‘molecular weight’. Using this scale, one can make a more standardized quantitative estimate of the relative sizes across the biological spectrum, showing, for example, that the ‘molecular weight’ of an adult human is about 25 orders of magnitude greater than the molecular weight of an amino acid. The main purpose of this figure is to show that high-resolution electron microscopy bridges a very important technique gap in the size spectrum for subcellular imaging at resolutions between those allowed by X-ray crystallography and by light microscopy.