an image on an electron-sensitive 
photographic plate. 
If pushed to the limit, electron 
microscopes can resolve objects as 
small as the diameter of an atom. 
Most electron microscopes used to 
view biological material can "see" 
down to about 10 angstroms — an 
incredible feat, for although this does 
not make atoms visible, it does allow 
researchers to distinguish individual 
molecules of biological importance. 
In effect, it can magnify objects up 
to 1 million times. Nevertheless, all 
electron microscopes suffer from a 
serious drawback. Since no speci- 
men can survive under their high 
vacuum, they cannot show the ever- 
changing movements that character- 
ize a living cell. 
The first electron microscopes were 
used to study crystals and were 
impractical for the study of cell struc- 
ture. Cell researchers had to learn 
how to cut extremely thin slices of 
cells, sometimes down to a thickness 
of only a few hundred angstroms, so 
that electrons could pass through 
them. Also, to ensure contrast be- 
tween different parts of the otherwise 
transparent cell, new staining tech- 
niques had to be devised, involving 
"heavy" metals that are absorbed to 
differing extents by various cell parts. 
The cell sections also had to be 
"fixed" in new ways, to preserve 
them, and embedded in new kinds of 
materials (mostly transparent plastic). 
Altogether, it was not until the early 
1950's that electron microscopes 
began to be used routinely. While 
microscopists were looking through 
their instruments at smaller and smaller 
particles in cells and attempting to 
understand their structure, another 
group of scientists was pursuing an 
entirely different, but equally impor- 
tant, line of research— biochemistry. 
Unit 
Equal to 
Used to Measure 
Centimeter 
1/100 meter 
Objects visible to the eye 
Millimeter 
1/10 centimeter 
Very large cells 
Micrometer 
(or micron) 
1 / 1 000 millimeter 
Most cells, large organelles 
Nanometer 
1 / 1 000 micrometer 
Sma organelles, large molecules 
Angstrom 
1/10 nanometer 
Molecules, atoms 
Units of size commonly used in cell biology. 
