The range in size of the members of the microbial world is tremendous. As a general rule, the obligate intracellular parasites are the smallest and the eukaryotes the largest.

■ Why do eukaryotic cells need to be larger than prokaryotic cells?

■ What factor limits the size of free-living cells?

The basic unit of length is the meter (m), and all other units are fractions of a meter.

nanometer (nm) = 10-9 meter = .000000001 meter micrometer (^m) = 10-6 meter = .000001 meter millimeter (mm) = 10-3 meter = .001 meter 1 meter = 39.4 inches

These units of measurement correspond to units in an older but still widely used convention.

1 angstrom (Â) = 10-10 meter 1 micron = 10-6 meter

Figure 1.13 Sizes of Organisms and Viruses


Limit of visibility

Relative sizes of organisms and viruses

Light microscope t

Electron microscope

Adult roundworm Helminth

_Eukaryotic cell


Bacillus megaterium

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