Essential Cell Biology 5th edition

The Prokaryotic Cell
11
0.2 mm
(200 µm)
visible with
unaided eye
x10
CELLS
20 µm
x10
20 mm 2 mm 0.2 mm
ORGANELLES
2 µm
x10
200 nm
light
microscope
x10
MOLECULES
20 nm
x10
2 nm
super-resolution
fluorescence
microscope
20 µm 2 µm 0.2 µm
x10
ATOMS
0.2 nm
electron
microscope
1 m = 10 3 mm
= 10 6 µm
= 10 9 nm
20 nm 2 nm 0.2 nm
(A)
(B)
electrons rather than a beam of light through the sample. Another type
of electron microscope—the scanning electron microscope—scatters ECB5 e1.08-1.09 electrons
off the surface of the sample and so is used to look at the surface
detail of cells and other structures. These techniques, along with the different
forms of light microscopy, are reviewed in Panel 1–1 (pp. 12–13).
Even the most powerful electron microscopes, however, cannot visualize
the individual atoms that make up biological molecules (Figure
1–9). To study the cell’s key components in atomic detail, biologists
have developed even more sophisticated tools. Techniques such as x-ray
crystallography or cryoelectron microscopy, for example, can be used to
determine the precise positioning of atoms within the three-dimensional
structure of protein molecules and complexes (discussed in Chapter 4).
THE PROKARYOTIC CELL
Of all the types of cells that have been examined microscopically, bacteria
have the simplest structure and come closest to showing us life stripped
down to its essentials. Indeed, a bacterium contains no organelles other
than ribosomes—not even a nucleus to hold its DNA. This property—the
presence or absence of a nucleus—is used as the basis for a simple but fundamental
classification of all living things. Organisms whose cells have a
nucleus are called eukaryotes (from the Greek words eu, meaning “well”
or “truly,” and karyon, a “kernel” or “nucleus”). Organisms whose cells do
not have a nucleus are called prokaryotes (from pro, meaning “before”).
Figure 1–9 How big are cells and their
components? (A) This chart lists sizes
of cells and their component parts, the
units in which they are measured, and the
instruments needed to visualize them.
(B) Drawings convey a sense of scale
between living cells and atoms. Each panel
shows an image that is magnified by a
factor of 10 compared to its predecessor—
producing an imaginary progression
from a thumb, to skin, to skin cells, to
a mitochondrion, to a ribosome, and
ultimately to a cluster of atoms forming
part of one of the many protein molecules
in our bodies. Note that ribosomes are
present inside mitochondria (as shown
here), as well as in the cytoplasm. Details of
molecular structure, as shown in the last two
bottom panels, are beyond the power of the
electron microscope.