Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

560 Chapter 9: Visualizing Cells
electron
gun
condenser
lens
beam deflector
scan
generator
objective
lens
video
screen
detector
electrons from
specimen
specimen
created. Negative staining is especially useful for viewing large macromolecular
aggregates such as viruses or ribosomes, and for seeing the subunit structure of
protein filaments (Figure 9–53).
Shadowing and negative staining can provide high-contrast surface views of
small macromolecular assemblies, but the size of the smallest metal particles in
the shadow or stain used limits the resolution of both techniques. An alternative
that allows us to visualize directly at high resolution even the interior features
of three-dimensional structures such as viruses and organelles is cryoelectron
microscopy, in which rapid freezing to form vitreous ice is again the key. A very
thin (about 100 nm) film of an aqueous suspension of virus or purified macromolecular
complex is prepared on a microscope grid and is then rapidly frozen by
MBoC6 m9.49/9.50
Figure 9–50 The scanning electron
microscope. In an SEM, the specimen is
scanned by a beam of electrons brought
to a focus on the specimen by the
electromagnetic coils that act as lenses.
The detector measures the quantity of
electrons scattered or emitted as the beam
bombards each successive point on the
surface of the specimen and controls the
intensity of successive points in an image
built up on a screen. The SEM creates
striking images of three-dimensional
objects with great depth of focus and
a resolution between 3 nm and 20 nm
depending on the instrument. (Photograph
courtesy of Andrew Davies.)
(B)
(A)
1 µm
(C)
5 µm
Figure 9–51 Scanning electron
microscopy. (A) A scanning electron
micrograph of the stereocilia projecting
from a hair cell in the inner ear of a bullfrog.
For comparison, the same structure is
shown by (B) differential-interferencecontrast
light microscopy (Movie 9.3)
and (C) thin-section transmission electron
microscopy. (Courtesy of Richard Jacobs
and James Hudspeth.)