Biology (Holt)

Real Life

The most powerful compound light microscopes have a total
magnification of up to 2,000, which is sufficient for viewing
objects as small as 0.5 μm in diameter. For you to see smaller
objects, the wavelength of the light beam must be shorter than the
wavelength of visible light. Electron beams have a much shorter
wavelength than that of visible light, so electron microscopes are
much more powerful than light microscopes.

Electron Microscopes
Electron microscopes can magnify an image up to 200,000, and
they can be used to study very small structures inside cells or on cell
surfaces. In electron microscopes, both the electron beam and the
specimen must be placed in a vacuum chamber so that the electrons
in the beam will not bounce off gas molecules in the air. Because liv-
ing cells cannot survive in a vacuum, they cannot be viewed using
electron microscopes.

Transmission electron microscopeIn a transmission electron micro-
scope, shown in Figure 4,the electron beam is directed at a very thin
slice of a specimen stained with metal ions. Some structures in the
specimen become more heavily stained than others. The heavily
stained parts of the specimen absorb electrons, while those that are
lightly stained allow electrons to pass through. The electrons that
pass through the specimen strike a fluorescent screen, forming an
image on the screen. A transmission electron micrograph (TEM),
such as the one of sperm cells shown in Figure 4, can reveal a cell’s
internal structure in fine detail. TEM images are always in black
and white. However, with the help of computers, scientists often add
artificial colors to make certain structures more visible.

SECTION 1Looking at Cells 53

In a transmission electron microscope, electrons pass through a specimen,
forming an image of the specimen on a fluorescent screen.

Figure 4 Transmission electron microscope

TEM of sperm

Magnification: 7,730

Thirty movies could be

stored on a disk the size

of a penny.

Using the scanning inter-

ferometric apertureless

microscope (SIAM)

researchers have viewed

features that are about

four atoms (1 nm) in diam-

eter. The technology could

also be used to code

information on storage

disks.

Applying Information

Would the SIAM likely

be more useful in under-

standing the overall

structure of the cell

or the structure

of biological

compounds

in the cell? 2A