Biology Reference
In-Depth Information
Can we see a membrane?
Since a membrane bilayer is less than 50
˚
thick, it would have to be about 20,000 times
bigger than it is, just to be seen as a speck by someone with excellent vision. Is it then possible
that someone can see a much larger cell?
Can we see a cell?
1,000,000
˚
Resolving power of the eye
100
m
m
70,000
˚
Erythrocyte
7
m
m
20,000
˚
Bacteria
2
m
m
20,000
˚
Mitochondria
2
m
m
200,000
˚
Liver cell
20
m
m
Even the relatively large liver cell would have to be ~5X bigger to be seen as a speck by the
human eye.
What can you see with a light microscope?
The resolving power of a good light microscope is wavelength-dependent. Shorter wave-
length (blue) light has a better resolving power than does longer wavelength (red) light.
Although the entire visible light spectrum is quite narrow, from 380
750 nm, it is possible
to push resolution of the light microscope down to about 300 nm or 3,000
˚
. Since a bio-
logical membrane bilayer is ~50
˚
thick, the membrane would have to be 60X bigger than
it is to be
directly
detectable, even by a light microscope.
Resolution of a membrane was not possible until discovery of the electron microscope
(EM) in 1931 by Ernst Ruska (
Figure 1.5
) who later won the 1986 Nobel Prize in Physics
for this achievement. The EM, however, did not become commonly employed until the
1950s. Most EM use on biological samples has a resolution of ~10
˚
and so can readily distin-
guish a biological membrane. Even higher resolution is possible in certain cases.
e
FIGURE 1.5
Ernst Ruska (1901
e
1988).
Courtesy of Siemens AG,
http://www.siemens.com/history/en/personalities/
