Biology Reference
In-Depth Information
picoseconds (10
12
sec). Both size and time ranges are so vast that multiple instrumentations
must be employed. Size domains are demonstrated by use of the electromagnetic spectrum
shown in
Figure 9.1
. The electromagnetic spectrum is the range of all possible electromagnetic
radiation frequencies. It has no distinct beginning, nor does it have a distinct end. It covers
wavelengths from the size of the universe down to a fraction of the size of an atom. The spec-
trum is continuous and infinite. However, for convenience, the spectrum is classified into
several, overlapping ranges. At the long wavelength end are radiowaves, followed by infrared,
the visible region, ultraviolet, X-rays, and at the shortest wavelengths, gamma rays. The shorter
the wavelength, the higher is its frequency and energy. While short wavelength radiation has
better resolving power, it is also a more destructive power. Long wavelength radiowaves can
be used for some crude imaging such as ultrasound to safely image a fetus still in the womb,
but would be of little use in imaging a membrane. Electromagnetic radiation between 380
nm and 760 nm can be detected by the human eye and so is referred to as visible light. Shorter
wavelength blue light has better resolving power than does longer wavelength red light. Note,
however, that even the wavelength of blue light is much longer (380 nm) than the thickness of
a biological membrane. Even including proteins, membrane thickness is generally
10 nm.
Therefore a biological membrane cannot be directly imaged with a light microscope, but
must employ much shorter wavelength radiation, primarily destructive X-rays. It is unfortu-
nate that non-harmful, long wavelength radiation has poor resolving power, while good reso-
lution can only be achieved with biologically harmful short wavelength radiation.
Also complicating studies of biological membranes are the very fast times associated with
many biological events (micro seconds to picoseconds).
Figure 9.2
shows the time ranges
<
FAST
10
14
10
10
10
6
10
2
10
-2
10
-6
SLOW
FIGURE 9.2
Time domains for several biophysical techniques. Frequencies are per second.
