A note on the use of electron microscopy in biology.

1. SYED MUHAMMAD KHAN (BS HONS. ZOOLOGY)
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Electron Microscopy
An electron microscope uses a beam of electrons to illuminate a specimen and
produce a magnified image.
An electron microscope (EM) has greater resolving power than a light-powered optical
microscope because electrons have wavelengths about 100,000 times shorter than
visible light photons. They can achieve better than 50-pm resolution and
magnifications of up to about 10,000,000X whereas ordinary, non-confocal light
microscopes are limited by diffraction to about 200 nm resolution and useful
magnifications below 2000X. The electron microscope uses electrostatic and
electromagnetic lenses to control the electron beam and focus it to form an image.
Electron microscopes are used to observe a wide range of biological and inorganic
specimens including microorganisms, cells, large molecules, biopsy samples, metals,
etc. It is often used for quality control and failure analysis. Modern electron
microscopes produce electron micrographs, using specialized digital cameras or
frame grabbers to capture the image.
TRANSMISSION ELECTRON MICROSCOPY (TEM)
It is a microscopy technique whereby a beam of electrons is transmitted through an
ultra-thin specimen, interacting with the specimen as it passes through. An image is
formed from the interaction of the electrons transmitted through the specimen; the
image is magnified and focused onto an imaging device, such as a fluorescent screen
or a layer of photographic film, or it may be detected by a sensor such as a CCD
camera. TEMs are capable of imaging at a significantly higher resolution than light
microscopes, owing to the smaller wavelength of electrons. This enables the
instrument's user to examine fine detail even as small as a single column of atoms,
which is tens of thousands of times smaller than the smallest resolvable object in a
light microscope.

2. SYED MUHAMMAD KHAN (BS HONS. ZOOLOGY)
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History: The first TEM was built by Max Knoll and Ernst Ruska in 1931. This group
developed the first TEM with resolving power greater than that of light in 1933 and the
first commercial TEM in 1939.
Applications: TEM forms a major analysis method in a range of scientific fields, in both
physical and biological sciences. TEMs find application in cancer research, virology,
materials science as well as pollution, nanotechnology, and semiconductor research.
Electron Beam: These microscopes use a beam of highly energetic electrons instead
of light to examine objects on a very fine scale. This allows the microscope to surpass
the resolution limits of optical microscopes and can magnify specimens up to
250,000X or more. Users can examine the topography of a specimen, its morphology,
composition, etc.
Components: A TEM is composed of several components, which include:
 Vacuum system in which the electrons travel
 Electron emission source for the generation of the electron stream
 Series of electromagnetic lenses
 Electrostatic plates
 Imaging devices are used to create an image from the electrons that exit the
system
Electromagnetic lenses and electrostatic plates allow the operator to guide and
manipulate the beam as required. Also required is a device to allow the insertion into,
motion within, and removal of specimens from the beam path.
SCANNING ELECTRON MICROSCOPE (SEM)
It is a type of electron microscope that produces images of a sample by scanning it
with a focused beam of electrons. The electrons in the beam interact with electrons in
the sample, producing various signals that can be detected and that contain
information about the sample's surface topography and composition. SEM can

3. SYED MUHAMMAD KHAN (BS HONS. ZOOLOGY)
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achieve resolution better than 1 nanometer. Specimens can be observed in high
vacuum, low vacuum. In Environmental SEM, specimens can be observed in wet
conditions.
Types of Signals: The types of signals produced by an SEM include secondary
electrons (SE), back-scattered electrons (BSE), characteristic X-rays, light (cathode
luminescence) (CL), specimen current, and transmitted electrons.
Detectors: Secondary electron detectors are standard equipment in all SEMs, but a
single machine would rarely have detectors for all possible signals.
Magnification Range: A wide range of magnifications is possible, from about 10 times
(about equivalent to that of a powerful hand-lens) to more than 500,000 times, about
250 times the magnification limit of the best light microscopes.
Working Principle: The working principle of a Scanning Electron Microscope:
 The signals result from interactions of the electron beam with atoms at or near the
surface of the sample.
 In the most common or standard detection mode, secondary electron imaging or
SEI, the SEM can produce very high-resolution images of a sample surface,
revealing details less than 1 nm in size.
Figure: Configuration of Electron Microscopes: Left – TEM. Right – SEM.

4. SYED MUHAMMAD KHAN (BS HONS. ZOOLOGY)
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Figure: Left – In TEM, electron rays pass through the specimen. Right – In SEM,
electron rays are reflected off from the surface of the specimen and are detected by
a detector.