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Chapter 6

BIL 150 Chapter Notes - Chapter 6: Scanning Electron Microscope, Electron Microscope, Optical Sectioning


Department
Biology
Course Code
BIL 150
Professor
Gaines Michael
Chapter
6

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Chapter 6 - A Tour of the Cell
Microscopy
oIn a light microscope (LM), visible light is passed through the
specimen and then through glass lenses, which refract (bend) the
light in such a way that the image of the specimen is magnified as it
is projected into the eye or camera
Brightfield (unstained) – passes light directly through
specimen
Brightfield (stained) – various dyes enhance contrast but
cells are fixed
Fluorescence – tagging with fluorescent dyes or antibodies
Phase-contrast – enhances contrast by amplifying variation
in density
Differential-interference contrast – like phase-contrast
exaggerate differences in density
Confocal – uses lasers and special optics for “optical
sectioning” of fluorescently-stained specimens
oMagnification is the ratio of an object’s image size to its real size
oResolution is a measure of the clarity of the image
oContrast is the difference in brightness between the light and dark
areas of an image
oThe electron microscope (EM) focuses a beam of electrons
through the specimen or onto its surface (instead of focusing light)
Resolution = inversely proportional to the wavelength of light
(or electrons)
Therefore, electron beams have much shorter wavelengths
than visible light
oThe scanning electron microscope (SEM) is use for detailed
study of the topography of a specimen

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The electron beam scans the surface of the sample, which
excites the electrons on the surface and these secondary
electrons are detected by a device that translates the pattern
of electrons into an electronic signal sent to a video screen
(appears 3D)
oThe transmission electron microscope (TEM) is used to study
the internal structure of call
Aims an electron beam through a very thin section of the
specimen that has been stained with atoms of heavy metals
(which attach to certain cellular structures), thus enhancing
the electron density of some parts of the cell more than
others
oAutoradiography – method to count radioactivity in a cell through
the measure of thymine (radioactive)
oComputerized Axial Tomography (CAT scan) – procedure which
combines many X-ray images with the aid of a computer to
generate cross-sectional views
oMagnetic Resonance Imaging (MRI) – radiology technique which
uses magnetism, radio waves, and a computer to produce images
of body structures
oPositron Emission Tomography (PET Scan) – imaging technique
that monitors metabolic, or biochemical activity by tracking the
movement and concentration of radioactive tracer
(fluorodeoxyglucose)
Cells – the basic structural an functional units of every organism
oAll cells are bounded by a selective barrier called the plasma
membrane
Functions as a selective barrier that allows the passage of
enough oxygen, nutrients, and wastes to service the entire
cell
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oInside all cells is semifluid, jellylike substance called cytosol in
which subcellular components are suspended
oAll cells contain chromosomes, which carry genes in the form of
DNA
oAll cells have ribosomes, tiny complexes that make proteins
according to instruction from the genes
Prokaryotic Cells (without nucleus (bacteria))
oIs smaller and has a greater surface area to volume ratio
Larger organisms do not have larger cells, just more cells
A high surface area to volume ratio is important in cells that
exchange a lot of material with their surroundings
Some cells have microvilli, which increase surface area
without a big increase in volume
oDNA is concentrated in a region that in not membrane-enclosed,
called the nucleoid
oCytoplasm is not a formless soup, but yet organized into different
regions
Eukaryotic Cells (with nucleus)
oGenerally much larger than prokaryotic cells
oEndomembrane System – includes the nuclear envelope, ER,
Golgi apparatus, lysosomes, various kinds of vesicles and
vacuoles, and the plasma membrane
The membranes of this system are related either through
direct physical continuity or by the transfer of membrane
segments know as vesicles (sacs made of membrane)
oOrganelles:
DNA is located in an organelle called the nucleus, which is
bounded by a double membrane
Nuclear Envelope – double membrane enclosing the
nucleus, filled with pores
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