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Cell Structure and Microscopy
In the biology lab, you frequently use a compound light microscope. It is called a compound
scope because magnification is the result of two lenses: the objective and the ocular.
The objective lenses, located on the rotary nosepiece, provide 4 different degrees of
magnification:
Name
Characteristics
Magnifying power
Scanning power
shortest objective, red stripe
4X
Low power
next shortest, yellow stripe
10 X
High-dry power
intermediate length, blue stripe
40, 43 or 45 X
Oil immersion
longest, black stripe
100 X
The ocular lens, located nearest to your eye, has a magnification power of 10X. The total
magnification is determined by multiplying the power of the objective by the power of the
ocular. (For example, 4 X times 10 X = 40 X).
Your microscope is referred to as a light microscope because the specimen is observed using
visible light. The light source is an incandescent bulb which is turned on by the toggle switch at
the side of the base.
There are two ways to adjust the light on your scope. The dial on the base of your scope turns
the light level from bright to dim. You can also use the iris diaphragm, just under the stage to
control the amount of light that enters the condenser.
Here is a diagram of a compound scope. You will need to become familiar with the different
parts and controls of the scope in order to effectively use it.
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Principles of Microscopy
Resolution
As two small objects are moved closer to each other, a point is reached where the eye is unable
to distinguish the objects as separate entities, and only a single object is observed. The
smallest distance at which two points can be seen separately is called the resolving power of
the lens, and the resolution of highest magnification lens on a light microscope is limited by the
shortest wavelength of visible light (400nm).
To be able to distinguish between objects that are closer together than the shortest
wavelengths of the visible light, electron microscopes must be used. Electron microscopes,
which use a beam of electrons rather than light, are able to resolve very, very small objects
such as structures of bacteria (like flagella) and viruses.
Depth-of-Field
The term depth-of-field refers to the vertical distance that is in focus at any one time. Higher
magnification lenses have a smaller depth of field – that is, only a thin horizontal slice of your
sample may be in focus at any one time. So, at high magnification, very small adjustments to
the fine focus can quickly take your sample completely out of focus.
Field of View
The term field-of-view refers to how much of the horizontal are of your sample you can see at
any one time. The field of view decreases with increased magnification. As you increase the
magnification, you are looking at progressively smaller subsections of your sample. That is why
you need to be sure to center what you are viewing before increasing the magnification.
Artifact
As you look at things under the scope, you need to be able to distinguish the things that you
are supposed to observe from artifacts – things inadvertently introduced (junk).
For example, if you take a sample of your cheek cells and place them on a slide with
physiological saline and stain, you will be able to examine this specimen under the scope, but
there may be other things in you sample that are not check cells (such as little bits and pieces
of things that you have eaten). The cells are what you want to view; the bits of your lunch are
the artifacts.
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Parfocal
The objective lenses on a microscope are said to be parfocal if you can change from one to
another and still have your specimen in focus without having to focus more than a little. This is
a very convenient characteristic of a set of lenses, because if you had to find your specimen
using a high-magnification objective, it would probably take quite awhile if you had to start
focusing ‘from scratch’. With parfocal lenses, if you get an object in focus at low magnification,
when you switch to a high magnification lens, your specimen will still be in focus.
Cell Structure: Introduction
In this laboratory activity you will study biology from the viewpoint of the individual cell.
Although the cell is considered to be the building block of all organisms, cells differ enormously
in shape, size and capability. Prokaryotic cells are less complex, are usually found only in
unicellular organisms and have more limited capabilities than eukaryotic cells. Multicellular
organisms are made up of highly integrated aggregations of specialized eukaryotic cells, but
some complex organisms consist of a single eukaryotic cell.
PURPOSE: To examine the similarities and differences between plant and animal eukaryotic
cells and prokaryotic cells using microscopic evidence.
PROCEDURE:
Part A. Basic Plant Cell Structure- Onion Epidermis
a. Obtain a piece of a single piece of sliced onion bulb. Snap the piece in half and, using
forceps, peel a bit of tissue-like, transparent epidermis from the inner layer. Mount the piece of
epidermis in tap water on a glass microscope slide. Be certain that it is flat and not doubled
over itself. Add a coverslip and gently press it down with a pencil eraser to remove as many air
bubbles as possible.
b. Observe the specimen under low power. If you see a confusing mass of overlapping cell
parts, you probably do not have a good piece of epidermis. Scan your slide for a single layer of
cells that resembles a brick wall. If you can not find such a view, then prepare another slide.
c. As you view onion epidermis cells, note that the cell membrane is tight against the cell wall
and too thin to be seen with a light microscope. Note also any signs of nuclei.
d. Remove the slide from the microscope. Add a drop of iodine stain to the slide immediately
next to the coverslip but not on top of it. With a small piece of paper towel, draw the stain
under the coverslip by placing the towel on the opposite side of the stain and touching the
water. This action should wick the water out from under the coverslip and draw the stain in.
e. Observe the epidermis cells again under low power. Note that the cells are now colored, with
the cell wall, nucleus and nucleoli stained more darkly than the cytoplasm. Try to distinguish
between the cytoplasm and the large central vacuole.
f. Observe the cells under high power to help you better distinguish cell structures.
g. Make a sketch of a few onion cells. Label the cell wall, cell membrane, cytoplasm,
central vacuole, nucleus and nucleolus.
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Elodea
a. Break off a single leaf near the tip of an Elodea plant and prepare a wet mount. Position the
slide so that you are observing the edge of the leaf near the tip. Observe the cells both under
low power and high power. If you are viewing at the edge of the leaf you should see only one
cell thickness. You should also see two types of cells. One type of cell has a smooth margin
facing the environment; the second type, called a spike cell, has a projection out into the
environment.
b. The thick cell walls and the green chloroplasts are the most conspicuous structures.
Sometimes a shadow of a nucleus can be seen. If you are patient and fortunate, you may see
movement of the chloroplasts around the periphery of the cell. This movement, called cyclosis,
is caused by cytoplasmic streaming.
c. Make a labeled drawing of a few Elodea cells to demonstrate your observations. LABEL cell
wall, cell membrane, cytoplasm, nucleus, and chloroplast
Part B. Basic Animal Cell Structure- Cheek Cells
a. Make a wet mount of cheek cell epithelium by gently scraping the inside of your cheek with a
toothpick and transferring the material to the drop of water on the slide.
b. Place a coverslip in the edge of the drop of water and gently lower the coverslip down on the
slide. If done slowly and carefully, there will not be air bubbles under the coverslip. If air
bubbles are present another preparation should be made. Observe the cells under both low and
high power.
c. Make another wet mount and stain with methylene blue. Do this by placing the stain on
one edge of the cover slip and drawing it through with a piece of paper towel held at the other
edge of the cover slip. d. Make a labeled drawing of a few check cells to demonstrate your
observations. LABEL cell membrane, cytoplasm, and nucleus.
Part C. Bacteria
a. Obtain a prepared slide of bacteria. Since these prokaryotic cells are so small you will have
great difficulty seeing them, even under high power. While searching for the specimens use the
color of the stain to help you locate them.
b. After locating and observing the bacteria, observe them using high power.
c. Observe that the stained bacterial cells are of three major forms, spherical-shaped (called
coccus), rodshaped (called bacillus) and spiral-shaped (called spirillum).
d. Sketch a few bacterial cells, showing the three major structural forms. Label the Types.
Note: All drawings MUST be done in the following manner:
a. On a piece of blank white paper, draw the specimen as seen under the microscope.
b. The drawing must be a minimum 4cm x 4 cm in size.
c. Beneath the drawing label the picture identifying the cells and give the magnification, such as "Cheek Cells - 40X" OR Onion Cells
with Iodine-100X."
d. LABEL all identifiable organelles by printing horizontally. Use a ruler to draw a line to the object. The line MUST TOUCH the
object labeled.
e. Remember that detail is important in microscopic drawings.