LABORATORY #5: The Microscope and Introduction to the Cell
Required Materials:
1. Light microscope
2. Dissection microscope & flower parts
3. Prepared slides of :
a. Neisseria gonorrhea - cocci
b. Treponema pallidum - spiral
c. Mycobacterium tuberculosis - bacillus
d. Nostoc – filamentous/colonial cyanobacteria
e. Human blood smear
4. Toothpicks for cheek smear
5. 0.3% methylene blue
6. Plain slides & cover slips
7. Pond water & eye dropper
8. Methyl cellulose
9. Lens paper
10. Oil
Introduction
This laboratory assignment introduces you to some of the microscopes used in biology. In your
lecture, you learned about the basics of microscopy and the major types of microscopes.
Unfortunately, we do not have access to a transmission electron microscope or a scanning
electron microscope, but you will become familiar with the simpler (and cheaper) light
microscope and dissecting microscope.
Most modern light microscopes use two sets of lenses (i.e. a compound light microscope) to
focus and magnify the light. The first set of lenses is the objective, which usually comes in
different powers (5X very lower power, 10X low power, 20X medium power, 40X high dry & 100
high oil). The microscopes you will be using have 4X, 10X, 40X and 100X objective lenses. The
second set is known as the ocular. This usually has a power of 10X. So the actual magnification
power is a combination of the two lenses.
A. Identification of parts: Take a look at the microscope on your table and the labeled picture
at the end of this lab. Identify these listed parts.
PART
In-base illuminator
Iris Diaphragm level
FUNCTION
Light source
Adjusts the amount of light that passes through the subject
Condenser lens
Stage
Nosepiece
Objective lens
Field pointer
Ocular (eyepiece)
Coarse adjustment knob
Fine adjustment
Focuses the light on the subject
Location of the subject
Holds and rotates the objective lenses
Magnifies the subject based on the power of the lens
Identifies object of interest to a second viewer
Contains a 10X power lens
Moves the stage up and down to coarsely focus on the subject
Also moves the stage to more precisely focus on the subject
B. Focusing: Use your letter ‘e’ slide for the following steps.
Use the following steps to focus the microscope and to increase the magnification on your
subject
1. Raise the nose piece using the coarse adjustment knob and clean the lenses of your
objectives with lens paper AND ONLY lens paper to prevent scratching
2. Place your slide on the stage with the subject centered over the condenser lens (hint:
use the location of the focused light to center)
3. Rotate the nose piece until the lowest power lens is in place. Using the coarse
adjustment, lowly move the nose piece down until you see the subject come into
approximate focus.
4. Adjust the iris until the light is correct for viewing the subject. As you increase in lens
power, you will need to allow more light to pass through the subject
5. Use the fine focus to bring your subject into clear focus
6. Increase you magnification on your subject by rotating the nose piece until the next
highest power lens ‘snaps’ into place. Adjust your fine focus with each change of lens
power. Observe your subject at 100X and 400X magnification using the 10X and 40X
objectives
7. Replace the “e” slide with that containing three strings and practice focusing on each of
these strings.
C. Resolution:
Resolution is the ability of a microscope to distinguish small objects. It is given as a value
representing the smallest distance between two points at which they are still distinguishable.
Scopes with the highest resolution power have the smallest resolution distances. Resolution is
directly related to the wavelength of the illuminating source (i.e. light or electrons) and it is
inversely related to the numerical aperture of the lens (i.e. the solid cone of light delivered to
the subject and received by the lens.
d = wavelength
2Xnumerical aperture
D. Inversion: Use the slide of the letter ‘e’ for the following
The image of a subject changes when viewed through a compound light microscope. How??
Record your observations below
E. Magnification:
In a compound microscope, magnification is done by the objective lens and the ocular lens. To
determine magnification, multiply the power of the two lens together.
Determine the total magnification that the lenses on your microscope would provide. In the
space below, list the objective lens and the total magnification associated with each one.
F. Diameter of field: Use a prepared millimeter slide for the following
The field of a scope is the illuminated are seen when an observe looks into the ocular lens. This
area is the shape of a large circle. The diameter of the field is any distance which goes across
this circle and passes through the subject. If the length of the diameter is known, it can be used
to estimate the size of the subject within it. The diameter of field is larger when using lower
power objective lenses. In other words, the field diameter decreases as you increase the power
of the lens. The amount the diameter of field decreases when using high power lenses is the
same as the amount the magnification increases. So if your magnification increases ten-fold,
the diameter decreases ten-fold.
low power magnification = diameter field high power
high power magnification diameter field low power
Measure the diameter of field using the ruler slide under 10X, 40X and 100X lenses and record
it below.
OBSERVING CELLS:
The Cell Theory states that all living things are composed of cells. During this lab, you will
observe an array of different cells types and some of their organelles. From this lab, you should
learn that there are a wide variety of cells in many diverse forms.
Record all your scientific observations in your lab notebook. For some cells, you may want to
draw what you see. Also, taking photos through the ocular with your smartphones are also
possible in most cases. Use these photos to help supplement your observations when studying
at home.
1. Use a pencil for all drawings
2. Drawing should be large enough for you to add pertinent details
3. Be sure to properly label all your drawings
Types of cells
Although cells come in many different sizes and shapes, there are a few general categories.
There are type types of cells in nature: Prokaryotic (lacking a nucleus and membrane bound
organelles) and Eukaryotic (having a nucleus and organelles).
To study these cells, you will use two types of microscope preparations in this course: prepared
slides and wet mounts
G. Wet mount: Use the provided pond water and prepare a wet mount by doing the following:
To prepare a wet mount, use a plain slide, water and a coverslip. Place the subject on the slide
and add a drop of water on top (if necessary). Place the edge of the coverslip at the edge of the
water and slowly lower the cover slip down on the subject. Go slowly to prevent bubbles! To
add a substance to the preparation, add it to one edge of the cover slip and remove some
water through capillary action by placing a paper towel at the opposite edge of the cover slip.
This will draw your substance under the coverslip and over the subject.
Make a wet mount with some pond water and add methyl cellulose to your prep. If you are
having difficulty drawing the methyl cellulose under your coverslip, you may add the substance
directly to you wet mount and then add the coverslip. But be careful not to add too much
methyl cellulose. This laboratory may also provide you with a microscope slide with a shallow
depression in the center. You may add your pond water and methyl cellulose to this depression
and cover it with a coverslip.
Observe your prep under the microscope using several objective lenses. Record your
observations in your lab notebook. Draw some of the subjects you observe in your notebook
and guess what they might be. Using your diameter of field measurements for each of your
lenses, estimate the average size of some of the objects that you see.
H. Prepared slides: Use the provided prepared slides or prepare the indicated wet mounts to
study various prokaryotic and eukaryotic cells
A. Prokaryotic cells - Eubacteria: Observe the prepared slides of bacteria in class under the
40X and 100X oil immersion lenses. Draw and label what you see in your field of view
with respect to cell shape and size.
B. Prokaryotic cells - Cyanobacteria: For the Nostoc slide, there are two distinct cell types
present. Can you observe them? Identify these two cell types in the drawing you made
in your lab notebook.
C. Eukaryotic cells:
A. Animal cells:
i. Human blood smear: Observe the prepared slide of a human blood smear under
the 40X and 100X oil immersion lenses Draw and label what you see in your field of
view with respect to cell shape and size.
ii. Human cheek smear: The cells on the inside of your cheek are epithelial cells. They
are flat, pancake-shaped cells stacked into successive layers, resulting in a tissue
called stratified. This stratified epithelial tissue is similar to that found in the
epidermis of your skin, with one important difference. The cells on in the inside of
your cheeks do not form a water resistant protein called keratin. This means that the
cells you scrape from the inside of your cheek are alive.
To prepare a human cheek smear: Obtain some of these cells by gently scraping the
inside of your cheek with a wooden applicator or toothpick. Add 1-2 drops of water
to a slide, then add the cheek cells to the water by swirling the toothpick in the
water. Add a drop of 0.3% methylene blue to the cell suspension. Carefully add a
coverslip. Observe the cells using your 40X or 100X objective lenses.
B. Plant cells – Elodea: Elodea is a multicellular aquatic plant. The leaves of this plant
exhibit the general characteristics of plant cells, including a cell wall (for support), a
large, central vacuole (for water storage) and many small chloroplasts (for
photosynthesis). If present in the lab, prepare and observe a wet mount of a leaf of
Elodea. Identify, draw and label the cell wall, central vacuole and chloroplasts (if
observable).
USING THE OIL IMMERSION LENS: For each prepared slide, observe your samples using the
100X oil immersion lens. To use the 100X oil immersion lens, rotate the nose piece so that the
subject is in between the 40X and 100X lenses. Place a small drop of immersion oil on the
coverslip over the subject and rotate the 100X lens into position. The oil will increase the
resolution of the subject when at higher magnifications. AFTER YOU ARE DONE, CLEAN THE
SLIDES CAREFULLY WITH SOAP AND WATER. DO NOT CLEAN THE OIL IMMERSION LENS!!!!
J. Dissecting scope: Use the provided flower parts
The dissecting scope is for low magnification of larger objects. It is a compound scope, but its
magnification varies from 15X to 30X. Obtain some flower parts and place them under the
dissecting scope. Using trial error, determine how to use the lights and lenses on this scope.
Compare and contrast the images of the light microscope with that of the dissecting scope.
Worksheet exercises
Using the following key, identify what you can from the electron micrographs of the plant and
animal cell given below
A.
B.
C.
D.
E.
F.
G.
H.
Nucleus
Nuclear envelope
Mitochondria
Plasma membrane
Cell wall
Central vacuole
Rough endoplasmic reticulum
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At the end of this lab, please clean up as follows:
1. Clean all glass microscope slides with soap and water. Dry and return them to the box.
2. Discard the plastic coverslips in the trash
3. Return the microscope as follows:
a. Ensure the objective nosepiece is in the empty position
b. Ensure the stage is at its lowest position
c. Ensure the light level is at its lowest level
d. Blot off excess oil from your oil immersion lens using LENS PAPER ONLY!!!!
4. Return your prepared slides to the correct trays