LAB EXERCISE: Microscopy and the Cell
Laboratory Objectives
After completing this lab topic, you should be able to:
1. Identify the parts of compound and stereoscopic microscopes and be proficient in their
correct use in biological studies.
2. Describe procedures used in preparing materials for light microscopy.
3. Describe features of specific cells and determine characteristics shared by all cells
studied.
4. Discuss the evolutionary significance of increasing complexity from unicellular to
multicellular organization and provide examples from the lab.
Introduction
According to cell theory, the cell is the fundamental biological unit, the smallest and
simplest biological structure possessing all the characteristics of the living condition. All
living organisms are composed of one or more cells, and every activity taking place in a
living organism is ultimately related to metabolic activities in cells. Thus, understanding
the processes of life necessitates an understanding of the structure and function of the
cell.
The earliest known cells found in fossilized sediments 3.5 billion years old (called
prokaryotic cells) lack nuclei and membrane-bound organelles. Cells with a membranebound nucleus and organelles (eukaryotic cells) do not appear in the fossil record for
another 2 billion years. But the eventual evolution of the eukaryotic cell and its internal
compartmentalization led to enormous biological diversity in single cells. The evolution
of loose aggregates of cells ultimately to colonies of connected cells provided for
specialization, so that groups of cells had specific and different functions.
This early division of labor included cells had specific and different functions. This early
division of labor included cells whose primary function was locomotion or reproduction.
The evolution of multicellularity appears to have originated more than once in eukaryotes
and provided an opportunity for extensive adaptive radiation as organisms specialized
and diversified, eventually giving rise to fungi, plants, and animals.
Given the fundamental role played by cells in the organization of life, one can readily
understand why the study of cells is essential to the study of life. Cells, however, are
below the limit of resolution of the human eye. We cannot study them without using a
microscope. The microscope has probably contributed more than any other instrument to
the development of biology as a science. Two types of microscopes are named according
to the source of illumination used: light microscopes and electron microscopes. We will
be using light microscopes exclusively in our study of cells, and we will view electron
micrographs of cell structures not visible with the light microscope.
Microscopes of one kind or another are used by all biologists in numerous subdisciplines:
genetics, molecular biology, neurobiology, cell biology, evolution, and ecology. The
knowledge and skills you develop today will be used and enhanced throughout this
course and throughout your career in biology. It is important, therefore, that you take the
time to master these exercises thoroughly.
The microscope is designed to make objects visible that are too difficult or too small to
see with the unaided eye. There are many different kinds of light microscopes, including
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phase-contrast, darkfield, polarizing, and UV. These differ primarily in the source and
manner in which light is passed through the specimen to be viewed.
The microscopes in biology lab are usually compound binocular or monocular light
microscopes, some of which may have phase-contrast attachments. Compound means
that the scopes have a minimum of two magnifying lenses (the ocular and the objective
lenses). Binocular microscopes have two eyepieces, monoculars have only one
eyepiece, and light refers to the type of illumination used, that is, visible light from a
lamp.
Place the microscope on your desk with the arm of the microscope nearest you. With the
aid of the illustration, locate the various parts of the instrument. Learn to understand the
function of each part. You should also become familiar with the terms frequently used in
microscopy, some of which are described in the following.
Figure 1. Compound light microscope.
Microscope parts and their functions
Eyepiece – Topmost series of lenses through which a specimen is viewed.
Body Tube – Holds nosepiece at one end and the eyepiece at the opposite, conducts light
rays.
Nosepiece – Revolving device; holds objectives for interchange.
Low Power Objective – 10 X lens holder; shortest lens holder which should always be
in position at the beginning of a lab and at the finish of the lab.
High Power Objective – 43 X lens holder.
Oil Immersion Objective – 97 X lens holder; used with oil in a special technique.
Stage – Holds and supports microscope slides.
Arm – Supports upper parts and provides carrying handle.
Coarse Adjustment Knob – Knob used to move the body tube for approximate focus
(use only with low power objective.)
Fine Adjustment Knob – Knob used to move the body tube for minute adjustment in
focusing.
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Inclination Joint – Permits tilting of parts attached to arm; should be used only with
permission of the instructor.
Pillar – Supports microscope arm and its attached parts.
Base – Bottom portion that supports the microscope; provides carrying support.
Condenser – Concentrates and directs light through specimen.
Iris Diaphragm – Controls area of illumination through the condenser.
Proper procedures in the laboratory
1. Use only lens paper and xylol for cleaning the lens. Usually lens paper is adequate, but
sometimes a solvent like xylol is required. Never use other tissues such as Kleenex for
cleaning the lens because of lint and the possibility of scratching the lens. Water is never
used on lens.
2. Do not remove microscope parts. Necessary repairs or cleaning will be done by a
trained technician.
3. Keep both eyes open when observing something under the microscope. This usually
reduces the chances of your getting a headache.
4. The low power objective is in position both at the beginning of the lab and when you
are ready to store the microscope.
5. Clean and dry the stage to prevent rust and corrosion.
6. Report any breakage or malfunctioning to the instructor.
Use of the microscope
To clean the lens:
Clean all exposed lens surfaces before you start to use the microscope. Only clean lens
paper should be used. Gently polish (1) upper lens of the eyepiece, (2) the lower surfaces
of the objectives, (3) the upper lens of the condenser, and (4) the surfaces of any
commercially prepared slide. Blank slides which you prepare in lab work need some less
expensive cleaning material such as a paper towel or tissue. Dirt or debris may
accumulate during your work, but locating this is easy. Dirt on the eyepiece will rotate as
you rotate the eyepiece. If there is dirt on the objective lens or the slide, it will move as
you move the part. Notify the instructor if you cannot eliminate the troublesome speck.
Xylol may be used with caution to clean the lens. Put a drop of the cleaning solvent xylol
on a clean piece of lens paper. Gently wipe the lens clean. Finish cleaning the lens with a
second piece of lens paper. With a third piece of lens paper, polish the lens.
To determine magnification:
If the eyepiece is marked 10 X and the objective in position is marked 10 X, the specimen
is magnified 100 times (10 X 10). If the eyepiece is 10 X and the objective in position is
43 X, then the specimen is magnified 430 times (10 X 43 = 430).
Working distance:
The distance between the specimen and the objective lens is the working distance. This
distance decreases as the magnification is increased. Therefore, the greater a specimen is
magnified, the greater is the chance for the specimen and the lens of the objective coming
into contact. This requires that the student exercise care when the high power objective or
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the oil immersion objective is in position. On a commercially prepared slide this could
mean the difference between a broken slide and one that is kept in good condition.
On most microscopes the low power objective is blocked by a stop screw from coming
too close to a slide. This objective is most commonly used so the stop is standard.
However, the high power does not have a stop and is clearly a potential threat for
breakage. Turn the adjustment knobs so that the body tube is moved away from the slide
to prevent breakage.
To place an objective into position:
Elevate the tube of the microscope with the coarse adjustment knob so that the objective
will be an inch or more from the stage. Turn the nosepiece and gently move the low
power objective into position until there is a click. Avoid 97 X.
The preparation to be studied under the microscope is usually mounted on a glass slide
which measures about 25 mm X 75 mm (1” X 3”) and is about 1.0 mm thick. A means of
holding the slide securely is provided either by clips or mechanical stage. Usually the
specimen is covered with a very thin coverslip which is made of glass or plastic.
To place a microscope slide on the mechanical stage, press forward on the lever which is
located at the rear left-hand corner of the mechanical stage. This will cause the slide
holder to open, and the slide may be placed within the holding area. The slide’s right and
rear edges will contact the corresponding edges of the mechanical stage. The bottom of
the slide should rest on the microscope stage and not on the mechanical and not on the
mechanical stage apparatus. Release the finger control lever, allowing the slide holder to
close, holding the slide within the mechanical stage. Using the mechanical stage control
knobs located on the side beneath the stage, move the slide until the specimen on the
slide is approximately over the center of the condenser opening in the stage. One knob
will move the slide left-right and the other knob will move the slide front-back.
To place a specimen for viewing:
Place the specimen on the center of the stage directly over the light source and in line
with the objective lens. Do this before looking into the eyepiece for viewing. Proper
positioning then can be accomplished.
The mechanical stage gives the student considerable assistance in operating the
microscope. Nearly any area of the specimen of particular interest may be brought to the
center of the viewing field for critical examination. The STAGE CONTROL KNOBS
are located below and on the right of the stage. On the left of the mechanical stage there
is a slide finger apparatus which can be pushed open to provide access to the slide and
slowly released to hold the slide in position. This is especially valuable when trying to
locate small and moving specimens. This apparatus will not function properly if the
bottom of the slide gets wet because there is too much drag or friction which keeps the
slide from moving freely. This is another reason not to tilt the microscope when using
fluid on the slide.
To focus, to adjust the focal plane:
Place the slide with the specimen you want to observe in the center of the stage. Be sure
that the specimen you wish to examine is in the center of the stage opening. The low
power objective should be in position, with its tip lowered about 5 mm above the stage.
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With your eye to the eyepiece and your hand on the coarse adjustment knob, slowly focus
upward until the specimen is in view. You should have to move the coarse adjustment
knob very little before the specimen is in view. Once the specimen is in fairly good focus,
it may be made more distinct by slight adjustments of the fine adjustment knob.
If you wish to observe the specimen under high power, decide upon the portion of the
specimen you wish to examine, and move this to the exact center of the low power field.
Rotate the high power objective in position. Do this slowly, making sure that as the
objective moves into position, it does not hit the stage or the slide.
Open and close the diaphragm several times to obtain the best lighting.
Most microscopes are parfocal, that is, once the specimen is in focus on low power, it
will remain in focus when you change to high power. A slight turn of the fine adjustment
knob will make the focus sharper.
Return the low power objective to position when you are finished.
Note that focus on the lamp will produce the ground glass or frosted glass effect. Refocus
to get rid of this problem.
To find and focus on the specimen:
Start the examination of any specimen under low power magnification, proceeding to
higher powers as specific points of interest are found during the low power viewing.
Small and/or moving specimens are more easily located by scanning the slide under the
microscope. This is accomplished by starting at one of the corners of the slide and
moving in a straight line, view an area from the area completely to the other side of the
slide. Lower the viewing area to the next line and view this to the opposite side and lower
again and view to the opposite side until the whole slide has been examined. If the
specimen is on the slide and in focus, it will appear. Several different slides may be
necessary to get a specimen.
To change the field of view for a specimen:
The field or field of view of a microscope is the area visible when the microscope is in
focus. When properly lighted and with no specimen in position, the field appears as a disc
of light. By moving the specimen within the light disc, different parts may be brought
successively into the viewing field. The greater the magnification, the smaller the field
will be.
To use the pointer:
The eyepiece has what appears as a black line. This is used to point out some feature of a
specimen, especially, if one is trying to show someone else a detail of a specimen or
object.
To use the iris diaphragm:
Sometimes in viewing minute structures or detail, it is best to reduce the light from the
condenser. Use the iris diaphragm to vary the amount of light passing through the
specimen. This can be done to the extreme so that a specimen appears white on a dark
field. As the power of the lens in the objective increases, the amount of light which is
passing through the objective is reduced. In other words, adjust the iris diaphragm when
the objectives are changed.
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Preparation of a wet mount:
Clean the microscope slide and coverslip with water and some clean paper toweling or
paper tissues. It is wasteful to use the expensive lens paper for cleaning blank slides.
Handle the slide and coverslip by the edges so that body oils and other bodily releases do
not get onto them.
If the specimen is dry or nearly so water will have to be added with it. Place this type of
specimen on the center of the clean glass slide. Put a single drop of water on the
specimen. If the water is too quickly absorbed, add some more. Then, following the
procedures illustrated below, cover the specimen in such a manner that the coverslip is
lowered onto the water-specimen gently to keep out air bubbles.
Water should not be found on top of the coverslip because it might contaminate the
objectives of the microscope.
For cultures which are fluid, place the drop from the culture medium onto the slide and
add the coverslip. There is no need to add water in the initial set-up.
If this preparation begins to dry, add a bit of water at the edge of the coverslip so that it
can seep under the coverslip and thus keep the specimen wet.
Always keep the bottom of the microscope slides clean and dry so that the microscope
stage is not messed up or the mechanical stage cannot move the slide smoothly.
Blank slides, those without labels, may be washed and dried with paper toweling or other
inexpensive materials.
However, commercially prepared slides, those with labels, must be cleaned with lens
paper to remove finger prints and smudges. Using any other materials may ruin the slide.
Stains:
Stain are used to color so that a specimen being viewed with light coming through it will
appear more distinct. Obviously, a specimen which is naturally colored will not need any
additional staining. The most commonly used stain in this laboratory text is diluted
iodine. Iodine is a very useful stain because it is easy to prepare, stores for long periods
without deterioration, and it colors the specimens which will be used in these exercises.
However, iodine is toxic and will kill most forms of life. There is no danger to the student
in these procedures ordinarily. Some care is necessary in handling the stain because it
will not wash out of many types of clothing.
VITAL STAINS are also used to stain and color certain specimens in a natural state. The
advantage of this type of stain is that it either does not kill or kills slowly so that one may
observe the living organism. In most instances the vital stain is no more harmful than
food coloring.
Methyl cellulose or protoslo:
Methyl cellulose is a harmless material which is used to slow some fast-moving
microscopic forms.
Spread a thin layer of methyl cellulose on a clean microscope slide. Use the flat side of a
toothpick to do the spreading.
Add a drop of culture medium containing the organism and mix the two by stirring with a
toothpick.
Add a coverslip.
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Let the preparation stand for at least a minute so that the methyl cellulose and fluid can
mix. If the preparation is too thick, tap the coverslip gently to spread out the mixture.
Protoslo is a clear material also used to slow fast-moving microscopic forms.
Neither one of theses slowing materials is colored. They will not add any color to a
specimen.
Dissecting scope:
When a specimen is large and thick, it is not possible or practical to view it under the lens
of a compound microscope, which requires that light pass through anything on view.
With the dissecting scope, it is the whole specimen which is to be viewed and which can
be viewed in three dimensions.
By using the dissecting scope, it is not necessary to have a special staining or to develop
difficult techniques of preparation so that the specimen can be observed. This type of
scope is not a substitute for the compound scope because it serves in a different capacity
as another tool in the biology laboratory.
Typically, there are two objectives and though their power of magnification varies, they
usually are 5 X or 10 X objectives. The objectives can be moved into position by rotation.
There is a metal plate that serves as a stage or platform on which the specimen is placed.
One side is white for use in any way convenient for viewing a specimen.
In most instances a specimen is lighted by some sort of lamp with the light directed onto
the specimen from above, and to light up or shade areas of a specimen.
EXERCISE 1. Basic Microscope Techniques
Materials
clear ruler
coverslips
prepared slides: letter and crossed thread
Kimwipes
lens paper
dropper bottle with distilled water
blank slides
Introduction
In this exercise, you will learn to use the microscope to examine a recognizable object, a
slide of the letter e. Recall that microscopes vary, so you may have to omit steps that
refer to features not available on your microscope. Practice adjusting your microscope to
become proficient in locating a specimen, focusing clearly, and adjusting the light for the
best contrast.
Procedure
1. Clean microscope lenses.
Each time you use the microscope, you should begin by cleaning the lenses. Using lens
paper moistened with a drop of distilled water, wipe the ocular, objective, and condenser
lenses. Wipe them again with a piece of dry lens paper.
*Use only lens paper on microscope lenses. Do not use Kimwipes, tissues, or other
papers.
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2. Adjust the focus on your microscope:
a. Plug your microscope into the outlet.
b. Turn on the light. Adjust the light intensity to mid-range if your microscope has
that feature.
c. Rotate the 4 X objective into position using the revolving nosepiece ring, not
the objective itself.
d. Take the letter slide and wipe it with a Kimwipe tissue.
Each time you study a prepared slide, you should first wipe it clean. Place the
letter slide on the stage, and center it over the stage opening.
*Slides should be placed on and removed from the stage only when the 4 X
objective is in place. Removing a slide when the higher objectives are in
position may scratch the lenses.
e. Look through the ocular and bring the letter into rough focus by slowly
focusing upward using the coarse adjustment.
f. For binocular microscopes, looking through the oculars, move the oculars until
you see only one image of the letter e. In this position, the oculars should be
aligned with your pupils. In the margin of your lab manual, make a note of the
interpupillary distance on the scale between the oculars. Each new lab day,
before you begin to use the microscope, set this distance.
g. Raise the condenser to its highest position, and fully close the iris diaphragm.
h. Looking through the ocular, slowly lower the condenser just until the graininess
disappears. Slowly open the iris diaphragm just until the entire field of view is
illuminated. This is the correct position for both the condenser and the iris
diaphragm.
i. Rotate the 10 X objective into position.
j. Look through the ocular and slowly focus upward with the coarse adjustment
knob until the image is in rough focus. Sharpen the focus using the fine
adjustment knob.
*Do not turn the fine adjustment knob more than two revolutions in either
direction. If the image does not come into focus, return the 10 X and refocus
using the coarse adjustment.
k. For binocular microscopes, cover your left eye and use the fine adjustment
knob to focus the fixed (right) ocular until the letter e is in maximum focus. Now
cover the right eye and, using the diopter ring on the left ocular, bring the image
into focus. The letter e should now be in focus for both of your eyes. Each new
lab day, as you begin to study your first slide, repeat this procedure.
l. You can increase or decrease the contrast by adjusting the iris diaphragm
opening. Note that the maximum amount of light provides little
contrast. Adjust the aperture until the image is sharp.
m. Move the slide slowly to the right. In what direction does the image in the
ocular move?
n. Is the image in the ocular inverted relative to the specimen on the stage?
o. Center the specimen in the field of view; then rotate the 40 X objective into
position while watching from the side. If it appears that the objective will hit the
slide, stop and ask for assistance.
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*Most of the microscopes have parfocal lenses, which means that little
refocusing is required when moving from one lens to another. If your scope is
not parfocal, ask your instructor for assistance.
p. After the 40 X objective is in place, focus using the fine adjustment knob.
*Never focus with the coarse adjustment knob when you are using the high
power objective.
q. The distance between the specimen and the objective lens is called the working
distance. Is this distance greater with the 40 X or the 10 X objective?
*Answer the appropriate questions at the end of the microscopy exercise.
Determine spatial relationships.
The depth of field is the thickness of the specimen that may be seen in focus at one time.
Because the depth of focus is very short in the compound microscope, focus up and down
to clearly view all planes of a specimen.
a. Rotate the 4 X objective into position. Take a slide of crossed threads, wipe it
with a Kimwipe, and place the slide on the stage. Center the slide so that the
region where the two threads cross is in the center of the stage opening.
b. Focus upward (move the stage up) with a coarse adjustment until both threads
are just out of focus. Slowly focus down using the fine adjustment. Which thread
comes into focus first? Is this thread lying under or over the other thread?
c. Rotate the 40 X objective into position and slowly focus up and down, using
the fine adjustment. Which thread comes into focus first? Is this thread lying
under or over the other thread?
*Answer the appropriate questions at the end of the microscopy exercise.
EXERCISE 2. The Organization of Cells
In this exercise, you will examine the features common to all eukaryotic cells that are
indicative of their common ancestry. However, you will observe that all cells are not the
same. Some organisms are unicellular (single-celled), with all living functions
(respiration, digestion, reproduction, and excretion) handled by that one cell. Others form
random, temporary aggregates, or clusters, of cells. Clusters composed of a consistent
and predictable number of cells are called colonies. Simple colonies have no
physiological colonies have physiological connections and specialization of groups of
cells. Multicellular organisms have large numbers of cells with specialized structure and
function, and no one cell can exist successfully by itself.
Unicellular Organisms
Materials
microscope slides
Amoeba culture
coverslips
Paramecium culture
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Procedure
1. Prepare wet mounts of both amoeba and paramecium samples.
a. Using a clean pipette (it is important not to interchange pipettes between culture
dishes), transfer a drop with several amoebas or paramecia to your microscope
slide. To do this, squeeze the pipette bulb before you place the tip under the
surface of the water. Disturbing the culture as little as possible, pipette a drop of
water with debris from the bottom of the culture dish.
b. Cover your preparation with a clean coverslip.
c. Under low power on the compound scope, scan the slide to locate an amoeba.
Center the specimen in your field of view, then switch to higher powers.
d. Identify the following structures in the amoeba:
Cell membrane is the boundary that separates the organism from its surroundings.
Ectoplasm is the thin, transparent layer of cytoplasm directly beneath the cell membrane.
Endoplasm is the granular cytoplasm containing cell organelles.
The nucleus is the grayish, football-shaped body that is somewhat granular in
appearance. This organelle, which directs the cellular activities, will often be seen
moving within the endoplasm.
Contractile vacuoles are clear, spherical vesicles of varying sizes that gradually enlarge
as they fill and then empty its contents into the surrounding environment. These vacuoles
serve an excretory function for the amoeba.
Food vacuoles are small, dark, irregularly shaped vesicles within the endoplasm. They
contain undigested food particles.
Pseudopodia (“false feet”) are fingerlike projections of the cytoplasm. They are used for
locomotion as well as for trapping and engulfing food in a process called phagocytosis.
Figure 3. Amoeba. An Amoeba moves using pseudopodia.
d. Follow the same procedure for Paramecium and identify the following
structures:
Cilia: minuscule cilia that envelop the paramecium and are used for locomotion.
Contractile vacuole: cavity of the paramecium that is able to contract.
Food vacuole: cavity of the paramecium responsible for digestion.
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Micronucleus: one of the less important central organelles of a paramecium.
Oral groove: canal of the paramecium used to ingest nutrients.
Gullet: cavity of the pharynx.
Ectoplasm: vitreous superficial layer of a paramecium.
Endoplasm: central part of a paramecium.
Large nucleus: the most important central organelle of a paramecium.
Canals of contractive vacuole: division of the contractile cavity of a paramecium.
Trochocyst: root of a vibrative cilium of a paramecium.
Figure 4. Paramecium.
*Draw examples of what you see for both Amoeba and Paramecium cultures.
Aggregate and Colonial Organisms
Materials
microscope slides
coverslips
Protococcus and Volvox cultures
Introduction
Unlike unicellular organisms, which live independently of each other, colonial organisms
are cells that live in groups and are to some degree dependent on one another. The
following organisms show an increasing degree of interaction among cells. Protococcus
(Figure 5) is a terrestrial green alga that that forms loose aggregates on the bark of trees
grows on the north sides of trees and is often referred to as “moss”. The size of the cell
groupings is random, and there are no permanent connections between cells. Each cell is
surrounded by a cell membrane and an outer cell wall. Volvox (Figure 6) is an aquatic
green alga that also is common in aquaria, ponds, and lakes. In this complex colony, the
individual cells are interconnected by cytoplasmic strands to form a sphere. Small
clusters of cells, called daughter colonies, are specialized for reproduction that the cells
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of this organism form a large complex colony. Approximately 500 to 50,000 cells
(depending on the species) are permanently united, there are cytoplasmic connections
between cells, and some cells are specialized for reproduction.
Procedure
1. Prepare wet mounts of both Protococcus and Volvox samples.
a. Identify the following structures in Volvox:
Individual cells all possess the following structures: cell wall, nucleus, vacuole,
chloroplasts, flagella (two per cell).
Cytoplasmic strands form connections between adjacent cells.
Daughter colonies are smaller spheres within the larger colony. These are produced
asexually, and when they are large enough, they will be discharged from the parent
colony into the surrounding environment.
Figure 5. Protococcus.
Figure 6. Volvox.
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*Draw examples of what you see for both Protococcus and Volvox cultures.
Multicellular Organisms
Materials
microscope slides
dropper bottles of water
toothpicks
coverslips
Elodea
methylene blue
Introduction
Multicellular organisms are composed of groups of specialized cells, called tissues that
together perform particular functions for the organisms. Tissues, in turn, may be grouped
to form organs, and organs may be grouped into organ systems. In this lab study, you
will examine some of the cells that compose the basic tissue types of plants and animals.
Procedure
Plant Cells
1. The major characteristics of a typical plant cell are readily seen in the leaf cells of
Elodea, a common aquatic plant (Figure 7). Prepare a wet mount and examine one of the
youngest (smallest) leaves from a sprig of Elodea under the compound
microscope.
2. Identify the following structures:
The cell wall is the rigid outer framework surrounding the cell. This structure gives the
cell a definite shape and support. It is not found in animal cells.
Protoplasm is the organized contents of the cell, exclusive of the cell wall.
Cytoplasm is the protoplasm of the cell, exclusive of the cell wall.
The central vacuole is a membrane-bound sac within the cytoplasm that is filled with
water and dissolved substances. This structure serves to store metabolic wastes and gives
the cell support by means of turgor pressure. Animal cells also have vacuoles, but they
are not as large and conspicuous as those found in plants.
Chloroplasts are the green, spherical organelles often seen moving within the cytoplasm.
These organelles carry the pigment chlorophyll that is involved in photosynthesis. As the
microscope light heats up the cells, cytoplasm and chloroplasts may begin to move
around the central vacuole in a process called cytoplasmic streaming, or cyclosis.
The nucleus is the usually spherical, transparent organelle within the cytoplasm. The
structure controls cell metabolism and division.
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Figure 7. Elodea. Elodea is an aquatic plant commonly grown in freshwater aquaria. The
cell structures may be difficult to see because of the three-dimensional cell shape and the
presence of a large central vacuole.
Animal Cells
1. Animals are multicellular heterotrophic organisms that ingest organic matter. They are
composed of cells that can be categorized into four major tissue groups, epithelial,
connective, muscle, and nervous tissue. In this lab study, you will examine epithelial
cells. Similar to the epidermal cells of plants, epithelial cells occur on the outside of
animals and serve to protect the animals from water loss, mechanical injury, and foreign
invaders. In addition, epithelial cells line interior cavities and ducts in animals. Examine
the epithelial cells (Figure 8) that form the lining of your inner cheek. To obtain a
specimen, follow this procedure:
a. With a clean toothpick, gently scrape the inside of your cheek several times.
b. Roll the scraping into a drop of water on a clean microscope slide, add a small
drop of methylene blue, and cover with a coverslip.
c. Using the compound microscope, view the cells under high power.
Observe that these cells are extremely flat and so may be folded over on themselves.
Attempt to locate several cells that are not badly folded, and study their detail.
2. Identify the following structures:
The cell membrane is the boundary that separates the cell from its surroundings.
The nucleus is the large, circular organelle near the middle of the cell.
Cytoplasm is the granular contents of the cell, exclusive of the nucleus.
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Figure 8. Human epithelial cells.
*Draw examples of what you see for both plant and animal cells.
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BIO201 Laboratory Assignment: Microscopy
Exercise for today:
Follow procedures as outlined in manual and answer the following questions:
Exercise 1. Basic Microscope Techniques
1. If you move a slide slowly to the right, in what direction does the image in the ocular
move?
2. Is the image in the ocular inverted relative to the specimen on the stage?
3. Working distance. Is the working distance greater with the 40 X or the 10 X objective?
4. Compute the total magnification of the specimen being viewed (the letter e and crossed
threads).
a) What is the total magnification of the letter e when viewed in focus with the 40
X objective?
b) What would be the total magnification if the ocular were 20 X and the
objectives were 100 X?
c) Focus on the region where the threads cross. Are both threads in focus at the
same time?
d) Rotate the 10 X objective into position and focus on the cross. Are both threads
in focus at the same time?
e) Does the 4 X or the 10 X objective have a shorter depth of field?
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5. Measuring the diameter of the field of view. (Using a clear ruler).
a) What is the diameter of the field of view? (Explain)
b) Measure the diameters of the field of view for the 4X, 10 X and 40 X
objectives:
4X = _________________ 10 X = ______________ 40 X = ______________
Exercise 2. The Organization of Cells
Draw examples of what you see for Amoeba, Paramecium, Protococcus, Volvox, Elodea
and human epithelial cells.
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