LAB 2 – Using A Microscope to Observe Cells

LAB 2 – Using A Microscope to Observe Cells
BIOLOGY 100 LAB PACKET LAB 2 – Using A Microscope to Observe Cells
In this lab you will learn about the compound light microscope and how to use it
to enlarge the image of cells. You will also review the main parts of a cell
underneath the microscope. You will use your textbook and the information in
this lab as a reference.
Since its invention, the microscope has been a valuable tool in the development of scientific theory. Magnifying
lenses have been known for as long as recorded history, but it was not until the advent of the modern
compound light microscope that the device was used in biology. The microscope used most frequently in the
biology lab is the compound light microscope. The compound microscope is used to magnify and resolve
fine detail within a transparent specimen (one through which light can pass). The compound light microscope
is so called because it has two separate lens systems; an objective lens, located near the specimen, and an
ocular lens, located near the eye. Both the objective lens and ocular lens magnify the image of the object.
The total magnification observed by the human eye is the product of the magnification of the two lenses
(objective lens magnification multiplied by the ocular lens magnification).
The microscope is used to create an enlarged view of an object so that we can observe details not otherwise
possible with the human eye. Because of the enlargement, resolution is often confused with magnification but
they are not the same. Magnification refers to the size of an image. Resolution is the ability to clearly see
detail. In general, the greater the magnification, the greater the resolution, but this is not always true.
As previously learned, cells are small structures and most require observation through a microscope. This lab
will help you learn about the basics of microscopy and the importance of this tool when observing cellular
details. You will use a microscope to observe and compare different types of cells including plant, animal, and
bacterial cells.
The lab activity is divided into two main sections as outlined below. While you are working through the lab
activities you should be answering the Lab 2 homework questions. All of the questions on the lab homework
sheet should be answered while completing the lab activities.
Activity I: Identifying the Parts of the Microscope
The microscope gives you the opportunity to see a world of things you cannot see with the naked human eye.
To observe the clearest image you need understand how to use the microscope. In order to use a microscope
properly you must be familiar with the optical and mechanical parts. Please be careful when using the
microscope, always be sure to carry the microscope properly and observe the proper protocol when viewing a
slide.
We will also use the following website as a supplemental guide to this section of the lab. Please feel free to
visit the site before and/or after completing this lab. This will help you become familiar with the microscope, its
parts, and how it can be used to enlarge cellular images. http://www.udel.edu/biology/ketcham/microscope/
Please review the following terms.
1 | P a g e BIOLOGY 100 LAB PACKET A. Magnification is the process of enlarging something only in appearance, not in physical size. Magnification
is important because it allows us to view very small objectives that could not be seen with the naked eye. The
total magnification is calculated by multiplying the magnification of each lens.
B. Resolution is the ability to see detail. It is important because it allows differentiation between cells and
cellular structures.
C. Field of View is sometimes abbreviated "FOV", it is the diameter of the circle of light that you see when
looking into a microscope. As the magnifying power increases, the field of view gets smaller. You can measure
this by placing a clear metric ruler on the stage and counting the millimeters from one side to the other.
D. Image Formation refers to what you see when looking through the microscope and how the image is
manipulated when using the microscope.
ACTIVITY I PROCEDURE: IDENTIFYING PARTS OF THE MICROSCOPE
Obtain a microscope from your instructor. Be sure to transport the microscope correctly when you are carrying
it to your lab bench. Review the microscope parts list below. Identify each of the components listed and
practice using each component described. Be sure you understand the function of the parts listed in this
description. Answer the corresponding lab homework questions.
When completing parts of this activity, you will need to place the letter e slide in the stage holder to answer
some of the questions. Always be very careful when you are placing a slide on the microscope stage. After
you have securely placed the slide on the microscope begin by using the scanning objective lens (4x) to locate
the slide contents. It is important to begin with this lens because it will provide the largest field of view and it
will be easier to find the object. Once your object is clearly focused using the 4x objective, move to the next
highest objective (10x objective). After the image is in focus with the 10x objective, then move to the 40x
objective. Do not move from one objective power to the next until your image is focused. NOTE: We
will not be using the 100x objective (oil immersion lens) in this class.
Microscope Parts List:
2 | P a g e BIOLOGY 100 LAB PACKET 1. Adjustment Knobs
Your microscope has two adjustment knobs which are used to focus the specimen to be studied. The
largest is the coarse adjustment knob. This is used for rapid (or coarse) focusing of the specimen when
using the scanning objective lens (lowest powered objective lens, 4X). The coarse objective knob is
rotated until the specimen is roughly in focus and then left alone. The fine adjustment knob controls
precise focusing of the object and should be used after the object has been “found” using the course
objective. Only the fine adjustment knob should be used with the higher magnification objective lenses.
Moving the fine adjustment knob also helps you to determine the third dimension (depth) of the specimen
you are studying.
2. Stage
The stage holds the slide to be observed. Moving the coarse and fine adjustment knobs moves the
nosepiece or the stage up or down to bring the specimen into focus (this depends on the specific
microscope used).
3. Mechanical Stage
Your slide is fixed into position on the stage with the mechanical stage. The slide is fastened into the
mechanical stage by using a small lever located on the mechanical stage. Two knobs located on the side
of the mechanical stage are used to move the slide around to locate your specimen; one knob moves the
slide from side to side (horizontally) and the other moves the slide forward and backward (vertically). The
mechanical stage permits precision movements of your slide, especially nice when using the high
magnifications.
4. Condenser
The condenser, located below the stage, contains a system of lenses that focuses light on your specimen.
The condenser may be raised or lowered using the condenser knob. Most microscopes have a built-in light
source. Use caution to avoid having the light cord hang where you might trip over it.
5. Iris Diaphragm
The iris diaphragm is located on the condenser. The lever of the iris diaphragm is used to adjust the
amount of light striking the object being studied. It is important that you know the proper use of the
condenser and iris diaphragm. A common problem with microscope use is having too much light which
obliterates the object (more or less like trying to see something while looking directly at the sun).
6. Objective Lenses
When proper illumination is provided the resolving power of a microscope depends on the quality of its
objective lenses. Our microscopes contain four objective lenses (see table below). These lens are
mounted on a rotating turret or nosepiece. As you rotate the turret you will feel the lens "click" into position
for proper viewing. If a lens is not in position you will observe only darkness as you look into the
microscope.
Objective Lens
Scanning Lens
Low Power Lens
High Power Lens
Oil Immersion Lens
Magnification
4x
10x
40x
100x
Note: The magnification of the oil immersion lens requires using the lens with special immersion oil for proper
resolution. This lens will not be used in this class, however, it is commonly used in other, higher level
biology classes which require greater magnification.
7. Ocular Lens
The ocular lens, or eyepiece, further magnifies the image formed by the objective lens. It does not improve
resolution. Your microscope will have either a monocular system (one ocular lens) or binocular (two ocular
lens) system. The magnification of the ocular lens is 10x. Recall that the total magnification of the lens
system is the product of the magnification of the ocular times the magnification of the objective
lens being used (ocular X objective).
3 | P a g e BIOLOGY 100 LAB PACKET Be sure you can correctly identify the location and function of the above structures on the actual
microscope and/or virtual microscope before moving the next section.
Activity II: Viewing Different Types of Cells.
Now that you are familiar with the basic properties of microscopes and their components you will practice using
the microscope to observe different types of cells. Remember, all cells are classified as either Prokaryotic or
Eukaryotic. Although there are many different types of Prokaryotic cells, ALL Prokaryotic cells have specific
characteristics that belong to the prokaryotic group (no nucleus, etc). The same is true of Eukaryotic cells.
There are many different types of Eukaryotic cells (including animal cells, plant cells, fungal cells, etc) and they
all look and function differently. However as Eukaryotic cells they all have certain characteristics in common
(i.e. they all store their genetic information in the nucleus, they all have organelles, etc).
This portion of the lab will allow you to look at representative Prokaryotic and Eukaryotic cells through the
microscope. Be sure to do all required activities so you can answer the corresponding lab questions on your
lab 2 homework sheet. You will be viewing the following cells in the following order.
Remember, when viewing the slides underneath the microscope, you ALWAYS begin the observation with the
scanning objective lens (4x) which will provide the largest field of view. After you have focused the slide using
the 4x objective, move to the next highest objective (10x objective). Move to the 40x objective only after you
have focused the object using the 10x objective.
Cell Review:
A. Plant Cells are Eukaryotic cells. There are many different types of plant cells. You will be viewing
Elodea and Onion Cells. Although they are both plant cells, you will observe differences between the
cells. B. Animal Cells are also Eukaryotic cells. You will be looking at human cheek cells. These cells have
different functional properties than elodea. C. Euglena is also a type of Eukaryotic cell but unlike most other Eukaryotic organisms, it is a single celled
organism. D. Bacteria are abundant in our environment and in/on our body. Unlike human cells bacteria are
Prokaryotic. This is particularly helpful when trying to kill the pathogenic bacteria that can make us
sick. ACTIVITY II PROCEDURE: VIEWING DIFFERENT TYPES OF CELLS
Each pair of students should obtain one of each of the above slides (elodea slide, onion cell slide, human
cheek cell slide, euglena slide, and a mixed bacteria slide). Observe 1 slide at a time using correct microscope
viewing protocol. View the slides in the order listed above. While viewing the slides you should be
answering the corresponding Lab 2 homework questions. 4 | P a g e 
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