Prokaryotes and Eukaryotes
Comparing Prokaryotes and Eukaryotes
Purpose
In this lab you will study the major structural similarities and differences between prokaryotes and
eukaryotes, the two primary categories of living organisms. This examination will introduce you to the
microscope and visualization procedures of cells.
Introduction
Scientists currently agree that there are three major groupings of organisms on Earth called domains: the
Bacteria, the Archaea and the Eukarya. Bacteria are believed to be direct descendents of the most
ancient organisms on Earth, later giving rise to Archaea and Eukarya (Fig. 1). Bacteria and Archaea are
called prokaryotes because they lack a true nucleus (pro=before, karyon=nucleus) and do not have
organelles, highly ordered structures found in eukaryotic cells that are dedicated to functions like
respiration, photosynthesis or protein packaging.
Figure 1 Three domains of life: Bacteria, Archaea and Eukarya
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Prokaryotes and Eukaryotes
It is easy to believe that prokaryotes are the most simple of the existing organisms. They are essentially
invisible to the naked eye, they are capable of functioning as solitary cells and they lack organelles.
However, prokaryotes are the most widely distributed organisms, inhabiting the most benign and the most
extreme environments, from hot sulfur vents in the sea floor that reach over 100 °C to arid deserts, frigid
Arctic icebergs and highly acidic human stomachs. All modes of metabolism are represented by the
prokaryotes: respiration, fermentation, photosynthesis and anaerobic respiration. This wide range of
metabolic ability has allowed for bioremediation of radiation and toxic chemical spills.
Eukarya are the eukaryotes, organisms whose cells have a nucleus and organelles. Plants, animals,
fungi and protists are all eukaryotes. While many prokaryotes and eukaryotes can exist in a multi-cellular
state, only the eukaryotes have demonstrated a division of labor among the cells, the development of
tissues. In tissues, an aggregation of cells performs very specific functions, leaving other functions to
other tissues.
Today, you will compare the cell structures of Bacteria and Eukarya.
Technique
Proper use of the compound microscope
Microscopes use visible light and lenses to magnify an object. There are two lenses on a microscope, the
ocular lens, which magnifies things by 10 times (10X), and the objective lens (Figure 1). There are
usually four types of objective lenses to choose from on a typical microscope: 4X, 10X, 40X, and 100X.
The combination of these two lenses allows for a magnification between 40 (10 X 4) and 1000 (10 X 100)
times. Familiarize yourself with the parts of a microscope using Figure 1.
Figure 1 A compound microscope
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Prokaryotes and Eukaryotes
Preparing the microscope
1. Carry the microscope to your bench top with two hands, one under the microscope and one
holding the microscope arm.
2. Set the objective magnification to the lowest power, usually 4X.
3. Slowly crank the stage down using the course adjustment knob.
4. Place your slide of interest on the stage.
5. Turn on the light source with the on/off switch and set the brightness using the rheostat. Mid-level
light will be best at this magnification.
Viewing an object
1. Look through the eyepieces with both eyes. The eyepieces are designed so that your eyes
should be about 1-2cm away from the lenses. The distance between the eyepieces is adjustable.
2. Slowly crank the stage up using the course adjustment knob. Once the object on the slide comes
into focus, use the fine focus knob to make the image clearer.
3. You can now change the objective lens to a higher magnification. Do not change the height of
the stage when doing this.
4. When you view the image at higher magnification, you should only need to adjust the fine focus,
not the course focus. If the image was in focus at a lower magnification, it should generally be in
focus at higher magnification.
5. When using the 100X objective, always place a drop of oil on the slide. The 100X objective lens
will sit in this oil droplet to limit any light refraction. Be sure to wipe off the oil with lens paper
(NOT a Kim-wipe) when you are done with the 100X objective.
Putting away the microscope
1.
2.
3.
4.
Wipe off any oil on the 100X objective with lens paper (NOT a Kim-wipe).
Set the objective lens to the lowest magnification.
Crank the stage all the way down.
Turn off the light source. Wrap the cord loosely around the microscope. Place it back in the
cabinet.
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Prokaryotes and Eukaryotes
Name: _______________________
Class: _______________
Period: ___________
Date: ___________
Procedures
A. Examination of Eukaryotic Cells
In this laboratory session, you will examine live and stained representatives of eukaryotic cells: protists,
plant cells and animal cells. You should note characteristics such as size, shape and cellular structures.
You should be able to identify key features of these three major groups by the end of the exercise. At
labeled stations around the room, you will find representatives of eukaryotes to view under compound
microscopes. Do not move the field or change the magnification. You should only need to adjust the fine
focus. Fill in Table 1 with your observations about these cells.
Organism
Table 1 Comparison of Prokaryotic and Eukaryotic Cells
Magnification and
Cellular
approximate size
Drawing of Cell
structures?
relative to field
Bacteria
(a prokaryote)
Amoeba
(a protist)
Elodea
(a plant)
Red blood cells
Cheek Cells
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Prokaryotes and Eukaryotes
Name: _______________________
Class: _______________
Period: ___________
Date: ___________
Prokaryotes and Eukaryotes
Post-lab Assignment
1. What is the difference in the average size of a prokaryote to a eukaryote?
2. What eukaryotic cellular structures are visible under a microscope?
3. What prokaryotic cellular structures are visible under a microscope?
4. If you were to look under a microscope and identify whether the cell was a prokaryote or a
eukaryote, what characteristics would you look for?
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