Lab Protocol: The Cell
Last week we practiced using a compound light microscope and a dissecting microscope and investigated how magnification affects field of view, illumination, depth of field and resolution. This week, our goal will be to compare the cells from different organisms representing the three domains of life. We will focus on the sizes and shapes of cells and on identifying the subcellular structures that we can see under the microscope.
Safety Precautions
● Be careful when handling glass slides, the edges may be sharp.
● Dispose of used cover slips in a glass disposal box.
● Observe proper use of the microscope; avoid handling the electric cord with wet hands.
● Do not use the coarse adjustment knob of the microscope at higher magnifications.
● Inform your teacher immediately of any broken glassware as it could cause injuries.
● Wash your hands with soap and water after handling live organisms.
●Used cotton swabs are considered biohazard; dispose of swabs in the biohazard trash container as soon as you
have used them.
● Methylene blue is a dye; be cautious not to ingest methylene blue.
Exercise 1 Comparing cell types by preparing a wet mount slide
In this simple experiment, you will prepare a wet mount slide to view and compare several different cell types using a light microscope. In lecture, you learned about the three domains of life, and you learned about prokaryotic and eukaryotic cells. Now it is time to see them in action!
To prepare a wet mount you will use a glass microscope slide, water, and a cover slip. A cover slip is important whenever viewing a specimen to prevent dirtying the microscope lens, to slow the evaporation of the sample, and to flatten out the sample. You will have a choice between four different organisms to observe. Ideally, each table will have four microscopes set up with one organism at each microscope.
The four microorganisms available for viewing are:
- Brine shrimp/cysts (an animal)
- Gleocapsa (a cyanobacteria)
- Paramecium (a protist) – add a drop of Proto-Slo before placing the coverslip.
- Chlamydomonas (an alga) – can you see the flagella?
A good technique for preparing the slide:
- Mix the solution containing the specimens by pulling up and blowing out liquid several times using a transfer bulb before placing a single drop of the liquid on the microscope slide.
- Place the coverslip on the microscope slide close to the droplet and lower slowly to cover the specimen (see figure below). It may take some practice to reduce the number of air bubbles trapped under the cover slip.
- Remove excess liquid using a Kim wipe. The slide is now ready to be placed on the microscope for viewing.
A good technique for finding the specimen under the microscope:
- Place the microscope slide on the stage and secure using the stage clip. Be sure to gently close the stage clip to avoid breaking the microscope slide.
- Turn on the light source and begin scanning the slide using the low power objective to search for organisms and brine shrimp eggs.
- Once a specimen of interest has been located, use a higher power objective lens to see increased detail. You may want to use a drop of Proto-Slo if the protists are moving too quickly.
- Shift to the highest magnification that allows you to see the entire organism or cell then you may begin determining the specimen’s size.
Now draw a diagram of the specimen being careful to draw its size accurately relative to the field of view. Record the total magnification used to view the specimen.
You are now ready to determine the specimen’s size using the technique described under the previous heading.
Total Magnification:
Name of specimen:
Size of specimen on diagram:
Diameter of field of view:
Actual size of specimen:
Exercise 3 Prepare your cheek cell slides
a. Take a clean cotton swab and gently scrape the inside of your mouth.
b. Smear the cotton swab on the center of the microscope slide for 2 to 3 seconds.
c. Add a drop of methylene blue solution (a dye) and place a coverslip on top.
d. Remove any excess solution by allowing a paper towel to touch one side of the coverslip.
e. View the slide at all magnifications.
f. Record your observations as drawings. Use color if present, label the magnification, and estimate the size of
the cells in your notebook. Record your observations (drawings, color if present, labels, magnification, and
size of cell) in your notebook.
