4 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.
Prokaryotes, unicellular organisms lacking a nucleus, include cyanobacteria (formerly blue-green algae. This name is now considered inaccurate because algae are eukaryotes. Cyanobacteria, like those shown in the figure below, are photoautotrophs—organisms that carry out photosynthesis by using light energy, water, and carbon dioxide from the air and converting to sugars, and providing oxygen to the atmosphere as a waste product. Cyanobacteria contain pigments capable of capturing light energy but do not contain chloroplasts. Cyanobacteria are single-celled organisms, but some can form colonies with differentiated cell types. These cells range from 1–40 micrometers in size. Not all bacteria can carry out photosynthesis there are many species of heterotrophic bacteria living virtually everywhere on Earth. These cells
are much smaller ranging from 0.5–8 micrometers. Eukaryotic cells have many more features and organelles and range in size from 10–500 micrometers
Plant cells are eukaryotic; they have subcellular organelles. Like the bacteria, they have a cell wall to help keep the cell rigid—in plants, the cell wall is composed of a complex carbohydrate called cellulose. Plant cells, like that shown in the figure below also have a nucleus with DNA, a large central vacuole full of water and other important substances for maintaining life such as carbohydrates, non-nutrients, wastes, and help maintain cell pressure.
Animal cells are eukaryotic and possess subcellular components in common with the plant cells. Organelles that plant and animal cells share in common include the nucleus, Golgi apparatus, mitochondria,
ribosomes, and the endoplasmic reticulum. These are all participants in protein synthesis. An illustration of an animal cell is shown below. There are some exceptions to these general components. For example, mature red blood cells (RBC) which have ejected their nuclei to have more room for hemoglobin, the protein that carries oxygen around the body. One of the easiest eukaryotic cells to obtain in the lab is the squamous epithelial cell (your cheek cells). These cells are arranged in a flat layer and are easy to remove and observe.
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 Preparing a wet mount and viewing under the microscope
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. A solution with brine shrimp eggs/ brine shrimp and a pond water sample will be available for students to prepare fresh slides. Students will mix the solution 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. One side of a cover slip is then placed on the microscope slide close to the droplet and lowered slowly to cover the specimen. It may take some practice to reduce the number of air bubbles trapped under the cover slip. Excess liquid can be removed using a Kim wipe. The slide is now ready to be placed on the microscope for viewing.
Scan 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. Do not determine the specimen’s size until you shift to the highest magnification that allows you to see the entire organism or cell.
Preparing a wet mount figure by Dalia Salloum created in BioRender licensed under Creative Commons Attribution-NonCommercial 4.0 International
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.
