Microscope Lab – Day 1 – Prokaryotes – Diluted Yogurt Bacteria
Name: _____________________________________ Period:__________
Understanding the nature of cell structure and function is important to an understanding of organisms. All organisms are composed of cells, whether they exist as single cells, colonies of cells, or in multicellular form. Cells are usually very small, and for this reason, a thorough understanding of subcellular structure and function has been possible only through advances in electron microscopy and molecular biology.
There are two general types of cells: prokaryotic and eukaryotic. These two words have their root in the Greek word karyon (nut), which refers to a cell's nucleus.
The prefix pro- means "before" or "prior to." Thus, prokaryotic means "before having a nucleus." Prokaryotic cells do not have a membrane-bound nucleus and their genetic material (DNA) is only loosely confined to a nuclear area within the cell. Bacteria, including the cyanobacteria (formerly known as blue-green algae), are prokaryotes.
All other organisms are eukaryotes. The prefix eu- means "true." The cells of eukaryotes have true, membrane-bound nuclei containing their genetic material.
Prokaryotic and eukaryotic cells also differ in several other ways. Eukaryotic cells are generally larger and contain additional specialized compartments (membrane-bounded organelles) in which cell functions such as energy production may occur Prokaryotic cells lack membrane-bound organelles; their cell functions are carried out in the cytoplasm.
During this two day laboratory you will investigate some of the structural of prokaryotic and eukaryotic cells.
SAFETY
Do not consume yogurt products in lab. Eating is not permitted in lab.
PROCEDURE
Part A: Prokaryotic Cells: Observing Bacteria
Most prokaryotic cells are extremely small (approximately 1 to 2 μm in diameter). Most are heterotrophic, depending on preformed food, but some are autotrophic and make their own food. Morphologically, bacteria are either round (coccus), rod-shaped (bacillus), or spiral-shaped (spirillum). To view them with the compound microscope, you must use an oil-immersion lens (100x objective). Even then, not much more than their basic shapes will be visible. With the aid of the electron microscope, however, you can study these prokaryotic cells more closely. You can even use special staining techniques to learn about their structure.
Example Cells
Figure: The cells of many familiar genera of bacteria include the (a) rod-shaped bacillus, (b) spherical coccus, and (c) helical spirillum.
You can use the compound microscope to study bacteria, but only their external features will be distinguishable. It is possible to identify the three morphological types of bacteria (coccus, spirillum, and bacillus) by observing their shape (Figure). You will also note that bacteria are often found in clusters or in chains.
Bacteria are classified as follows:
Arrangement: Shape:
paired = diplo round = coccus
chained = strepto rod: bacillus
clusters = staphylo spiral = spirillus
1. Clean slides and cover slips for dust and other particles.
2. Place a very small portion of plain yogurt onto the slide.
3. Add one drop of water to the yogurt and place the cover slip on top.
4. Under low power, find a section where the yogurt is pretty thin; this is where you will find the bacteria. Sketch view on worksheet. The diaphragm setting should be very low (small) because these bacteria are nearly transparent.
5. Switch to high power (40X for most microscopes) for a better view of the bacteria. Sketch view on worksheet.
6. Identify any bacteria you might find. For example, a common inhabitant of yogurt is a paired, round bacteria or diplococcus.
7. Clean the slides thoroughly after usage.
Draw simple sketches of three different prokaryotes, if you can find 3, focusing on shape of the cells. Sketch the prokaryotic cells in the spaces below. For each, note whether the bacteria are spherical (coccus), rodshaped, (bacilus), or spiral-shaped (spirillum). Also, tell me the arrangement that you see of the bacteria. Don’t forget to write in the total magnification.
Specimen: __________________________
Magnification: _______________________
Shape and Description: ________________
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Specimen: __________________________
Magnification: _______________________
Shape and Description: ________________
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Specimen: __________________________
Magnification: _______________________
Shape and Description: ________________
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How many different kinds of bacteria could you find?
What other foods have bacteria living in them?
What are doctors looking for in strep throat samples (cultures)? Think of the full name of the bacteria.
What did you notice about these cells? [Hint: Mr. B mentioned it at the end of October/ beginning of November.]
How do you think these bacteria cells will be different from your cheek cells?