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Microscopic Observation of Cells

Page history last edited by Derek Weber 12 years, 4 months ago

A.  Learning Objectives:

In this lab, students will:

• observe the similarities and differences between bacterial, protist, animal, and plant cells.

• recognize the different shapes and characteristics of prokaryotic cells.

• recognize the typical structures in animal cells.

• recognize the typical structures in plant cells.

• learn to recognize and use the parts of a compound microscope.

• determine magnification power.

• focus a microscope.

• make a wet mount.

 

B. Textbook Correlation: 

Please review Chapter 4: General Features of Cells when preparing for the lab.

 

C.  Introduction to observing cells using a microscope: 

Please write a two paragraph introduction to the microscope and cells.  Include a description of the cell theory, a history of the microscope, and describe the two basic types of cells.  When discussing the types of cells, make sure to include the six kingdoms of life and a brief description of each.

 

IMAGE OF PROKARYOTIC CELL HERE
IMAGE OF EUKARYOTIC CELL HERE
figure legend
figure legend

 

 

Study the following list when preparing to use a microscope:

1. Carry a microscope by supporting it in both hands in an upright position. One hand should grasp the microscope arm while the other provides a flat platform for its base. Place it down on a flat surface that has been cleared of debris.

2. If your microscope has a rheostat, which adjusts light intensity, make sure it is turned to minimum light intensity before turning on the light.

3. Clean all lenses by wiping with grit-free lens paper. Never use a paper towel or any other material to clean the lens to avoid making permanent scratches.

4. When you adjust focus, always begin with the lowest power first. Avoid contacting the lens to the microscope slide on the stage.

5. If your microscope is not working as you expect, notify your lab instructor immediately. Do not attempt to make repairs by removing or adjusting parts.

 

The Basic Parts and Functions of the Compound Microscope

As you examine the compound microscope, review Table 1, in which each part is listed and described. In the last column of the table, indicate which number on the microscope diagram corresponds to the location of that particular part.

 

Magnification in the Microscope

With two-lens systems, there is a simple method to figure the total magnification when using each objective. The total magnification is equal to the magnification of the ocular multiplied by the magnification of the objective.

 

Resolution. 

Resolution refers to the ability to distinguish two points as separate points. As you might think, resolution is dependent upon the distance between the two points. When using a microscope or any optical instrument, the closer the points are, the more difficult it becomes to see them as separate points. Numerical aperture is related to resolution, although magnification has very little to do with it. The higher the numerical aperture, the better the resolution will be. An object can be magnified larger and still be blurry, because although magnification has increased the resolution has not improved.

 

Focusing the Microscope:

There are some basic steps to follow when focusing the microscope.

1. Always begin with the objective that has the lowest magnification—the scanning objective. Turn the revolving nosepiece until this objective is over the stage.

2. Use the coarse adjustment knob to move the stage so that it is closest to the lens.

3. Place a slide on the stage and secure it with the clips.

4. While looking through the ocular, slowly move the stage away from the lens using the coarse adjustment knob.

5. The diaphragm can be used to increase or decrease the amount of light coming through the stage opening.

6. Be sure that you have the object clearly focused at this point.

7. Now you are ready to move to the next objective. The term parfocal is used to describe a microscope’s ability to remain in focus no matter what objective is used. The key to this parfocal ability is to always begin with the scanning objective.

8. Keep in mind that as you move up in magnification, you will not be able to use the coarse adjustment knob. The objectives are too long and will crack the slide. Use only the FINE ADJUSTMENT KNOB.

9. Rotate the revolving nosepiece to the next objective—low power. Use only the fine adjustment knob to sharpen the focus.

10. Now that you have the slide in focus at this power, move the revolving nosepiece to the next objective—high power—and repeat the process described in step 9.

11. The oil immersion lens can only be used with immersion oil. It is commonly needed to see very small objects such as bacteria.

Your First Observation: Observing Prokaryotic Cells

Prokaryotes have existed for at least 3.8 billion years and are thought to be the first cells on earth.  Bacteria are the most common prokaryotes.  There are more bacteria on and within our bodies than we have human cells.  Most bacteria are non-pathogenic (non-disease causing) and actually are used in making a variety of useful products.  Bacteria can take a variety of shapes and sizes.  Three basic shapes commonly found among members of this kingdom include bacillus, coccus, and spiral.  The term bacillus (bacilli plural) describes a bacterium that is rod shaped or cylindrical.  An example of a bacillus shaped bacterium includes Clostridium botulinum, the causative agent of botulism and the source of the drug Botox.  A bacterium that is symmetrically spherical is termed coccus.  Examples of cocci shaped bacteria include Streptococcus pyogenes, the causative agent of strep throat, and Staphylococcus pneumoniae, the causative agent of both bacterial pneumonia and meningitis.  Spiral shaped bacteria have one or more twists.  A spirochete is a bacterium with multiple twists that gives it a corkscrew-like appearance.  Treponema pallidum is a spirochete that causes the STD syphilis.

 

Bacteria can also take a variety of arrangements.  Recall that these are single celled organisms.   They reproduce through cell division.  In some cases, after the cells divide, they remain in contact with one another.  For example, the arrangement of two cells that remain in contact is termed diplo-.  Bacteria arranged as long chains are called strepto- and bacteria that are clustered are described as staphylo-.    For example, if you observe bacillus shaped cells arranged in a chain, you would describe them as steptobacillus. 

 

Beside shape and arrangement, bacteria are commonly classified by their cell wall structure.  Some bacteria have a thickened cell wall structure made of the carbohydrate peptidoglycan.   This type of cell wall is referred to as a Gram (+) cell wall.  Other bacteria have a very thin layer of peptidoglycan that is surrounded by an additional outer membrane.  These bacteria are called Gram (-) bacteria.  The origin of these names comes from a staining technique developed by Hans Christian Gram.  In this technique, bacteria with the thickened cell wall are colored purple and the bacteria with the outer membrane are colored red/pink.  This method is used extensively as a diagnostic tool to identify bacteria.

Procedure:

1. Observe the slides that demonstrate the three different shapes of bacteria. Upload your photos and describe the observed shape and arrangement.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

2.  Observe the Staphylococcus aureus specimen.  Upload your photos and describe the observed shape and arrangement.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Practicing Your Craft:   Observing Yeast

It is time to practice using the microscope. We will use a member of the Fungi kingdom called Saccharomyces cerevisiae in this exercise.  Members of the Fungi kingdom are eukaryotic and can be unicellular (yeast) or multicellular (molds).  Their cell walls contain the polysaccharide chitin.  They play the role of decomposers in the ecosystem and are essential for recycling nutrients back to the soil.  In this exercise, we will observe the yeast S. cerevisiae using three different objective lenses. 

 

Upload your photos at each  magnification and explain the differences that you have seen using each objective.

 

S. cervisiae 

Total Magnification =    

S. cervisiae

Total Magnification =

 

 

S. cervisiae

Total Magnification =

 

 
 

 

Preparing A Wet Mount

Most of the microscope slides that you will be studying in the lab will be prepared slides. That is, they have been sectioned, dehydrated, and stained by professional histologists and purchased for your educational use. Staining brings out the structural details of specimen.

 

In some cases, a wet mount slide may be prepared in order to view objects that do not require sectioning and dehydration for observation. The advantage is its simplicity and quickness, providing a sample that can be observed within minutes of obtaining it. Clinical labs use this procedure with body fluids, such as blood and urine. In this exercise, you will learn how to make a wet mount slide.

 

Protist cells:

Many protists inhabit fresh water and are characterized by their mode of movement.  The three basic motile structures found in protists include flagella, cilia, and pseudopods.  A few are pathogenic like Plasmodium vivax, the causative agent of malaria and Giardia intestinalis, the causative agent of giardiasis.  Most are free living and will be the focus of this part of the lab.

 

Follow the instructions below to prepare and observe a wet mount:

 

1. With the slide on the bench top, place a drop of pond water onto the center of the slide.

 

2. Using the figure on the right as a guide, place one edge of the coverslip near the preparation, allowing the coverslip to slightly contact it. Then gently lower the coverslip onto the preparation; do not drop it, or air bubbles will form in the preparation that will make it difficult for viewing. If excess fluid pushes outside of the coverslip, blot it with a paper towel to keep your microscope stage dry.

 

3.  Many of the microorganisms that live in fresh water are characterized by their motility.  Start with the scanning objective to provide the greatest field of view.  After finding a specimen, observe at low power and draw what you find.  Describe the mode of movement observed.

 




Plant cells:

1. Use your tweezers to remove a small leaf from the growing tip of an Elodea sprig.

2. Make a wet mount.

3. Locate the cells with low power. Then focus on a small number of cells using high power.

4. Take a phot of your specimen and locate the following structures and label them in your photo below:

chloroplasts: small bright green organelles

cell wall

central vacuole: this will appear as a clear area without chloroplasts

nucleus: it is slightly larger than the chloroplasts and is clear or very light brown – NOT green

 

 

 

 

5. Also observe cytoplasmic streaming in these cells as indicated by chloroplasts moving about the cell, usually around the outer edge of the cell.   Obtain a 10-20 second video clip of this phenomenon  and describe what the viewer is observing.

 

 

 

 

Animal Cells:

1. Using the broad end of a toothpick, lightly scrape the inside of your cheek.

2. Stir the scraping in a drop of methylene blue on a microscope slide. Cover with a coverslip.

3. Place the slide on the microscope stage and find the cells using low power. Then switch to high power.

4.  Obtain a photo of your specimen.  Identify and label the nucleus, cytoplasm, and plasma membrane.

 

 

 

D. Presentation

For your presentation, discuss the different kingdoms observed today (Bacteria, Protist, Fungi, Plant, Animal).  In your presentation include the type of cell found in the kingdom (prokaryotic/eukaryotic), cellularity (single of multi-). common environmental niche and other distinguishing characteristics.  Show your specimens as part of your presentation.  For example, for fungi, you may want to discuss how molds reproduce by spores produced by and show your image of  the mold Penicillium to demonstrate.

 




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