BSC1005-CellsAndMicroscopyLab
Cells and Microscopy
In this exercise, you will examine several types of cells, including plant and animal cells, protists, diatoms and other pond inhabitants. Cells range in size from 5 micrometers (e.g., a bacterium) to several centimeters in diameter (e.g., a chicken egg). The cells that you will observe today are in the range of 100-500 ųm and are too small to be observed with the naked eye. Before you can observe cells, you must first become familiar with use of the compound microscope.
Using the Compound Microscope
The compound microscope is a precision instrument used to observe objects too small to detect with the unaided eye. In this exercise, you will learn to use the microscope correctly. You will also observe several different kinds of cells.
Locate the following parts on your microscope and learn the function of each one.
1. Base
2. Arm
3. Ocular lens (eyepiece). The magnification of the ocular lens is 10x (10 times). You have a binocular microscope, meaning that it has two ocular lenses. Note the pointer placed in one of the eyepieces.
4. Revolving nosepiece – holds the objective lenses, often with the following
magnifications:
· scanning lens 4X
· low power lens 10x
· high power lens 40x
5. Note that the total magnification of an object being viewed is the power of the ocular lens multiplied by the power of the objective lens in use.
6. Course focus adjustment knob – the large knob on the side of the microscope that is used with the scanning lens to locate a specimen.
7. Fine focus adjustment knob – the smaller knob on the side of the microscope that is used to obtain a sharp focus.
8. Mechanical stage – area where the slide is placed and held by the slide holder. The slide holder is controlled by a pair of knobs located at the edge of the stage. One knob moves the slide from side to side and the other moves the slide forward and backward.
9. Condenser with iris diaphragm – located below the stage. The condenser can be raised or lowered with a knob. The condenser concentrates the beam of the light from the illuminator toward the specimen and therefore should be all the way up, flush with the opening in the stage. The iris diaphragm can be adjusted to control the amount of light entering the objective. Adjustment of the position of the condenser and the diameter of the iris diaphragm provides the best light for viewing a particular specimen. If the specimen is thin or transparent, close the iris diaphragm, reducing the opening. If the specimen is thick or darkly stained, open the iris diaphragm.
10. Substage illuminator and light switch – located in the base of the microscope.
Photo courtesy of Gary Hill
Revolving nosepiece
Ocular lenses
Stage
Condenser
Iris diaphragm
Base
Course focus adjustment
Fine focus adjustment
Light switch
Objective lens
Arm
Handling the microscope
Always handle the microscope with care. Read and observe the following rules of microscopy:
1. Use both hands to transport the microscope. Support the base with one hand while holding the arm with the other. Ocular lenses are removable and thus can be dropped and damaged if you carry the scope with one hand.
2. Use only lens paper to keep the lenses clean. Do NOT wipe the lenses with anything other than lens paper or you will scratch them.
3. Always maximize the working distance between the objective and the stage before placing or removing a slide. This helps you avoid ‘ramming’ the objective into the slide.
4. Always begin focusing with the lower power lens in place. Once in focus, then you can switch to the next higher power lens.
5. Never use the course focus knob when a high power lens is in place.
6. Always use a cover slip when observing a wet mount preparation.
7. Before storing the microscope, remove the slide from the stage, rotate the lower power objective into place and maximize the working distance. Fold the electrical cord and cover the microscope with its dust cover.
Tips for effective scientific illustration
When you examine specimens through the microscope, you will observe many more details if you draw what you are seeing. There are a number of guidelines for drawing specimen.
· Always use pencil. Do not use colored pencils.
· Use unlined paper for your scientific illustrations.
· Keep your pencil sharp. Unsure of your draftsmanship? Sketch lightly first, then
darken the drawing later. USE THE ENTIRE AVAILABLE SPACE FOR EACH
DRAWING. Don’t be shy.
· Don’t leap into the drawing without first looking at the specimen. Study the specimen
under scanning power then examine it more closely under higher power.
· Label all identifiable structures. Make sure that the line points directly to the
structure.
· Draw only what you see. Do not label a structure that is not visible.
· In the bottom right hand corner of the page, list all pertinent information such as the identity of the specimen and the total magnification.
· Do NOT draw a circle representing the field of view.
· Take your time and really look at the specimen that you are drawing!
If you are unsure about what you see under the microscope, ASK YOUR LABORATORY INSTRUCTOR to help you identify the material. Don’t wait to find out you’ve just drawn a piece of sludge, when you were looking for an amoeba.
If you look closely, you will notice that an earthworm does not really look like a fat piece of brown spaghetti. It has delicate segments, a mouth, an anus, bristles and all kinds of interesting details. LOOK FOR THOSE DETAILS! And draw them.
Right: Wrong:
clitellum
Drawing from a prepared slide:
· Gently polish the slide with a Kimwipe before you put it on the microscope stage. Start on low power and work your way up.
· Switch back and forth from low to higher powers so that you see both The Big Picture and the Tiny Details.
· When you’re finished, place the slide back in its proper tray on the display table.
Observations of living cells
A. Elodea
Procedure:
1. Examine the video link. The cell will appear rectangular due to the rigid border of each cell known as the cell wall. The numerous green circular structures inside are the chloroplasts. Chloroplasts are the site of photosynthesis. As the light warms the slide, the chloroplasts appear to circle around the central vacuole (see video link above). This is called cyclosis or cytoplasmic streaming.
2. Draw two Elodea cells in the space below. Label the cell wall and chloroplasts.
Analysis / Data:
B. Onion epidermal cells
Procedure:
1. Examine the onion slide. The cell nucleus should be readily visible as a stained dot along the border of the cell . The nucleus contains the cell’s DNA. These cells also have a rigid structure due to the cell wall.
2. Draw two onion cells in the space below. Label the nucleus and cell wall.
Analysis / Data:
C. Human epithelial cells
Procedure:
1. Examine the cheek slide. The cell nucleus is the large circular structure in the center of the cell. The nucleus contains the cell’s DNA. Due to the lack of a cell wall the border is not rigid. The cell membrane is a semi-permeable flexible barrier.
2. Draw two cells in the space below. Label the nucleus and cell membrane.
Analysis / Data:
D. Pond water organisms
Procedure:
1. Examine the pond water video.
2. Draw and name four different organisms below. Pond water can contain a wide variety of organisms. Do your best to identify the organisms using the illustrations that follow as a guide. The organisms you see may not be illustrated.
Questions
1. Why do the Elodea cells have chloroplasts while the onion cells do not?
2. How does the shape of the Elodea contrast with that of the Onion cell?
3. How does the shape of the Elodea contrast that of the Human Cheek Cell?
4. What structure accounts for the differences in shape?
Representative Pond Water Organisms
Algae and Diatoms
Volvox Anabaena Micrasterias
Euglena
diatoms and dinoflagellates
Diatom Diatoms Scenedesmus
Ciliates
Stentor
Vorticella
Paramecium
Amoebas
Flagellates
Animals
copepod
Daphnia
rotifer
amphipod ostracod Hydra
oligochaete flatworm nematode
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