Download Observing Living Cells

Name: ________________________
Date: _________________________
OBSERVING LIVING CELLS
Pre-Lab Discussion
Cells are as basic to biology as atoms are to chemistry. All organisms are made of cells. Organisms are either
unicellular (single-celled), such as most bacteria and protists, or multicellular (many-celled), such as
plants, animals, and most fungi. Because most cells cannot be seen without magnification, people's
understanding of cells and their importance is relatively recent.
Human understanding of nature often follows the invention and improvement of instruments that extend
human senses. The development of microscopes provided increasingly clear windows to the world of cells.
Light microscopes, the kind used in your classroom, were first developed and used by scientists around 1600.
In a light microscope, visible light passes through an object, such as a thin slice of muscle tissue, and glass
lenses then enlarge the image and project it into the human eye or a camera.
In 1665, an English scientist named Robert Hooke observed "compartments" in a thin slice of cork (oak bark)
using a light microscope. He named the compartments cells. Actually, Hooke was observing the walls of
dead plant cells. Many more observations by many other scientists were needed to understand the
importance of Hooke's discovery. By 1700, Dutch scientist Anton van Leeuwenhoek (LAY vun hook) had
developed simple light microscopes with high-quality lenses to observe tiny living organisms, such as those
in pond water. He described what he called "animalcules" in letters to Hooke and his colleagues.
For the next two centuries, scientists, using microscopes, found cells in every organism they examined. By
the mid-1800s, this evidence led to the cell theory—the generalization that all living things are composed of
cells, and that cells are the basic unit of structure and function in living things. Later, the cell theory was
extended to include the concept that all cells come from pre-existing cells.
Cells contain structures called organelles that carry out life processes. Cells can be classified by the types
of organelles they contain. In plant and animal cells, similarities and differences exist because of varied life
functions. In this investigation, you will compare the structures of a typical plant cell (Elodea) and a typical
animal cell (human).
Problem
How are plant and animal cells alike? How are they different?
Materials (per group)
Forceps
Medicine dropper
Elodea leaf
Water
Microscope
Glass slide
Coverslip
Toothpicks
Methylene blue stain
Paper towel
Lens paper
Safety
Always handle the microscope with extreme care. You are responsible for its proper care and use. Use
caution when handling glass slides as they can break easily and cut you. Always use special caution when
working with laboratory chemicals, as they may irritate the skin or cause staining of the skin or clothing.
Never touch or taste any chemical unless instructed to do so.
Procedure: Part A – Examining Plant Cells
1. Read all of the following directions for Part A before you do anything.
2. Obtain all of the necessary materials and bring them back to your lab table.
3. Take a microscope from the storage area and place it about 10 cm from the edge of the lab table. Plug
in the microscope in the nearest power outlet.
4. Carefully clean the eyepiece and objective lenses with lens paper.
5. Place a drop of water in the center of a clean glass slide.
6. With forceps, remove a leaf from the Elodea plant and place it on the drop of water on the slide. Make
sure that the leaf is flat. If it is folded, straighten it with the forceps.
7. Carefully place a coverslip over the drop of water and the Elodea leaf.
8. Place the slide on the stage of the microscope with the leaf directly over the opening in the stage.
9. Using the low-power objective lens, locate the leaf under the microscope. Turn the coarse adjustment
knob (the bigger one) until the leaf comes into focus.
10. Switch to the high-power objective lens. CAUTION: When turning the high-power objective lens, you
should always look at the objective from the side of your microscope so that the objective lens does not
hit or damage the slide.
11. Observe the cells of the Elodea leaf. In the circle below, draw and label a detailed picture of what you
see. Write a brief (2 sentence) description of the colors, shapes, and other patterns you notice in each
of the Elodea cells. Do not forget to record the magnification of the microscope.
Magnification: _______ X
Description of Cells:
Procedure: Part B – Examining Animal Cells
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Read all of the following directions for Part B before you do anything.
Carefully clean and dry your slide and coverslip using the lens paper.
Place a drop of water in the center of your glass slide.
Using the flat end of a toothpick, gently scrape the inside of your cheek. CAUTION: Do not use force
when scraping the inside of your cheek. Only a few cells are needed. The end of the toothpick will have
several cheek cells stuck to it even though you may see nothing but a drop of saliva.
Stir the water on the slide with the end of the toothpick to mix the cheek cells with the water. Dispose
of the toothpick in the trashcan.
Put one drop of methylene blue stain on top of the drop of water containing cheek cells. CAUTION: Use
care when working with methylene blue to avoid staining hands and clothing.
Wait one minute, and then carefully place a coverslip of the stained cheek cells.
To remove the stain from under the coverslip and replace it with clear water, place a piece of paper
towel at the edge of one side of the coverslip. Then place a drop of water at the edge of the coverslip of
the opposite side. The stained water under the coverslip will be absorbed by the paper towel. As the
stain is removed, the clear water next to the coverslip on the opposite side will be drawn under the
coverslip. Discard the paper towel after it has absorbed the stained water. See the figures below for
help with this procedure.
9. Place the slide on the stage of the microscope with the center of the coverslip directly over the opening
in the stage.
10. Using the low-power objective lens, locate a few cheek cells under the microscope. NOTE: You will need
to reduce the amount of light coming through the slide in order to see the cells more clearly. Adjust
the diaphragm as necessary.
11. Switch to the high-power objective lens. CAUTION: When turning the high-power objective lens, you
should always look at the objective from the side of your microscope so that the objective lens does not
hit or damage the slide.
12. Observe some cheek cells. In the circle on the next page, draw and label a detailed picture of what you
see. Write a brief (2 sentence) description of the colors, shape, and other patterns you notice in each of
the cheek cells. Do not forget to record the magnification of the microscope.
Magnification: _______ X
Description of Cells:
13. Carefully clean and dry your slide and coverslip.
14. Return your microscope to the storage area.
Analysis and Conclusions:
1. What is the general location of the nucleus in an Elodea cell?
2. What is the general location of the nucleus in a human cheek cell?
3. Based on what you observed in this lab, how are plant and animal cells similar in structure?
4. Based on what you observed in this lab, how are plant and animal cells different in structure?
5. Why are stains such as methylene blue used when observing cells under the microscope?
Critical Thinking and Application:
6. What is an advantage of using a wet-mount preparation instead of a dry-mount preparation in the study
of living cells?
7. In general, the tissue of a tree has a harder “feel” than does the tissue of a dog. The presence or
absence of what cell structure can be used to explain this difference?
8. Explain why it would not work to use an oak leaf in this investigation.