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LAB 3 – CELLS
Cells are the basic functional units of all living organisms. They may exist singly, as in unicellular
organisms, or in aggregates, as in multicellular organisms. Living things are grouped into three domains.
Bacteria and Archaea contain all prokaryotic cells (organized nucleus absent). The domain Eukarya
contains all eukaryotic cells (organized nucleus present) that are found in plants, animals, fungi and
protists.
Both animal and plant cells occur within multicellular organisms. In a multicellular organism each
different cell type has a specialized function. Similar cells that work together form tissues. Epithelium,
mesophyll, and blood are examples of different tissues. In this lab you will look at epithelial cells in both
plants and animals. Epithelial cells form the covering of the outer body surfaces or the linings of the inner
surfaces. These cells are specialized for secretion of various substances and for protection. Mesophyll cells
are found only in plants. It is the tissue that is responsible for photosynthesis and storage in leaves. Blood
cells are found only in animals. Blood is a tissue in which separate cells are maintained and transported by a
liquid called plasma. In humans erythrocytes (red blood corpuscles) contain hemoglobin for oxygen
transport. As an erythrocyte matures, the nucleus and organelles of the cell are expelled, and the cell
becomes entirely filled with hemoglobin. Leukocytes are a diverse collection of white blood cells that work
together to fight infection and maintain immunity. Platelets are small cell fragments that are important in
blood clotting.
Objectives
In this investigation you will examine and identify similarities and differences in animal and plant cells.
By observing the microscopic characteristics of eukaryotic cells you will be able to infer the functional
significance of specific cell structures and cell types.
Prelab
1.
2.
3.
4.
What is the basic structural feature that distinguishes plant and animal cells from bacteria?
Why do multicellular organisms need to have many different cell types?
In what way do blood cells resemble unicellular organisms? Why are they considered a tissue?
Explain why mature erythrocytes can’t be classified as cells.
Hypothesis
What are the microscopic structural features that will allow you to determine if a cell is from plant
tissue or animal tissue? List at least two observable structures.
Materials
Red onion
Iodine solution
Sprigs of Elodea
Microscope slides and coverslips
Straight-edged razor blade or scalpel
Flat-edged toothpick
Compound microscope
Prepared slides of human blood
Prepared slides of three unknown tissues
Procedure
A. Epithelium
1. Onion cells
Onion bulbs are condensed, underground stems that will give
rise to an entire plant under the appropriate conditions. The curved
pieces that flake away from a section of onion are called scales. On
the inside of each scale is a thin, transparent layer called the
epidermis. Place a dime-sized piece of this tissue into a drop of
iodine solution on a glass slide. Try to smooth out any wrinkles, then
gently drop a coverslip onto the tissue.
Examine the epidermis first with the low power objective of your
microscope. In the space to the right describe how the tissue looks. If
the circular field of view is 1500 μ across its diameter, how long is
one cell? How many cell layers are there? Switch to high power and
observe the ultrastructure of an individual cell. On the data sheet
make a labeled diagram of one onion cell as observed under high
power.
2. Cheek cells
The cells lining the mouth in humans are called squamous
epithelium. They form a flat sheet. Place a small drop of iodine
solution on a glass microscope slide. Use a toothpick to gently scrape
the inside of your mouth to dislodge a few cells from the sheet of
tissue. Gently swirl the toothpick into the drop of iodine solution on
the slide and add a coverslip. Discard the toothpick.
Examine the cells first with the low power objective of your
microscope. In the space to the right describe how the tissue looks.
Estimate the size of the cells. Switch to high power and observe the
ultrastructure of an individual cell. On the data sheet make a labeled
diagram of one cheek cell as observed under high power.
B. Mesophyll.
Obtain a single leaf of Elodea from the young leaves at the tip of
the stem. Place it into a drop of water on a microscope slide and add
a coverslip. No iodine will be used since we want the cells to remain
alive.
In the space to the right describe how the tissue looks.. Estimate
the number of layers of cells and the size of the cells. Switch to high
power and observe the individual cells for several minutes. Describe
the relative size, shape and movement of the chloroplasts. On the
data sheet make a labeled diagram of one mesophyll cell as observed
under high power.
C. Blood
Observe a prepared slide of human blood under low power. The
specimen has been treated with Wright’s stain which causes
erythrocytes to appear pink. Leukocytes appear purple, and platelets
appear as dark blue/purple cell fragments. Switch to high power.
Move the slide around to observe the many different kinds of blood
cells. On the data sheet make a labeled diagram of several different
blood cells as observed under high power.
Notes
D. Unknowns
Examine each of the three prepared slides of unknown plant or animal tissue. Focus first on low
power, then switch to high power and move the slide around to observe different cell types within each
tissue sample. On the data table below indicate whether each specimen is from plant or animal tissue
and the reason for your decision.
Data
A. Classification of unknowns
Unknown
Classification
Reason for Classification
A
B
C
B. Plant and animal tissues
1. Onion cells
Label – nucleus, cell wall, central vacuole
cytoplasm
Size of cell _____________________________
Number of cell layers _____________________
2. Cheek cells
Label – nucleus, cell membrane, cytoplasm
Size of cell _____________________________
3. Mesophyll
Label – nucleus, cell wall, central vacuole
cytoplasm
Size of cell _____________________________
Number of cell layers _____________________
4. Blood
Label – erythrocytes, various leukocytes,
platelets
Sketches
Analysis
A. Epidermis
1. Describe the appearance of the onion tissue (not individual cells) as seen under low power.
2. In an onion cell, describe the location of the nucleus and cytoplasm. How can we infer the presence
of a central vacuole from their location?
3. What could account for the granular appearance of the cytoplasm?
4. Describe the appearance of the cheek tissue as seen under low power. Why aren’t the cells
connected on the slide as they are in your mouth?
5. In a cheek cell, describe the location of the nucleus and cytoplasm.
6. In what observable ways are cheek cells and onion cells similar?
7. Explain why the cheek cells have an irregular shape compared to the boxy shape of the onion cells.
B. Mesophyll
1. Describe the appearance of the mesophyll tissue as seen under low power.
2. What does a single chloroplast look like? Where are they located in the Elodea cells?
3. Describe the movement of the chloroplasts. What causes this movement?
4. In what ways are onion epidermis and Elodea mesophyll tissues similar?
5. Describe at least two ways that the tissues differ.
C. Blood
1. Describe the relative number of each cell type in blood tissue.
2. How many different types of leukocytes did you observe? How did you distinguish the different
types of white blood cells?
3. In what ways are blood cells similar to the cheek cells you observed?
4. How does blood tissue differ from cheek epithelium tissue?
Sources of Error
Inexperience is the main source of error in this lab. Describe ways in which this problem may have
caused errors in your observations and data gathering.
Conclusion
1. All cells have a plasma membrane. Why is this structure difficult to observe in the plant cells we
studied?
2. What two cell components could you observe (especially if you stained the cells) under high power
in all plant and animal cells except red blood corpuscles?
3. Describe three differences you observed under the microscope between plant and animal cells.
4. All plants, including onions, are photosynthetic. Which parts of the onion plant have cells with
chloroplasts?
5. How is the shape and arrangement of the cells in the onion tissue related to its function as an
epithelial tissue?
6. How are the Elodea cells related to their function in a mesophyll tissue?
7. Why is blood considered to be a tissue?