Lab: Cells
What do animal and plant cells look like?
Introduction
Cells are the basic functional units of
all living organisms. They may exist singly or in
aggregates. When cells join together to take on a specialized function
within a larger
organism, they form a tissue.
There are 2 major divisions into which
all cells fall - prokaryotic (no organized
nucleus or organelles with membranes) and eukaryotic (nucleus and membraned
organelles present). Bacteria make up the former division while the
cells of plants,
animals, fungi, protozoa, and algae comprise the latter.
Animal and plant cells share many characteristics,
which you will observe in this
lab. They also differ in several important ways. In this lab you will:
1. examine
characteristics common to both plant and animal cells;
2. identify
the differences between plant and animal cells;
3. observe
the microscopic characteristics of eukaryotic cells;
4. classify
several unknown specimens as plant or animal cells.
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Prelab Preparation
Both animal and plant cells may occur
unicellularly or within multicellular organisms.
Because they often take on specific functions within tissues, animal
cells
are frequently more specialized than plant cells. Epithelial cells and
blood cells are
examples of different tissues.
In this lab, you will look at epithelial
cells in both plants and animals. Epithelial cells
form the skin of the outer body surfaces and the linings of the inner
surfaces.
These cells are specialized for transportation of substances and protection.
The
individual cells of these layers may be shaped like cubes, columns,
or be flat -
depending on their location and function.
Blood cells, although also common in
higher animals, appear very different
from other cells due to their specialization for transporting oxygen
or fighting infection.
Blood is a tissue in which the cells are maintained and transported
by a liquid plasma.
Red blood corpuscles, also known as erythrocytes contain hemoglobin
a protein that
transports oxygen through the body. As an erythrocyte matures, the nucleus
and
organelles of the cell diminish. Eventually they are forced out of the
cell. One possible
explanation for this behavior is the diminished tasks required of an
adult blood cell. Its
entire function centers around simply floating through blood vessels
with oxygen
attached to its outer coating of hemoglobin. White blood cells, also
known as
leukocytes, are a diverse collection of cells. Platelets are small cell
fragments that
are important in blood clotting.
1. How is the basic structure of plant
and animal cells different from the basic
structure
of bacteria?
2. As the cells of an organism become
more specialized is the diversity of the
cells in
that organism increased or decreased? Explain.
3. Almost all tissues found in plants
and animals, including skin, muscle, bone,
fat, and
nerve, the individual cells are arranged in an orderly pattern,
often in
direct contact with their neighboring cells.
a.
How are blood cells arranged uniquely?
b.
Why are they still considered a tissue if their arranged so uniquely?
4. Can an erythrocytes be classified as
complete cells when:
a. immature?
b.
mature? c.
Explain a & b.
Procedure
Part 1: Plant Cells
Onion bulbs are organized tissue that,
under the appropriate conditions, will
give rise to an entire plant. The curved pieces that flake away from
a slice of onion are
called scales. On the underside of each scale is a thin membrane called
the epidermis.
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A. Obtain a piece of
onion and remove one of the scales B. Add a drop of iodine solution onto the membrane and place a coverslip over it. C. Examine the epidermis first with the low power objective of
your microscope. Try to determine the number of 5. How many layers thick is the
epidermis? |
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7. Look inside an individual onion cell
and try to locate the nucleus and cytoplasm.
Look inside nearby
onion cells. There should be a common pattern to the
positioning of
the nucleus and cytoplasm within each cell. Describe this pattern.
D. Now examine your specimen under high power.
8. What does the nucleus look like under
high power?
9. Make a diagram of several cells as
observed under high power. Label the
following
structures in one cell: nucleus, cell wall,
cytoplasm.
E. Locate a single leaf of Elodea (from the young leaves at the tip)
and carefully pull
it off the rest of the sprig (whole plant).
Prepare a wet mount of it by placing the
leaf on a slide, adding a drop of water,
and covering with a coverslip.
10. What does Elodea look like under low power?
F. Examine the chloroplasts under high power.
11. What does a single chloroplast look
like?
12. In living Elodea cells you can often
see chloroplasts moving around the cell
near
its outer cell wall. Why might these chloroplasts be free to move about
inside
Elodea cells?
13. Consider the size, shape, color, and
positioning of nuclei for both onion
epidermis
and Elodea cells.
a.
In what ways are these two cells similar?
b.
In what ways are these two cells different?
14. List 2 characteristics that can be
used to help identify (classify) a specimen as a
plant?
Part II: Animal Cells
As you observe the different types of
cells, think of how differences in cell
structure might be related to specialization of cell function. To observe
specialization
in animals you will study slides of two common cells; epithelial cells
and blood cells.
| G. To make a slide of epithelial cells from the human
oral cavity you need to obtain a toothpick,
some methylene blue stain, a slide and
a coverslip. First GENTLY scrape the inside
of your cheek with the tip of a toothpick. Next,
place a drop of methylene blue stain onto the
slide. Now swirl the used tip of the toothpick in the drop of stain (cheek cells will be washed off into the drop of stain). Finally place a coverslip over the drop of stain. Examine the cells first under low power and then under high power, and compare to the diagram of typical epithelial cells shown to the right. |
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15. Inside the mouth, these cells are
joined together in a sheet. Why are they
scattered
here?
16. Draw a few cells and label the cell
membrane, nucleus, and cytoplasm.
| H. Obtain a prepared slide of blood
and examine it under low and high
power. This specimen has been treated
with Wright Stain, which causes erythrocytes
to appear pink. About 1 % of the pink-stained
structures are reticulocytes, immature
erythrocytes that still have their nuclei. The
nuclei in these cells will be stained deep blue or purple and the endoplasmic reticulum will look like a blue mesh. Leukocytes appear blue and platelets are violet or purple. |
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17. How many different cell types can
you see?
18. Blood cells and epithelial cells are
both examples of animal cells.
Consider
the size, shape, color, and presence of organelles for both types
of
cells.
a.
List two ways in which these two types of animal cells are different?
b.
List one way in which these two types of animal cells are alike?
19. Were you able to observe organelles
such as a nucleus, mitochondria, vacuoles,
etc. in either type of animal cell?
I. Obtain 3 slides of unknown specimens and examine each under low
and
high power.
20. Fill in the data table for your unknowns.
Postlab Analysis
21. List two characteristics you observed in animal cells but not in
plant cells.
22. List two characteristics you observed in both animal and plant cells.
23. White blood cells maintain their nuclei throughout their whole lives
while red blood
cells lose their nuclei once
they mature. Give one possible explanation why white
blood cells might maintain
their nuclei throughout their lives.
24. Briefly describe the basic structure of the following cells related
how the structure
of each cell helps it perform
its function.
a. Onion
cells
b. Blood
cells
25. Onions are classified as green plants. Why then was the onion epidermis
you
looked at not green?