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A Closer Look
REA DI
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ACTIVIT Y OVERVIEW
SUMMARY
A reading elaborates upon the basic structures common to all cells. The roles of the cell
membrane, cytoplasm, and nucleus are emphasized. The relationship between cell biology and disease is presented.
KEY CONCEPTS AND PROCESS SKILLS
1.
All living things are composed of microscopic units called cells.
2.
Cells of different organisms have similar structures, such as the cell
membrane. These structures function similarly in different organisms.
3.
The function of the cell membrane is to control what can enter or leave the
cell. Cell membranes are selectively permeable; some particles pass through
but not others.
4.
The cell membrane compartmentalizes the cytoplasm from its external
environment; the nuclear membrane protects the genetic material in the
nucleus.
Key Vocabulary
cell
cytoplasm
cell membrane
nucleus
cross-section
organelles
Teacher’s Guide
C-173
Activity 42 • A Closer Look
MATERIALS
For the teacher
*
1
overhead projector
For the Extension
For the teacher
2
green peppers or apples
different colored clays or soft, chewable candies
plastic knives or dental floss
*
1
Transparency 42.1, “Same Cell—Different Slices”
1
overhead projector
*Not supplied in kit
TEACHING SUMMARY
Getting Started
1.
Review what students have learned so far about cells from their observations
and from the cell membrane modeling activity.
Doing the Activity
2.
Students complete the reading.
Follow-Up
3.
Discuss the similarities and differences among cells.
Extension
Students model a cross-section of a cell.
BACKGROUND INFORMATION
Common Cell Structures
Some subcellular structures, such as the cell membrane and the genetic material, are
common to all cells due to their indispensable roles. The genetic material is on a chromosome (introduced in the “Our Genes, Our Selves” unit of Science and Life Issues) that
is free in a bacterial cell; the chromosomes are enclosed in a nucleus in animals, plants,
fungi, and protists. Organisms that have nucleated cells are referred to as eukaryotes,
while organisms that lack nuclei are referred to as prokaryotes. Certain other organelles
that characterize eukaryotic cells—such as mitochondria, chloroplasts, lysosomes,
cilia—are not present in all cells, or are found in dramatically varying numbers, because
of the diverse requirements of different cell types.
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Science and Life Issues
A Closer Look • Activity 42
TEACHING SUGGESTIONS
GETTING STARTED
1.
Follow up the role of membranes by discussing:
Why is the nuclear membrane so important to the
nucleus? Membranes within the cell create compartments where specialized activities can occur.
Review what students have learned so far
The nuclear membrane provides one example, as it
about cells from their observations and
isolates the genetic material on the chromosomes
from the cell membrane modeling activity.
from the rest of the cell. In fact, several other major
Prepare students for the reading by reviewing concepts introduced in the last few activities. Questions
that may provide some review include: What
organelles, such as mitochondria, chloroplasts,
lysosomes, and Golgi apparatus, are also membrane-bound.
organisms are made of cells? All organisms, such as
FOLLOW-UP
plants, animals, and microbes, are made of cells.
What kinds of things do all cells do? How did we
investigate this question? In Activity 39, “Cells
Alive!” students investigated the ability of cells to
respire, demonstrating that cells perform life functions. What does the cell membrane do? All cells
have a cell membrane that acts as a barrier, allowing
only some substances to enter and leave the cell.
DOING THE ACTIVIT Y
2.
Students complete the reading.
n Teacher’s Note:You may choose to have students
complete the reading either as homework or as an
3.
Discuss the similarities and differences
among cells.
Use the reading as the basis for a discussion of the
unity and variety of cells. All human cells contain
certain common elements, in particular the cell
membrane, nucleus, nuclear membrane, and cytoplasm. (One exception is the mature red blood cell
in mammals, which lacks a nucleus.) Differences in
internal structure and the diversity of cell roles
within the human body will be further explored in
Activity 46, “Disease Fighters,” when students
examine various blood cells.
in-class exercise. Develop the idea that different
Also introduce the idea that humans and many other
cells contain many structures that help them func-
multicellular organisms are made of many kinds of
tion. The role of specific structures (such as chloro-
cells. These different cells have different structures
plasts and cell walls in plant cells) within different
and perform different functions for the body.
groups of organisms is addressed in Unit E, “Ecology,” of Science and Life Issues. A number of
websites provide beautiful photos, short
movies, and information on
cells. The SALI page of the SEPUP
website provides links to several
of the many sites available.
Also discuss the use of electron microscopes to
obtain much greater magnifications of cells. In
order to view the insides of cells by transmission
electron microscopy, very thin sections must be
obtained from specimens embedded in plastic and
treated with special stains. Transparency 42.1,
“Same Cell—Different Slices,” provides a view of a
Teacher’s Guide
C-175
Activity 42 • A Closer Look
generic protist that has a flagellum, but is not pho-
intact cells or to view internal structures of broken
tosynthetic. Point out that the nucleus is observed
cell preparations.
only in those sections of the cell that actually pass
through the nucleus. You can extend the discussion
to consider different shapes and sizes of nuclei in
different shapes of cells. Draw students’ attention
to the section of the flagellum. Point out that even
this tiny structure has substructure visible at high
magnification. The tubular structures visible in the
cross-section of the flagellum perform a sliding
action that is responsible for the motility of the fla-
You can demonstrate this idea by first making crosswise and lengthwise slices of two apples or green
peppers. Students can see that the structure of the
apple or pepper changes depending on whether it is
cut through the center or the end. Similarly, when
they focus up and down with the light microscope,
or when a thin section is made, some structures may
not be represented.
gellum. While it is not necessary that students
Students can make models of cells by using clay or
remember this detail, you can use it to make the
soft chewable candies of different colors. They
point that cells have many structures that carry out
should take a pea-sized bit of clay and form a round
cell activities. Scanning electron microscopy, as
or ellipsoid nucleus. Then they can wrap a larger
used to obtain some of the images provided in Fig-
piece of clay around the nucleus. Since cells come in
ure 2 on page C-62 in the Student Book, provides a
many shapes, have different students model differ-
view of the surface of the cell.
ent shapes: spherical, torpedo-shaped, star-shaped
(like an amoeba), cuboidal, or flat pancakes. Stu-
Extension
dents can then use plastic knives or dental floss to
Students model a cross-section of a cell.
Many students have difficulty understanding the 3dimensional nature of cells and interpreting what
they see through a light microscope or in an elec-
slice through their model cells in different locations. Have them note differences between the different slices, in terms of overall shape and whether
the nucleus is visible.
tron micrograph. Single cells, such as protists or
As students explore their models, you may want to
human cheek cells, can often be observed through
discuss the following question with the class: Some
a light microscope as unstained or stained whole
microscope slides contain cross–sections of speci-
mounts, and need not be cut into thin sections.
mens. However, most microbes do not need to be
However, for light microscopy of tissues, the cells
cross–sectioned to be seen through a classroom
must often be embedded in a solid material, such as
microscope. What happens to a microbe when it is
wax or plastic, and sliced into sections for viewing.
pressed between the slide and the coverslip? How
The cells shown in the Introduction to the activity
would this affect what you see through the micro-
on page C-58 in the Student Book were prepared in
scope? Use this question to foreshadow the pre-
this way. For transmission electron microscopy,
pared slides students will observe in Activity 43,
very thin sections must be prepared, even when sin-
“Microbes Under View.” Since the microbes in that
gle cells are to be investigated. Scanning electron
activity are so small, many of these single–celled
microscopy can be used to view the surfaces of
organisms fit onto a single slide. However, not every
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Science and Life Issues
A Closer Look • Activity 42
specimen looks identical (particularly the Amoeba)
b. These flat cells form an even covering on the sur-
because of the various angles at which the organ-
face of areas like the inside of the mouth.
isms have been preserved. You can demonstrate this
Cell 3.
idea by making additional clay cells in different
shapes and then pressing them flat. Point out that
c. These round human cells are unusual because
the angle at which the cell is preserved will change
they do not have a nucleus. They are full of a pro-
its appearance on the slide.
tein that carries oxygen to all parts of the body.
Cell 4.
Conclude by emphasizing the point that interpretation of microscopic images of cells requires an
d. These cells are able to crawl around the body to
understanding of the effect of viewing 3-dimen-
attack bacteria and other foreign material. Ruf-
sional structures as if they were flat and, in some
fles on the cell membrane lead the way as the
cases, of viewing only a thin slice of a cell. When
cells move.
scientists view cells under the microscope, they
Cell 1.
must consider how the cells were prepared to make
the slide when they make conclusions about what
they see.
2.
Based on its description, which of the four
cells described in Question 1 is a nerve cell?
Which is a red blood cell? Which is a white blood
SUGGESTED ANSWERS
TO ANALYSIS QUESTIONS
1.
cell? Which is a skin cell? Explain how you were
able to match the type of cell with its function.
Observe the pictures of cells in Figure 2,
Cell 2 is a nerve cell. Signals travel down the
“Animal Cells.” These photos of four different ani-
long cell process (axon), causing a chemical sig-
mal cells were taken with an electron microscope.
nal (neurotransmitter) to be released at the end
Cells 1, 2, and 4 were taken with a scanning elec-
of the cell (synapse). This particular neuron is
tron microscope which shows the surface (and not
called a neurosecretory cell, because it releases
the inside) of the cell. This type of microscope mag-
its signaling molecules, called hormones, into
nifies the cells much more than the microscopes you
the bloodstream.
use in class. You can see that the cells have quite different shapes: some are rounded, while others are
elongated, flat, or ruffled. These shapes depend on
the cells’ functions in the body. Try to match each
cell with one of the following descriptions.
a. These cells have long branching parts that send
signals to distant parts of the body.
Cell 2.
Cell 4 is a red blood cell. The shape of the cell
helps it to squeeze through the tiny capillaries
of the circulatory system. (It also increases the
surface area available for the diffusion of oxygen.) Hemoglobin, the protein that carries oxygen, contains iron and is responsible for the red
color of oxygenated blood. Students will learn
about the function of red blood cells in Activity 50, “Fighting Back.”
Teacher’s Guide
C-177
Activity 42 • A Closer Look
Cell 1 is a white blood cell. The cell is moving
tance of cellular neurobiology to the study of
to the right. The membrane forms ruffles,
Alzheimer’s disease, and the unregulated cell
which surge ahead in the direction in which the
growth characteristic of cancer.
cell is moving as the cell probes the surrounding blood plasma for foreign invaders. Students
4.
will learn about the functions of white blood
The cell membrane separates the cell from its
cells in Activity 50, “Fighting Back.”
surroundings. Everything that enters or leaves
the cell must cross the membrane. In addition,
Cell 3 is a skin cell. The technical term for cells
the cell membrane mediates communication
that line bodily surfaces is “epithelial.” Their
between cells.
flattened shape is a clear functional adaptation.
3.
Explain why membranes are so important to cells.
Give one example of how the study of cells helps
5.
“Looking for Signs of Micro-Life.” Did you observe
treat diseases.
any structures within the microbes that you drew?
Student answers will vary, but are likely to cen-
What do you think these structures are?
ter around the examples given in the reading.
While student answers will vary, it is possible
For example, learning to influence the forma-
that students observed the nucleus in some of
tion of red blood cells reduces the need for
the microbes.
blood transfusions, and the study of the normal
functioning of the immune system assists in the
fight against AIDS. Other possible answers
include the role of developmental biology in
preventing avoidable birth defects, the impor-
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Science and Life Issues
Look back at your drawings from Activity 36,
6.
Reflection: Which of the questions studied by cell
biologists is most interesting to you? Why?
Student answers will reflect their own areas of
interest.
©2006 The Regents of the University of California
Same Cell—Different Slices
Science and Life Issues Transparency 42.1
C-179