Cell Biol Educ 3(1): 11-14 2004
DOI: 10.1187/cbe.04-01-0027
© 2004 American Society for Cell Biology
WWW.Cell Biology Education
Robert Blystone
Department of Biology, Trinity University, San Antonio, Texas 78212
Each quarter, Cell Biology Education calls attention to several
Web sites of educational interest to the life science community. The journal
does not endorse or guarantee the accuracy of the information at any of the
listed sites. The sites listed below were last accessed on December 26,
2003.
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CELL SIZE
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The interplay of size, growth, division, and diffusion by cells is a common
topic in Introductory Biology classes, though often covered superficially.
This issue's column will focus on teaching resource materials about cell size.
The review begins with a list of four recent articles on cell size that are
available as open access PDF files. These articles provide an overview as to
the latest thinking about the topic and will be discussed at the end of this
article.
- C. M. Coelho and S. J. Leevers (2000). Do growth and cell division rates
determine cell size in multicellular organisms? J. Cell Sci. 113,
2927-2934.
(http://jcs.biologists.org/cgi/reprint/113/17/2927.pdf)
- S. J. Day and P. A. Lawrence (2000). Measuring dimensions: the regulation
of size and shape. Development 127, 2977-2987.
(http://dev.biologists.org/cgi/reprint/127/14/2977.pdf)
- I. J. Conlon and M. C. Raff (2003). Differences in the way a mammalian cell
and yeast cells coordinate cell growth and cell-cycle progression. J. Biol.
2(1): Article 7.
(http://jbiol.com/content/pdf/1475-4924-2-7.pdf)
- E. Hafen and H. Stocker (2003). How are the sizes of cells, organs, and
bodies controlled? PloS Biol. 1, 319-323.
(http://www.plosbiology.org/plosonline/?request=get-document&doi=10.1371%2Fjournal.pbio.0000086)
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GIVING DIMENSION TO THE CELL
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The Biology Project http://www.biology.arizona.edu/cell_bio/tutorials/cells/cells2.html
The Biology Project at the University of Arizona is a good place to begin
our review of Web resources on cell size. In a brief tutorial on "Size
and Biology" (Figure 1),
the sizes of animal and plant cells are related to bacteria, virus, and
molecules. Information is also provided about light and electron microscopy as
they relate to visualizing cells.
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Figure 1. Relative sizes of cells. Image courtesy of Dr. William Grimes, Department
of Biochemistry, University of Arizona.
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Cells Alive http://www.cellsalive.com/howbig.htm
The Web site Cells Alive has been providing interesting Web-based
educational graphics for ten years. Figure
2 provides a representation of the navigation screen for viewing
cell dimensions. By clicking on the magnification button, various items from
pollen to E. coli are given size context by relating each to the head of a
pin. Image courtesy of Jim Sullivan, Quill Graphics, 568 Taylor's Gap Road,
Charlottesville, VA 22903.
Biology 1000—Kean University http://www.kean.edu/~biology/cellcalc.html
Dr. Bruce Reid of Kean University has created a simple method for
calculating cell size using a ruler and microscope.
Figure 3 illustrates how the
microscope magnification is determined with a transparent ruler (upper view).
Cells are then counted (lower view) and resultant data entered into an
interactive calculator that gives the average cell size in millimeters and
microns. Image courtesy of Dr. Bruce Reid, Dept. of Biology, Kean University,
Union, New Jersey.
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GEOMETRY, DIFFUSION, AND CELL SIZE
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Many lab activities that explore cell size and shape are based on the
geometry of the cell; to be more specific, the relationship between surface
area and volume. Working with cell size in this manner is one of the few times
that geometry is discussed in introductory biology classes. Three sites are
listed below that address geometry issues of cell sizing.
World Builders http://curriculum.calstatela.edu/courses/builders/lessons/less/les4/cellsize.html
Dr. Elizabeth Viau of Cal State Los Angeles asks the question: "How
big can cells get?" She provides some graphics that explore cell
geometry in a straight-forward lab activity. Three geometric shapes are
explored with calculations.
Secondary Classroom Science Concepts http://educ.queensu.ca/~science/main/concept/biol/b02/b02lapl4.htm
The same approach as above is echoed in a lab sizing activity developed at
Queens University in Ontario: "Could the world really be overtaken by a
giant amoeba?" is the catch phrase used to draw students into the
exercise.
Fellows Collection: Access Excellence @ the National Health Museum http://www.accessexcellence.org/AE/AEC/AEF/1996/deaver_cell.html
James Deaver provides a nice hand-on exercise where the student creates a
model cell out of paper. Figure
4 gives an idea of how graph paper can be used to better visualize
surface area to volume relationships in cells. Image courtesy of James Deaver,
Access Excellence.
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EMPHASIZING DIFFUSION
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Another approach for visualizing limits to cell size is to focus on
diffusion. The following two URL's outline an exercise based on an indicator
reaction. Phenolphthalein-treated agar blocks are cut into cubes and placed
into a sodium hydroxide solution. With the agar blocks serving as a metaphor,
visual evidence is provided of how far the basic solution diffuses into the
agar "cell."
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Figure 5. Users explore the architecture of a cell, adjusting size and selecting
cell-surface scenarios. (Used by permission of McGraw-Hill.)
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Cell Size Lab - Life Science Resources - Lapeer County, Michigan http://chem.lapeer.org/bio1docs/CellSize.html
In addition to the agar exercise, you may wish to explore the main site
(http://lapeer.org/)
which is a community resource for schools and education. Nineteen different
educational entities provide Web resources on a countywide level. Ten
libraries are also listed as part of this very interesting community site.
How Big Would You Want to Be If You Were a Cell - The SMILE Program http://www.iit.edu/~smile/bi9226.html
The cell diffusion exercise at this Web site is essentially the same as the
one above. The main site
(http://www.iit.edu/~smile)
is quite extensive. With a project titled "SMILE" (Science and
Mathematics Initiative for Learning Enhancement), a team head by Dr. Porter
Johnson of the Illinois Institute of Technology of Chicago has assembled over
900 activities and lesson plans involving science and mathematics teaching.
This innovative site is worth browsing for teaching ideas, primarily at the
6-12 grade levels.
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PUTTING THE IDEAS TOGETHER
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Textbook publishers such as McGraw-Hill develop interesting support
materials for their products. At a commercial Web site titled "Johnson
Explorations" the idea of cell size, diffusion, and surface
modifications are brought together in an interactive exercise developed for
the Raven and Johnson Introductory Biology textbook (ISBN 0-07-303120-8).
Johnson Explorations - Cell Size http://www.mhhe.com/biosci/genbio/biolink/j_explorations/ch02expl.htm
The site opens with a round cell. There are interactive controllers for
villi, "dimples", and cell shape. The surface area to volume ratio
as well as diffusion efficiency can be read out.
Figure 5 is an example of an
adjusted cell. Image courtesy of McGrawHill publishers. (pending) Students can
create a number of cell size and cell surface scenarios with the interactive
model.
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SUGGESTED CELL SIZE LAB
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T. Ryan Gregory of the Department of Entomology of London's Natural History
Museum has recently assembled what he calls a Cell Size Database. The database
contains an extensive collection of vertebrate erythrocyte (red blood cell)
sizes arranged by Class, Order, Family, Genus, and species. The database may
be found at the following Web site.
Cell Size Database http://www.genomesize.com/cellsize/
Figure 6 represents the
information available within the database. Image courtesy of Dr. T. Ryan
Gregory, The Natural History Museum, London. Mammalian erythrocytes are
generally round with diameters varying from as little as 2.1 µm to as much
as 10.8 µm. Other classes of vertebrates have erythrocytes that are
generally elliptical with some amphibian species ranging up to 70 by 41 µm
in size. In addition to the average size of the cell, the volume is also
given. If an instructor is looking for a database that provides information
suitable for statistical analysis by students, this would be an excellent
place to go. A variety of hypotheses could be developed about Family and Order
variation in erythrocyte size. With so many vertebrate species having red
cells with diameters in excess of 10 µm, one wishes to see a corresponding
database for mean capillary diameters for the same species. A very delicious
study could be performed relating capillary size to red cell diameter.
The database contains information about the Mammalian family of organisms
known as Camelidae (the camel group). Many people mistakenly think that the
erythrocytes of this mammalian family have nuclei, which in the mature form,
they don't. Rather, the cells of this mammalian family are elliptical instead
of round. Llamas have red blood cells that are 9 by 4.5 µm with a mean
volume of 28 µm3. Camel red blood cells are 8.1 by 4.3 µm
with a mean volume of 27 µm3. The llama-like vicuna erythrocytes
have mean dimensions of 7.1 by 3.9 µm with a volume of 31
µm3.
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FUTURE DEVELOPMENTS IN CELL SIZE LABS
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The lab exercises referred to in this review suffer from a common
shortcoming. None of these exercises explore the relationship between growth
and cell division with that of cell size. None of them try to address
signaling pathways that lead to the characteristic size of the cell under
study. This very rich area of study may be summarized by the earlier cited
paper (see reference one in the first section of this article) of Coelho and
Leevers (2000) and represented in Figure
7. Image courtesy of Dr. Sally Leevers of the Cancer Research UK
London Research Institute.
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Figure 7. Representation of relationship between cell growth and cell division
(Coelho, 2000). Reproduced by permission.
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Besides posing these four possible pathways leading to reproducible cell
size, Coelho and Leevers describe the interplay between various cyclins and
their regulating kinases. They also explore the rich literature concerning the
insulin/P1 3-kinase signaling pathway relationship with cell growth in
Drosophila imaginal discs. Day and Lawrence (2000) likewise review the
literature dealing with fly wing cell growth (please see reference two in the
first section). They also recall the literature about the relationship of
ploidy and cell size in salamander. Tetraploid salamanders have cells that are
twice the size of diploid salamanders, which leads to interesting results for
learning demonstrated in maze tests. Tetraploids have fewer neurons and take
longer to learn a route through a maze. The cell size may be different but the
body organ size is the same.
Conlon and Raff (2003) have reported that in rat Schwann cells, cell growth
is independent of cell size. They also argue that for Schwann cells the
hypothesized cell-size checkpoints operate very differently than has been
reported for yeast cells. Hafen and Stocker (2003) ask "Why is an
elephant bigger than a mouse?" Their review reminds readers that
ribosome biogenesis is an important regulator of multicellular organism cell
size.
Web teaching resources dealing with cell size are confined mainly to the
physical dimensions of cells. It would be extremely useful to expand the
teaching resources to include the signaling events and relationships to the
cell cycle. If anyone is aware of such teaching resources please contact me at
rblyston{at}trinity.edu.
If you want to comment on the selections or suggest future inclusions, please
send a message to the above address.
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CELL SIZE
GIVING DIMENSION TO THE...
