Download Which one Will Dry out Last?

7
Structure and Function
Which One Will Dry
Out Last?
Cocci
Bacilli
Spirilla
Bacterial cells have three common shapes—cocci, bacilli, and spirilla. It is important
for bacteria to have a moist environment so they do not dry out. Under hot, dry conditions, which of the bacterial cell shapes is most likely to dry out last if each cell has
the same volume? Circle your answer.
A Cocci
B Baccilli
C Spirilla
D They would all dry out at the same time.
Explain your thinking. What “rule” or reason did you use to select your answer?
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Uncovering Student Ideas in Life Science
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7
Structure and Function
Which One Will Dry Out
Last?
Teacher Notes
Purpose
The purpose of this assessment probe is to elicit
students’ ideas about loss of water through
cells. The probe is designed to reveal whether
students recognize the relationship between
shape, surface area, and volume.
Related Concepts
Cells, cell shape, bacteria, single-celled organisms, volume, surface area
Explanation
The best answer is A—cocci. The answer is best
explained through geometry. Cocci, sphericalshaped bacteria, have the lowest surface area to
volume ratio. Given the same volume, spheres
have the smallest surface area of any shape. The
low ratio of surface area to volume results in less
water loss through the cell. The ratio of surface
area to volume is important to cells because
more contact with the environment through
the surface of a cell relative to its environment
increases water loss. Long, thinner cells, like the
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spirilla, have a greater ratio of surface area to
volume and therefore would dry out first.
However, bear in mind this is the “best”
answer, based on general shape, but shape
alone cannot be used to predict the rate of drying for some bacteria. Bacteria have a variety
of physical and biochemical defenses against
desiccation. For example, two bacilli of the
same size and shape can have very different
rates of drying. Moreover, many bacteria can
form very resistant spores that defy predictions
based on shape alone.
Curricular and Instructional
Considerations
Elementary Students
In the elementary grades, students observe a
variety of plant and animal cells and describe
their shapes. They learn about the need that
all living things have for water. In mathematics they learn about and distinguish between a
variety of three-dimensional shapes.
N a t i o n a l S c i e n c e Te a c h e r s A s s o c i a t i o n
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7
Structure and Function
Middle School Students
Middle school students deepen their knowledge of cells, including single-celled organisms.
They learn that cells are mostly made of water
and that cells must control the passage of water
in and outside of the cell. They learn about the
basic characteristics of bacteria and develop
a notion that bacteria are much smaller than
the familiar cells they have observed. In mathematics, they develop an understanding of surface area and volume and apply this to several
different phenomena in science. For example,
water will evaporate more quickly from an
open container that has an opening with a
large surface area than from a container of the
same volume with a smaller opening. They also
deepen their knowledge of ratios at this level
and use ratios to describe relationships.
High School Students
High school students deepen their understanding of the factors that affect the passage of
water in and out of a cell. They learn about the
cell membrane in more detail. They explore the
ratio of surface area to volume in cells, organs,
and organisms and use it to explain several
important biological ideas. Using mathematics, they can compare surface areas of different shapes and predict what happens when the
surface areas increase while the volume stays
the same.
Administering the Probe
This probe is most appropriate for high school
students. Show students microscopic photos
of the three different shapes of bacteria. Please
note that it is difficult to illustrate volume in a
two-dimensional drawing. Make sure students
know that the volume is the same for all three
cells in this example by explicitly stating this
to the students. It is the shape that changes.
Related Ideas in National
Science Education Standards
(NRC 1996)
K–4 The Characteristics of Organisms
• Organisms have basic needs. For example,
animals need air, water, and food; plants
require air, water, nutrients, and light.
Organisms can survive only in environments where their needs can be met.
5–8 Structure and Function in Living
Systems
• All organisms are composed of cells—the
fundamental unit of life. Most organisms
are single cells.
• Cells carry on the many functions needed
to sustain life.
9–12 The Cell
• Cells have particular structures that underlie their functions. Every cell is surrounded
by a membrane that separates it from the
outside world.
Related Ideas in Benchmarks
for Science Literacy
(AAAS 2009)
K–2 The Cell
• Most living things need water, food, and
air.
3–5 The Cell
• Some living things consist of a single cell.
Like familiar organisms, they need food,
water, and air; a way to dispose of waste;
and an environment they can live in.
6–8 Scale
• Some properties of an object depend on its
length, some depend on its area, and some
depend on its volume.
Uncovering Student Ideas in Life Science
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7
Structure and Function
6–8 The Cell
• All living things are composed of cells,
from just one to many millions, whose
details usually are visible only through a
microscope.
• About two-thirds of the weight of cells is
accounted for by water, which gives cells
many of their properties.
9–12 Scale
 Because different properties are not affected
to the same degree by changes in size, large
changes in size typically change the way
that things work in physical, biological, or
social systems.
9–12 The Cell
• Every cell is covered by a membrane that
controls what can enter and leave the cell.
Related Research
• This question is similar to one used by
Livne (1996) in which high school biology students were asked about the cooling of milk. They were given a spherical
baby bottle and a cylindrical baby bottle
and told that they each contained the
same amount of milk. They were then
asked which bottle of milk would cool
off fastest, or whether they would cool at
the same time. Fewer than 50% predicted
the cylindrical bottle would cool faster
because of the greater surface area. Most
predicted they would cool at the same
rate because their volumes were equal.
The students were then presented with
the question about which type of bacteria
are most resistant to drying. About 40%
recognized the ratio of surface area to volume and correctly predicted the spherical bacteria (cocci) would be most resistant. However, the majority of students
said they would dry out at the same time
because they had the same volume.
• Stavy and Tirosh (2000) identify the
intuitive rule “same A, same B” to explain
why students choose that the bacteria
would dry out at the same time. Their
reasoning is that if the bacteria have the
same volume, then they have the same
resistance to drying.
Suggestions for Instruction and
Assessment
• To further address the relationship between
surface area and volume and issues related
to scale, consider combining this probe
with “Whale and Shrew” in Uncovering Student Ideas in Science, Vol. 2: 25
More Formative Assessment Probes (Keeley,
Eberle, and Tugel 2007).
• After students have made their predictions and explained their answers, show
3-D models that have nearly identical volumes and approximately the shapes of the
bacteria—for example, a Styrofoam ball, a
wooden rod, and a piece of thick rope. Ask
students to imagine these are the bacterial
cells and that they have the same volume.
Ask if they would change their predictions
after looking at the models. Alternatively,
provide students with modeling clay. Have
them create the three shapes from the same
volume of clay, then calculate and compare
the surface areas of each shape.
• For high school students, connect the surface area to volume relationship to the idea
of cells needing to diffuse material quickly
in and out for gas exchange, food, and so
on, in addition to maintaining water content. Because most cells normally exist in
an aqueous environment, water loss is less
of an issue than the ongoing need to bring
in materials for food or to exchange gases.
These needs are also related to the relationship between surface area and volume.
• Give students a ball of clay and have them
mathematically calculate the relationship
 Indicates a strong match between the ideas elicited by the probe and a national standard’s learning goal.
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N a t i o n a l S c i e n c e Te a c h e r s A s s o c i a t i o n
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7
Structure and Function
between surface area and volume. Have
them shape the clay into a rectangular
solid and perform the same calculation.
Then have them reshape the clay into a
long ropelike cylinder shape and perform
the calculation again. Have them compare
all three of the volumes (they are the same),
the surface areas, and the ratio of surface
area to volume. Have them compare their
results to the three bacterial cell shapes in
the probe.
• As a postassessment, this probe can be
modified to ask students to predict and
explain the order in which these cells
would dry out if exposed to environmental
conditions that promote desiccation.
• Challenge students to come up with other
examples of when a sphere is the best shape
for a particular circumstance. Contrast
this with examples of when it is better to
have a large exposed surface area.
Related NSTA Science Store
Publications, NSTA Journal
Articles, NSTA SciGuides,
NSTA SciPacks, and NSTA
Science Objects
American Association for the Advancement of Science (AAAS). 2001. Atlas of science literacy. Vol.
1. (See “Cells and Organs” map, pp. 74–75.)
Washington, DC: AAAS.
Konicek-Moran, R. 2008. Dried apples. In Everyday
science mysteries: Stories for inquiry-based science
teaching, 99–106. Arlington, VA: NSTA Press.
Maguire, L., L. Myerowitz, and V. Sampson. 2010.
Exploring osmosis and diffusion in cells. The
Science Teacher 77 (8): 55–60.
SciPack: Cell Structure and Function
Related Curriculum Topic Study
Guides (in Keeley 2005)
“Cells”
“Scale”
References
American Association for the Advancement of Science (AAAS). 2009. Benchmarks for science literacy online. www.project2061.org/publications/
bsl/online.
Keeley, P. 2005. Science curriculum topic study:
Bridging the gap between standards and practice.
Thousand Oaks, CA: Corwin Press and Arlington, VA: NSTA Press.
Keeley, P., F. Eberle, and J. Tugel. 2007. Uncovering
student ideas in science, vol. 2: 25 more formative
assessment probes. Arlington, VA: NSTA Press.
Livne, T. 1996. Examination of high school students’ difficulties in understanding the change
in surface area, volume, and surface area/volume ratio with the change in size and/or shape
of a body. Unpublished master’s thesis, Tel Aviv
University, Tel Aviv, Israel.
National Research Council (NRC). 1996. National
science education standards. Washington, DC:
National Academies Press.
Stavy, R., and D. Tirosh. 2000. How students (mis-)
understand science and mathematics: Intuitive
rules. New York: Teachers College Press.
Uncovering Student Ideas in Life Science
Copyright © 2011 NSTA. All rights reserved. For more information, go to www.nsta.org/permissions.
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