Using the
Past in the
Class
Learning from historical
models of cell membranes
Cameron Johnson and Julie A. Luft
S
cience is a human endeavor that changes over time
and occurs within a social environment. There has
been a long-standing interest in the role of history
as a tool for teaching science concepts. Historical accounts
of scientific investigations and original studies are mechanisms for including the history of science in the classroom.
Unfortunately, using history has been a “minority tradition in science education” (Matthews, 1994, 49). History is
rarely seen as a mechanism to help students understand
the nature of scientific process, a concept within a scientific discipline, or the importance of an event in science.
However, discussing historical events can help students
appreciate science as it is practiced. “To gain that insight
[into science], children need to understand the nature of
the problems as they appeared at the time; they need to see
the false starts and unproductive lines of speculation that
were followed, on occasion; they need to appreciate the
part played by individual creativity, personal ambition,
and social pressures. . . . Only then can a faithful picture
of the nature of scientific theory building be established”
(Hodson, 1993, 701). By including the history of science in
the curriculum, we can help students understand that the
process of science does not mysteriously produce the
“truth,” nor is science void of human judgment.
52
T h e S c i e n c e Te a c h e r
By learning about the history of science, students develop an understanding of the human aspect of science—
how it changes over time and occurs within a social environment. Both the National Science Education Standards
(National Research Council, 1996) and the Benchmarks for
Science Literacy (American Association for the Advancement of Science, 1993) call for the inclusion of the history
of science in science instruction. For example, the National Science Education Content Standard G (History
and Science) says, “Usually, changes in science occur as
small modifications in extant knowledge” (National Research Council, 1996, 201). Students should come to appreciate the modification of theories, experiments, and
explanations by scientists studying various phenomena.
Watson and Crick’s pursuit of the structure of DNA is a
case in point. They modified the model of DNA as more
information became available from related studies.
The national documents also suggest that students
should understand the role of culture in the advancement of scientific ideas. Students should realize that
some topics in science are (or are not) studied or
accepted because of social pressures. For instance, the
Copernican model of planetary motion was originally
rejected because it was against the teachings of the
FIGURE 1
Student-drawn examples of Evert Gorter and F. Grendel’s cell membrane models.
a.
b.
Catholic Church, which believed the Earth to be at the
center of the Universe. The Copernican model suggested
that the Universe was extremely large and that the Earth
was not in the center.
Including the history of science in the classroom is not
always easy. It takes time to find relevant historical
events and to understand the societal pressures existing
at various times in history. Fortunately, there are several
models for incorporating the history of science into the
curriculum. One easy-to-implement, problem-based
learning model directs teachers to select a topic and
identify salient studies that represent the advancement of
the concept or theory over a period of time (Wilkerson
and Gijselaers, 1996). The period of time can range from
twenty to hundreds of years. Studies should be selected
that have evidence and conclusions that can be understood and shared with students.
When we use this model, we select three or four key
studies that represent changes in experimentation and
knowledge. Depending upon the instructional goals of the
lesson, studies with conflicting evidence are sometimes selected. When possible, information about the scientists is
also shared during class discussions to give added context.
Historical milestones: A sample lesson
We have used the following lesson in secondary life science classes, but the format of the lesson is appropriate in
any secondary science classroom. This lesson presents historical milestones that led to the current understanding of
the structure of the cell membrane.
Study 1: Gorter and Grendel (1925)
At the start of the lesson, students were placed in groups
of four. As a class, we discussed Dutch researchers Evert
Gorter and F. Grendel’s (1925) experimental methods
used for studying red blood cells and their resulting data,
but we did not explain their model of the cell membrane.
Gorter and Grendel lysed red blood cells, emptied their
contents, isolated only the membranes, and calculated the
surface area of the red blood cells. Gorter and Grendel
knew that the membrane was made up of phospholipids,
which had hydrophilic heads and hydrophobic tails. They
also knew that water was present on both sides of the cell
membrane. From their calculations, they found that twice
as many phospholipids existed than were necessary to
cover the entire cell at one time.
Students were then instructed to create a model of a
cell membrane that corresponded to the presented information. They first worked individually to solve the
problem. They developed a rationale for their models
and then drew diagrams of the cell membrane. When
students were satisfied with their responses, they
shared their drawings and conclusions with their group
members. After each small group had compared rationales and models, a large group discussion was held in
which each group shared one of their models. While
the representative models were examined, students defended their conclusions based on the evidence and determined if they needed additional information. In this
social setting, students began to understand the tentative nature of their explanations, enhanced their ability
to look at data and construct explanations, and, if
needed, revised their model.
Two typical student responses are found in Figures 1a
and 1b. While they meet most of the criteria set by
Gorter and Grendel, they do not agree with their model.
Figure 1a shows a bilayer of phospholipids with the first
(outside) layer oriented with the heads toward the outside of the cell, exposed to water, and the tails pointed
into the bilayer, away from water. The second layer has
the phospholipids oriented horizontally to the first layer,
with the heads exposed to the water inside the cell and
also acting to shield the tails of the first layer and the
tails of the second layer embedded
behind the interior heads. Figure 1b
has a honeycomb pattern wherein
each cell has only a monolayer of
phospholipids oriented with the Explore blood cells
heads pointing into the cells and the at www.scilinks.org.
tails pointing out. This design was Enter code TST1112.
N ove m b e r 2 0 0 1
53
justified by the statement that the heads
Gorter and Grendel’s were exposed to the
actual cell membrane water inside the cell
model.
while the tails were
shielded from water by
contact with the tails of
the adjacent cell’s phospholipids. Students also
stated that the remaining phospholipids were
likely to exist somewhere inside the cell.
The Gorter and Grendel model was actually a bilayer model (Figure 2). The presence of two layers explained why twice as much phospholipid was isolated
as predicted. The model orients the heads facing the
water in both layers and the tails facing each other.
This model was presented to students only after the
large group discussion, and it was used to draw out
additional questions, which initiated the next presentation of historical data.
FIGURE 2
FIGURE 3
Student examples of James Danielli and
Hugh Davson’s cell membrane model.
a
b
Study 2: Danielli and Davson (1935)
In the same groups, students were then told about the
work of physical chemist James Danielli and physiologist Hugh Davson from University College, London
(1935). They researched mackerel eggs and observed the
movement of water-soluble molecules across the cell
membrane. They lysed cells to isolate cell membranes
and identified the types and amounts of certain molecules in cell membranes. Danielli and Davson (1935)
ultimately concluded that:
u
u
u
Both water and water-soluble molecules can cross
the membrane;
Protein exists in the cell membrane; and
The amount of protein is almost equivalent to the
amount of phospholipid.
The next challenge to students was to create a new
model that included protein and explained how water
crosses the membrane without getting the phospholipid
tails wet. Students repeated the process they had used before—drawing individual models, discussing and defending their models, and sharing representative models in the
large group setting.
Figures 3a and 3b are examples of student work. Figure 3a shows a layer of protein inside the bilayer, with
occasional channels built of protein. The channels enable
water and water-soluble molecules to cross the membrane
without the tails getting wet. Figure 3b interspaces phospholipid and protein molecules so that a pair of proteins
surrounds each pair of phospholipids. Water passes
through the pair of proteins.
54
T h e S c i e n c e Te a c h e r
The Danielli and Davson (1935) model proposed that
the phospholipid bilayer was covered on both sides by
a layer of protein that is commonly called the proteinsandwich model (Figure 4). Water and water-soluble molecules cross the membrane at protein-lined pores. The
pores made the membrane permeable, while the tails of
the phospholipids were protected and did not get wet.
Study 3: Singer and Nicholson (1972)
The previous two studies set the stage for University of
California, San Diego, researchers S. Jonathon Singer and
Garth L. Nicholson’s study of plasma membranes (1972).
Their model incorporates the work of previous researchers and used new findings that resulted from the invention of the electron microscope (EM). Singer and
FIGURE 4
Danielli and Davson’s actual cell membrane
model (the protein-sandwich model).
FIGURE 5
FIGURE 6
Student-generated model explaining The Singer and Nicholson Fluid Mosaic
protein movement based on S. Jonathon Model.
Singer and Garth L. Nicholson’s study of
plasma membranes.
a
b
Nicholson examined the cell membrane under the EM
and reported that the membrane was 7 or 8 nm thick,
with the bilayer measuring 5 nm thick and proteins being
7 or 8 nm wide. In addition, they found that membranes
readily fuse together. Specifically, if a section of membrane with one type of protein (protein A) was fused with
a second section containing protein B, the proteins quickly
intermixed. This provided evidence that proteins move
within the membrane. The new challenge to the students
was to create a model that explained how proteins could
be incorporated into the bilayer while maintaining an
overall thickness of 7 or 8 nm and allowing proteins to
move around within the membrane.
While most student groups did not have trouble explaining where and how to position the protein molecules
within the membrane to satisfy the thickness problem,
explaining the movement of proteins was a more difficult
challenge. Figure 5a illustrates a student-generated model
explaining protein movement wherein protein molecules
leave the membrane entirely and re-associate with the
membrane in a new position. The student model depicted
in Figure 5b proposes that the protein molecules are
cleaved by one amino acid (AA) at a time. Each individual amino acid is small enough to travel between the
tails and then reassemble themselves into a protein in a
new position. Another explanation proposed that a ring
of phospholipids, like a life ring, surrounds each protein
molecule, and the protein and its associated phospholipids
float around and through the other phospholipids.
The Singer and Nicholson Fluid Mosaic Model
(Singer and Nicholson, 1972) suggested that phospholipids are in a bilayer, with proteins occurring as irregular
globs that float around, within, and through the phospholipids (Figure 6). The proteins allow water and water-soluble molecules to pass through the membrane.
The lipid layer is fluid, meaning that both proteins and
individual phospholipids can move around. This is currently the accepted cell membrane model.
Looking back on history
After students worked through all of the presented research, they wrote a reflection on how their understanding of the cell membrane changed and what they learned
about the process of science. These simple reflections gave
information about students’ knowledge of the membrane
and the nature of science. The collected reflections, drawings, and rationales were assessed in three areas: models
and explanation; conclusions about the nature of science;
and working productively in the class. The rubric (Luft,
1997) for this activity is shown in Figure 7 (page 60).
There are many benefits to incorporating the history of
science into the classroom. Students can develop a critical
understanding of what science is—a human construct
subject to human limitations in understanding, technology, and social constraints. The human component in the
construction of knowledge is often overlooked, but vital
in science instruction.
Another benefit, as found in this specific lesson, is
that students can learn about the process of modeling. By
generating and evaluating their own models that represent the stated findings, students can experience the decision-making process involved in developing a model,
the creation of inaccurate models, and the forming of
models that have sufficient supporting evidence. In esN ove m b e r 2 0 0 1
55
FIGURE 7
Rubric to understanding the cell membrane and the process of science.
Item
Description
Points
Total score
Models and explanations
The model and explanations are revised
based upon the information presented
and the use of logic. The developed
models are clearly supported by
explanations that include the provided
evidence and account for evidence not
provided. When appropriate, following
each study the student states questions
for further study. All three models are
present along with clear explanations
for each model.
5—Exceptional
Student work is representative
of the description.
Total Score =
weight of the
item X score
Conclusions about the
process of science
Working in a team and
in the class
1—Needs improvement
Few components in the description
are met by the student
A description that is written in
complete sentences, and articulates
several factors about the process of
science. For example, the student
should include comments about
science as a human endeavor, the
nature of scientific knowledge, and
how scientific knowledge can change.
5—Exceptional
Student develops explanations and
models and shares these with his/her
peers. The student asks questions of
his/her peers and in the class
discussions. The student is an active
participant in an effective way, which
may not always be verbal.
5—Exceptional
sence, students begin to understand the benefits, pitfalls,
and qualities of models.
Finally, when teachers incorporate history into lessons,
the method of doing so is critical. It is not enough to
simply include historical accounts or relay historical
stories; these will just be perceived as add-ons to the curriculum. Historical accounts should be critical to the
curriculum and essential in assisting students to learn
about science and to develop their scientific thinking
skills. Ultimately teachers guide students to develop their
knowledge in skillful ways, and the history of science is
one tool to help students understand the complex processes involved in science.u
Cameron Johnson (e-mail: cameronj@
sunnysideud.k12.az.us) is a science teacher at
Sunnyside High School, 1725 Bilby Road, Tucson,
AZ 85706; and Julie A. Luft (luft@u.arizona.edu)
is an associate professor at the College of Education at the University of Arizona, 835 Education,
Tucson, AZ 85721.
56
3—Meets requirements
Most of the criteria in the
description are met by the student
T h e S c i e n c e Te a c h e r
3—Meets requirements
1—Needs improvement
3—Meets requirements
1—Needs improvement
References
American Association for the Advancement of Science. 1993. Benchmarks for Science Literacy. New York: Oxford University Press.
Danielli, J.F., and H. Davson. 1935. A contribution to the theory of
permeability of thin films. Journal of Cellular Comparative Physiology 7: 393–408.
Gorter, E., and F. Grendel. 1925. On biomolecular layers of lipids on
chromocytes of blood. Journal of Experimental Medicine 41: 439–443.
Hodson, D. 1993. In search of a rationale for multicultural science
education. Science Education 77: 685–711.
Luft, J.A. 1997. Design your own rubric. Science Scope 20(5): 25–27.
Matthews, M.R. 1994. Science Teaching: The Role of History and Philosophy of Science. New York: Routledge.
National Research Council. 1996. National Science Education Standards. Washington, D.C.: National Academy Press.
Singer, S.J., and G.L. Nicholson. 1972. The fluid mosaic model of
the structure of cell membranes. Science 175: 720–731.
Wilkerson, L., and W.H Gijselaers. eds. 1996. Bringing ProblemBased Learning to Higher Education: Theory and Practice. San
Francisco: Jossey-Bass Publishers.