Learning the cell-theory in Junior High and High School.
Yoko Takei
Introduction
Observations and experiments are necessary to learn natural science,
especially for students with a visual impairment. We have to elaborate the methods
of observations and experiments, to make students realize the phenomenon by
touching. However we are not able touch the micro world, which is accessible only
with microscopes, and also the macro world like celestial bodies. Nevertheless I
would not end nature science in “airy discussion”; rather I have improved my
teaching by elaborating the methods used by the late Mr. Masahiro Aoyanagi and
Mrs.Yoshiko Toriyama. Here I would talk about my practice on “Learning the
Cell-Theory” in the field of biology.
The organism consisting of cells
As I say “Today we’ll touch cells”, the astonished students respond “what?”,
for they are aware of the smallness of the cells. To make them imagine “the
organism consisting of cells”, we use different kinds of citrus (like pomelo, or
“Som-oh” in Thailand) for an introduction. We give them the whole citrus, let them
touch it, and then we break it off into pieces but keeping its peel, so that every each
student would have the one piece. And we instruct them; “Take a section of the
citrus and then peel the thin skin as you do if you were to eat it.” There are some
students who know only the ready-to-eat citrus (citrus with its thin peel off),
because their parents do it for them. In those cases we train them to prepare by
themselves, teaching them which part of citrus should be taken off or eaten.
Since eating is not the purpose of the lesson, we make them observe it
carefully
beforehand.
I
usually
have
them
observe
with
their
hands
the
spindle-shaped vesicles, and tell them those vesicles are cells. One time I had them
count all the vesicles. Usually students give up when they are halfway done
counting, but this experience is very important.
They understand much better than
listening to the word; “many cells”. After the touching and counting, they would
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write in their report “the bodies of creatures consist of innumerable cells”, with
sincere reality...
By the way, a mature human body is said to consist 60 trillions of cells, but
students cannot truly understand it. To give them just a little bit of concreteness, I
usually explain it using the following example;
Teacher: When you have 60 trillions of 1 Yen coins, how much is it?
Student: 60 trillion Yen! Wow!!
-- Now students may have a more vivid image.
Sizes and Shapes of cells
I also use eggs, as a tangible cell. By telling them ”This is also a cell”, I
give each student an egg of a quail. I suggest every teacher to prepare several
different kinds of eggs so that students understand what “egg” is (Sometimes, when
a student is only aware of one kind of egg, they may think that particular one is an
egg and not the others). In my class I pass around an egg of an ostrich (only shell),
that I have obtained few years ago. After all of us have confirmed that an egg is a
supersized cell, which contains almost only nutrition, we discuss about different e
kinds of eggs, having students give out examples of eggs from the biggest to the
smallest
(Ostrich > Hen > quail > salmon > herring (herring roe) > cod (code roe).
Code roe will become tangible when cooked. Although a human ovum is measured
0.14 mm in diameter, it is a huge cell in the human body. Emphasizing this fact, I
also tell them that 5 millions of red blood cells (it is just an example of
non-ovum-cell) are contained just in 1 cube mm (an amount equivalent to one dot of
a 6 dot Braille), I make students imagine how small normal cells are.
After learning about the different sizes of cells, we talk about its form.
Recent textbooks contain images of “various cells”, and Braille text books also have
them in tactile form.
I recommend teacher to use them. You could also make a
model of red blood cell and nerve cell using clay. Have the student touch the model
and ask “How do you explain this shape with your words?” Let them think about it.
I believe it is important to let them think by themselves; avoid teachers describing it
right away, like “center of the both sides are biconcave disks…”
The Structure of cells.
After the students have learned that there are many sizes and shapes in cells,
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we learn about the „Structure of cells“. Some ggood examples (“good” meaning they
are typical example) of plant cells and animal cells are presented in the recent
textbooks,
in color. We have to tell students how real cells have no colors, except
for chloroplast. Also ask students “Since there are no colors on the cells, it is not
easy to observe them with eyes. What can we do to make them easier to see?” When
asked that, students would think for a while and then reply that it would be better to
color the cells. Usually I am careful not to let them just repeat what I say like; “the
core will become red using by the acetocarmine solution”.
Structure of optical microscope.
The students also learn the structure of optical microscope. Giving them the
real one to touch and make them observe; where should the prepared slide be set,
which wheels are used to adjust the distance, and
where would the light pass. The
students who can recognize light are instructed to look into the eyepiece, or have
them look at the screen which projects the image caught by the video camera
attached to the microscope and have them
manipulate the microscope (have them
adjust the mirror and diaphragm). The totally blind students are instructed to
understand by the sound signal of the light probe. Set the light probe on the
eyepiece using a stand, and make students understand the position of the lighting
source. Have them adjust the mirror and the diaphragm to change the direction of
the light. When the position of the mirror is changed, the tone of the light probe
would also change, so they would understand that the light passes straight through
the lens barrel. When the diaphragm is changed, the tone of the light probe would
also change; therefore students will be able to understand its function (see Image 1.)
I have students prepare the slides themselves using the surface of an onion.
This is to have them understand how the sample to be observed with optical
microscope must be thin enough so that the light would pass through. Strong smell
of acetocarmine solution would also make the impression of dyeing solution
unforgettable. In some extent I explain how to make the ideal prepared slides, but in
any cases they do not have to make excellent slides. The most important thing here
is that the slides are prepared by the students themselves. Therefore I never cut the
onion, which the students have prepared. Have them drop the acetocarmine solution
from an eyewash bottle, and cover it with glass. It is ok even if the slice of an onion
is wrenched or stretched out of cover glass, or if the bubbles are seen under the
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glass. The purpose of this activity is for students to understand that the sample
should be cut as thin as a film of an onion, so that the light would pass through it.
Image
1:
Observation
of
the
light-passage
through
the
microscope with light probe (the light source is set on left side
in this image. When students adjust the mirror, the tone of the
light probe which is placed on top of the eyepiece would
change).
The Bouncing Egg
A quail egg, which the shell has been dissolved quickly by hydrochloric acid,
would swell extremely when it is soaked in water. Let students play with them by
bouncing it on the desk; I named this the bouncing egg.
After having them bounce
it, prick it with a pin so that the water would spurt. Compare the size and the
elasticity of the eggs before and after being soaked in water; how cell membrane
transmit the water is well understood. I do not explain the osmotic pressure since
they are only junior high students, but their vague understanding of it would help
them later in their learning in high school.
One may be advised to use the concentrated hydrochloric acid diluted 2times,
namely 6M. 12 M of hydrochloric acid would cause smoke which stings human eyes
and nose. Yet, 2M of hydrochloric acid would not quickly dissolve the shell, and as
a result the protein in the egg will be deteriorated. 6M of hydrochloric acid can
dissolve the shell quickly without causing the stinging smoke, and therefore allows
students to observe dissolution of the shell (sound of bubbles) in a close distance.
However be very careful handling it by preparing goggle for students and so forth,
because 6M of the hydrochloric acid is only original concentrated hydrochloric acid
being diluted 2 times.
To make a full swollen egg, soaking it in water for more than half a day is
recommended, therefore it is better to prepare the day before the experiment.
When the egg is given out to the students, tell them how it was soaked for a day;
like the cooking-show on TV. For osmotic pressure experiment for high school
students,
I prepare dialysis tubes, rubber caps and glass pipes. JASEB News Letter
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Image 2: Quail Eggs.
Right: the shell-dissolved Egg
Left: the full swollen Egg after being soaked
for several days.
The process of the cell-division
To have students think on their own and understand the lesson, I have made
„cell division cards“. Draw 3 stages of cell division and its interval terms before
and after the division, and use Minolta Paper so that totally blind can also use them.
Cut them into a size of a credit card and give them to the students.
Each student
will have a set of cards; which would have six cards (see Image 3). Tell them to set
the cards which consists an image of interval before cell division on the left and
cards which consists an image of interval after on the right.
Then have them
shuffle the rest. Thereafter they are instructed to observe the cards carefully, and to
sort them according to their own notion on the order of stages in cell division. It
should not be too difficult, because it is simply looking at images and grouping the
similar images which sufficiently suggest the right order.
The most important part
is having students think and come up with reasons why they have decided to sort the
cards in such order. Have everyone present their reasons; among the way, some
students may notice his misunderstanding and say, “Oh, now I get it!” – and at the
end most of all students would have the cards sorted out in the correct order. Next,
have students compare the each images on the card and ask them what kind of
changes are there between the first and the second card, the second and third card,
and so on. They may give out an answer like, “On the second image there is a linear
figure in the nuclear which was not there on the first card”, or “Now the nucleic
membrane is described in a dotted line.” These answers are literally the changes
seen, so, take these expressions given out from the students, make them into a
dialogue and organize and supplement their expressions (with proper words and
terms).
The expressions given out by the students may be poor at first, but would
change as we discuss and have them write down the words discussed in their
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notebooks. “The linear figure, which appeared in the nuclear” is “chromosome”, and
“the nucleic membrane which was described in a dotted line” would be paraphrased
by the expression; “the nucleic membrane has disappeared and the nuclear has lost
its shape.” It is important to let students observe the figures carefully and express
the differences seen by using their own words.
This is always better than the
teacher explaining the figures and its implications at the beginning. What the
teachers should do is to listen to what the students have to say, praise them, and ask
another question which would lead them to think and explain in the proper direction.
Image 3: Cards of cell division
(1 set, 6 cards)
The movement of chromosome
I would like to introduce the “chromosome model”, which I have developed
to supplement the content of cell division cards mentioned above. Prepare a string
which is used in handicrafts and cut it into 10cm (thickness should be about 1cm).
Then attach board magnets on both ends and sew a thread at the center. Make a
couple and set it parallel on the white board (or other steel boards). When one pulls
the thread at their center from both sides, they should make a figure like “< >” (see
image 4). By making students follow this procedure, they would understand the
movements of chromosomes, which they could not understand by looking at images.
Since the shape of chromosomes change drastically especially in the middle term
and the late term of cell division, it is usually difficult for students to understand
just by images.
The best way to make phenomena in movements comprehensible is
to replicate that same movement in front of them.
Image. 4:
Replicating the movements of chromosomes in the later
term of cell division.
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Nuclear Phase, Number of Chromosomes, Meiosis
I would like to introduce another self-made model. Use strings as
chromosomes and make a set of homologous chromosomes in length of 15cm, 10cm
and 4cm. Attach a hook at the point of kinetochores. Make one more strings in the
length of 15cm, so the set consists of 4 pair of homologous chromosomes (2n = 8).
Give each student a set, and make them observe and talk about it. Carry on
the lesson through dialogue; change questions according to their reaction. This is
efficient for learning about the nuclear phases, the number of chromosomes and
meiosis.
Image 5:
The model of chromosomes (one can reproduce the
division of chromosomes by detaching the hooks,).
Image 5:
A student observing chromosomes with a self-made
model.
For example...
Teacher: Say anything you‘ve noticed, even anything trivial.
Student: There is something like strings, so many....
Teacher: So many? How many are they?
Student 1: Eight.
Student 2: Sixteen.
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Teacher: Eight or sixteen?
Student 3: When the hooks are detached, there are sixteen.
Teacher: Ah. So there are hooks.
So count them without detaching the hooks. Do
not detach the hooks. Count the shape of X of an alphabet as one
Student 2: Then, there are eight.
Teacher: Exactly. There are eight strings. Anything other things you have noticed?
Student 1: They are in various lengths.
Teacher: Various? Are they all different from each other?
Student 2: Some strings have the same length.
Student 3: Two strings have the same length.
Teacher: Excellent! Let’s put them in order from the left being the shortest to the
right being the longest
Student 1: There are 4 strings that are the longest.
Teacher: Exactly. Did everyone notice that?
Are they really the same?
Student 2: No, they are different; the positions of the hooks are different!
Student 3: The two have the hook in the middle, and another two does not have it in
the middle.
Teacher: Wonderful! You’ve noticed many great things! This is a model of
chromosomes.
The chromosomes are made from strings and the hooks represent
kinetochores. If the lengths are the same, and the position of kinetochores is the
same, they would have the same shape. So how many same size same shape
chromosomes were there?
Student 3: Two each.
Two each and there are four pairs.
Teacher: Yes, that’s right.
We call the chromosomes that are in the same size and
shape homologous. So we have 4 pairs of homologous chromosomes, right?
In high school level biology, we have to teach the meiosis following the
somatic cell division. Normally the students get confused when they come across the
meiosis However, by having my students make this hand-made model and replicate
and compare the somatic cell division and meiosis, it helped students to understand
clearly.
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