The biological cell
what we teach, what they learn
Jyotsna Vijapurkar
This is a standard middle school curriculum topic: The biological cell described as the structural and functional unit of an organism. That is what we teach, and that is what we presume students learn. Simple enough, right? Wrong!
If you think about it, this statement is a brief summary of what is intended to be taught – that cells make up a living thing (they are its structural unit). Different kinds of cells may have different structures and different functions. That physiological processes take place inside them, and when students have no idea of those processes, even if seemingly simple ones of secretion of, say, saliva, this phrase ‘functional unit’ has no meaning.
As part of the curriculum development project at the primary and middle school level at the Homi Bhabha Centre for Science Education, we investigated students’ understanding of several fundamental concepts and the best way to teach them. Students often have their own alternative ideas that need to be addressed if teaching is to be meaningful. Successful teaching helps students move from their often scientifically incorrect ideas to the scientifically correct ones. The biological cell was one such topic we investigated.
There has not been a whole lot of research on students’ understanding of the biological cell.
When we embarked on our study, we came across some eye openers, that were particularly surprising as these students had already been introduced to the biological cell in their school. In our initial survey with students of grades 6 and 7, to establish the baseline, we found that they thought cells filled our bodies, not that they made them up. In the diagrams below, by students of grades 6 and 7, the skin itself is not made of cells, but holds cells inside.
They thought cells in bones were hard (students often attribute macroscopic properties to the microscopic constituents of objects).
Coming to functions, they thought that stomach acid was produced between the cells of the stomach. Research in Israel decades ago had found that even in high school (grade 10) students have greater difficulty understanding the functional aspect.
Students across middle school and even bachelor’s level visualized cells as two-dimensional entities. I too had visualized them this way in my school days. A second year B.Sc student of biology told us she had thought they were 3D but when she first looked through the microscope, she ‘realized’ they were 2D! This loss of the third dimension is common across subjects – the Earth as a circle, the cells as flat, the lens as shown sideways in optical diagrams as 2D. Textbook pictures and drawings are most unimaginative! And never clarify that the drawings are 2D projections of 3D entities. The task of creating a 3D mental model from a 2D projection is very challenging. To begin with, 2D projections of many different shapes can be identical – an oval, a sphere, a disc can all appear circular in 2D (making circular shadows of objects of these different shapes will drive this point home).
Given these issues, teaching about biological cells simply as a definition, ‘structural and functional units’, with absolutely no quantitative description, is an entirely meaningless exercise.
Is there a different way?
Make an effort to move away from 2D images. It’s hard to shake this mental model of flat cells even after seeing cells through the microscope, as in the example of the B.Sc student mentioned earlier. As they say, a picture is worth a thousand words, and in this case simply aids the formation of a wrong model of a cell in the student’s mind.
Make models of cells instead. Use models to teach and get students to make them. Move away from the textbook.
Teach processes inside the cell and the structure of the cell together, so that its 3D structure is conveyed. Focus on some physiology – the contents of a cell, the membrane that holds all that fluid inside. Structure and function go hand-in-hand, there is no reason to teach them as separate, disconnected ideas. Teach about what the cell holds – cytosol and organelles. The 3 dimensional nucleus, for example. All this aids in forming a clearer 3D picture.
You can also drive home the point about 2D projections – we just took a transparent plastic bag filled with water, held it up in front of students and invited them to come draw it on the board.
When we asked students to make cell models (bear in mind we had to get them to unlearn what they had learned before coming to our classes), we gave them all sorts of material to use, including modelling clay. They took the clay and flattened it – to reproduce what they thought was 2D from their texts. Here are some photographs of students’ models (as our teaching progressed, they slowly took on a 3D character). Point to note: We never spelt out that cells are 3D objects, yet we succeeded in conveying it through our teaching strategy.
In the photograph above, on the left is a neuron a student made initially, and later models took on a 3D character. The last model above is of rows of cells (onion skin cells perhaps).
When you use the microscope to show onion skin or a leaf membrane which are commonly used material to show cells, make sure you focus on the deeper layer(s) too. The very act of moving the lens closer to the slide gives a sense of depth – the 3rd dimension. Needless to add, let the students handle the microscope and feel the movement themselves.
The photographs are examples of models students made as teaching progressed. Note the increasing 3rd dimension as students got a better and better understanding. Note the one marked ‘b’. The student has constructed a model of a plant cell, complete with cell wall. Now this brings me to another aspect often disregarded in science classes. When a commonly used word is used in a technical sense, it takes on a completely different meaning. But that is never explicitly addressed, so students still use it or understand it in the non-technical sense. The cell wall has to be the enclosing surface of the cell (technical usage), not a boundary like the walls of a room (everyday usage). Discuss such language issues explicitly. Use a model of the cell as in ‘e’ and ask them to point out where the cell wall would be.
To reiterate, the students we worked with had come to us after their lessons on the biological cell, and with a strongly formed incorrect picture. But if you teach it with 3D models, and teach structure and function together, and address these issues of language and use the microscope as discussed above, and not passively just to see cells, the students won’t get the wrong picture in the first place! The strategies outlined above worked successfully even when they had a robust incorrect idea. And these strategies make for more fun teaching as well, quite a change from the dull diagrams, labelling exercises and definitions that otherwise constitute a majority of our classes.
Our research was reported in the following publication: Vijapurkar, J., Kawalkar, A., Nambiar, P. (2014). What do Cells Really Look Like? An Inquiry into Students’ Difficulties in Visualising a 3-D Biological Cell and Lessons for Pedagogy. Research in Science Education, 44:307-333.
The author retired as a professor from the Homi Bhabha Center for Science Education, TIFR, Mumbai, where she worked on curriculum development and classroom practice focussing on inquiry oriented teaching. She can be reached at jvijapurkar@gmail.com.
