Whenever I teach osmosis I carry out the typical practical activity of soaking potato (and other vegetables) in solutions of varying solute concentration. They are cut into blocks of similar size and shape, weighed, and then soaked for 24 hours. In the lesson with my students the difference in size of the blocks is visually striking. But, the difficulty comes with understanding a graph of the results. This is a post about resolving these problems with a class of 16-year-olds.
Here is a sketch graph of our results (sorry for the wonky y-axis):
At least in my experience, many students have trouble interpreting the graph because they confuse it with a time graph. They see the change in mass of the potato blocks over time, and despite me making it explicit that the x-axis is different, they can't shift the idea from their mind.
They need to perceive the difference, and make the distinction themselves.
As such, I always juxtapose the graph with a time graph, that is with the same y-axis (percentage change) but a different x-axis. I have students draw the sketch graph directly below the other graph and with the same size axes (see the final lesson image below). The time scale, I tell them, is between the two points that they were weighed (i.e before being soaked, and after 24 hours).
I then ask them to predict the shape of three lines for three potato blocks:
In a hypotonic solution (higher water potential)
In an isotonic solution (equal water potential)
In a hypertonic solution (lower water potential)
And then I show them my answer and we discuss the difference between their prediction and my answer.
Typically my students two types of lines, ones that consist of straight lines (not curved), and ones that are curved (like above). But neither type of student, in my experience, can explain why they have drawn what they have. It always seems that those students that draw curves like mine have just become accustomed to seeing this pattern in graphs and come to expect, without knowing the mechanism.
And so here I need a system dynamics model to help them. The great thing about system dynamics models is that you can add and remove components depending on what you want the model to convey—this is not the only way of modelling it, and I may do it differently depending on the class and their needs.
(I use system dynamics models with all my classes, with students from 11–18 years old).
In my case, I wanted a model that conveys the meaning of the curves in the graph above. If you want to see how I build models with students, see this post.
What does the model show? Firstly, it shows how the flow of water in and out of cells depends on solute concentration. Let's begin with the stock "water inside cells", notice how the outflow of water depends on the solute concentration outside. The relationship is "+" which means "same", which means that the higher the solute concentration outside, the higher the outflow.
(I prefer to teach with water potential, but this lesson was part of a syllabus that used the solute definition. If it were water potential, the relationship would be "-", which means "opposite", which means that the lower the water potential outside, the higher the outflow.)
The outflow of water from the cells accumulates in the stock "water outside". In this simplified model, there is a positive (same) relationship between this accumulation and the inflow of water back into cells. This can be thought of as the pressure of water (which contributes to increasing the water potential).
As my students and I work through the model in dialogue. Students come to see that as water leaves the cells, the pressure factor tends to increase the water entering the cell, until an equilibrium in flow rates is reached. When students think about these factors simultaneously, they begin to find meaning in the curve of the osmosis time graph above.
After perceiving these differences and making the distinctions, its time to return to the original graph of percentage change in mass versus solute concentration. And, in my experience, students now understand it much better and we can discuss how we can use it to estimate the concentration of solutes in the potato and other vegetables.
If you're interested in how I explain the mechanism of osmosis, then see this post. And If you've liked this then check out my book. Download chapter 1 here—English edition—edición española—or check out my other posts.
Christian Moore-Anderson (about me)
@CMooreAnderson (Blue Sky)
@CMooreAnderson (X)
