Plants as inspirational contexts 1 – Osmosis

Plants are truly amazing organisms – able to survive anywhere without being able to run away!

In this article, using content from an online course developed by SAPS and STEM Learning, we look at how plant examples can be used to inspire and engage students and to do this, we’re using a topic that many students find uninspiring: osmosis.

For many teachers, a key tool used when teaching about osmosis is the practical investigation where potato tissue samples are immersed in sucrose solution.   For our students, this reliance on potato tissue can be uninspiring and can develop misconceptions such as:

• osmosis only occurs in potatoes
• potatoes, rather than being composed of cells, are like lumps of bread that soak up water like a sponge
• osmosis happens slowly

There are many amazing plant responses, vital to their survival, that are brought about by osmosis, and we can use these as spectacular conceptual hooks with which to engage our students, helping them to understand osmosis as a dynamic cellular process. It’s also important to note that osmosis is not only important to plants and that understanding it is key to understanding other areas of Biology, such as the kidney or the process of tissue fluid creation.

Students need to be able to apply their understanding of key biological processes to new contexts, and many students find this difficult.  By using plants in your scheme of work, you can give your students additional opportunities to practice this skill.

Why plants can’t survive without osmosis

Osmosis can produce some very rapid and spectacular plant responses, and the Venus fly trap is a great example of this.

The Venus fly trap uses modified leaves to trap insects, providing the plant with a source of nutrients such as nitrogen. Successive stimulation of a trigger hair on the inside of the trap causes water to enter or leave cells in the trap by osmosis, flipping the curvature of the trap from concave to convex, rapidly closing the trap.

Osmosis plays a vital part in many key biological processes, essential for plant survival:

•  response
• growth
• support
• transport

We’ve put together a summary of the roles osmosis plays in plants here, which would also make a useful revision resource for students.

Download the summary.

You can also find this summary available as a download on any of our osmosis teaching resource pages.

Osmosis quiz

Try our quiz, based on the resource above. There may be several correct answers to each question.

Combining subject knowledge and context

In a review of 17 experimental studies undertaken in eight countries, Bennett et al.(2007) found that using an interesting or familiar context to set the scene when teaching about science was found to result in more “more positive attitudes to science in both girls and boys and reduce the gender differences in attitudes”

The types of context most commonly used are:

• relevant to students’ lives and interests at present;
• relevant to situations students may encounter at some point in their lives;
• related to technological developments likely to be of interest to students;
• relevant to students’ possible future careers

and at advanced levels of study:

• linked to recent scientific research and innovations;
• linked to industry.

Bennett, J., Lubben, F. and Hogarth, S., 2007. Bringing science to life: A synthesis of the research evidence on the effects of context‐based and STS approaches to science teaching. Science Education, 91(3), pp.347-370.

Adding context

How do you and your colleagues find inspirational contexts for teaching? Do you have a shared space where you collect your ideas? Speak to a colleague and find out where they look for ideas.

Using a context which feels relevant to students can be motivating and can help them to see the importance of what they are studying, help make abstract ideas more concrete, and capture their curiosity

Popping giant cells!

This short practical investigates the cells present within the bell pepper pericarp tissues, and the importance of cell turgor to plants. The pericarp is the flesh of the pepper. The practical aims to reinforce the importance of the movement of water into and out of cells by osmosis to the survival of plants. You can use any colour of bell pepper, though you may find that the cells are less pronounced in green peppers.

You need:

• Bell pepper
• Hand lens or microscope
• Mounted needle

If a pepper is sliced in half, specialised plant cells, called giant cells can be seen on the inside of the pepper. They’re so big that they can be seen without a microscope!

If the inner surface of the pericarp is examined using a hand lens or a binocular microscope, the giant cells can be seen as long, thin structures, up to 5mm long.

If the point of a mounted needle is gently pushed against one of the giant cells, the cell will burst and cell contents will exude from the inside of the cell. This is a very simple and elegant demonstration of the fact that these giant cells are turgid and full of water.

The giant cells of the capsicum are thought to play an important part in the mobilisation and storage of water within the pericarp or fruit tissue. Their ability to undergo large changes in cell turgor is important for the survival of the fruits and hence seeds of this species. They also help to produce the succulence which makes them such an attractive food source.

This investigation provides first-hand experience of turgid plant cells and reinforces the concept of movement of water into and out of cells by osmosis. The importance of osmosis in maintaining turgor pressure, which is in turn important for cell shape and, in the case of this edible fruit, juiciness, adds interesting context.

Osmosis in bell pepper pericarp

Traditionally, sections of potato are used to investigate osmosis. Potato tissue is familiar to all students, but some students will struggle to envisage the potato tissue as cellular and this could be a barrier to learning.

Rather than being able to explain the results of their investigation in terms of the net movement of water molecules into or out of the cells of potato tissue by osmosis, some students may envisage the potato soaking up the sucrose solution like a sponge and those students would be completely baffled as to why some potato cylinders gained mass while others lost mass.

Osmosis can be investigated using pieces of capsicum pericarp, instead of potato tissue. Introducing the students to the capsicum’s turgid giant cells before measuring the change in mass of sections of capsicum pericarp in different sugar solutions over time can help students to envisage osmosis as the movement of water molecules in and out of individual cells within a tissue.

Checking for understanding

When students have collected their data in the red pepper osmosis experiment, how will you check whether they understand what has happened?
Explaining the movement of water in osmosis requires students to clearly order their thoughts, and is a good opportunity for you to check for misconceptions.

Students could be asked to work together to annotate their results graph using key terms such as: osmosis, diffusion, water molecules, sucrose solution, dilute solution, concentrated solution, concentration gradient, concentration, percentage increase in mass, percentage decrease in mass, no change in mass, flaccid, turgid, cells, tissue, into the cell, out of the cell.

Alternatively, you could ask students to draw a diagram of a test tube of sucrose solution containing a piece of red pepper tissue in each of the three zones of the graph, with arrows to indicate net movement of water molecules.

Summary

By using bell pepper tissue with its giant cells that are visible to the naked eye, and by carrying out the cell-popping investigation which introduces the idea of the turgor pressure of cells, your students may be more likely to envisage the movement of water in and out of the cells in the pepper tissue by osmosis, allowing them to apply their knowledge when interpreting their experimental data.

The use of fascinating or familiar contexts in Biology teaching can bring teaching and learning to life and increase its relevance to the lives of learners. Osmosis is a process that is vital to the survival of plants and produces some spectacular and surprising plant responses. Practical investigations using plant materials can provide easily resourced, interesting contexts that will engage and inspire your pupils to find out more about Biology.

This article was written using reworked content from the course Teaching Biology: Inspiring Students with Plant Science codeveloped by SAPS and STEM Learning.