In any science Module, we present concepts that many students initially assume are difficult. It is a continuing challenge to make these concepts come alive so that students not only understand them at the moment but remember them later.
For example, life science concepts are really fascinating – if you understand them. But students often find them difficult and irrelevant – and therefore boring. Part of the problem is that life science truly is complex. In addition, much of it deals with things we can’t see. To understand life, you must understand cells and molecules, which are microscopic and mostly hidden inside us or other organisms.
So, how do we make the microscopic meaningful? How do we show students that cells and molecules are dynamic – that they perform important functions that keep us all alive, and they are not just the flat, inactive, boring structures pictured in textbooks?
We use hands-on activities, of course! In the Cell Structure
Module, we combine four different types of activities to bring cell structure and function to life.
1) Tabletop models
First, we ask students to identify the organelles, or structures inside cells, using larger-than-life tabletop models of plant and animal cells. They locate each organelle and label it on a diagram as they listen to a description of what the organelle does. Using these large models, students see that cells are active, three-dimensional structures with their own internal organization.
Second, students look at prepared slides of real cells under a microscope. They begin with slides of large single-celled paramecia and tiny bacteria, in which they see differences between simple (prokaryotic) and complex (eukaryotic) cells. Then, they compare prepared slides of Elodea leaf and human skin cells to see differences in plant and animal cells. They use software to project these images onto the computer screen and photograph them. Using the microscope gives them skill with a valuable – and fun – piece of equipment, and seeing the cells enlarged on-screen gives them an even better sense of the importance of magnification in studying cells.
3) Wet mounts
Third, students make wet mounts of living cells. They float a tiny, thin slice of onion tissue in a drop of water on a slide, stain it with a drop of iodine so the cells will be visible, and add a cover slip. Then they view it under the microscope to observe the living cells.
4) Naked-egg experiment
The most exciting activity in the Cell Structure
Module is probably the naked-egg experiment, which demonstrates the concept of osmosis. The egg is the largest single cell in any animal’s body, and, of course, it is microscopic in humans. In fact, throughout the Module, students have been learning that cells are microscopic, so it comes as a surprise when they learn that the huge chicken egg is also a single cell. In this experiment, they use chicken eggs that have been treated with vinegar to dissolve their hard shells. This leaves eggs with flexible cell membranes that are outside and visible.
They place one egg in distilled water and another in sugar water and store them in a refrigerator overnight. During the next class period, they observe the eggs and measure their mass and circumference.
They see that the egg in distilled water absorbs water, becoming larger and more rigid. The sugar-water egg loses water and becomes flabby. In each case, water flows in the direction that equalizes water concentrations inside and outside the cell membrane. In this experiment, students demonstrate that water really does travel through cell membranes. They can see, touch, and measure the changes brought about by osmosis.
Of course, during the Module students also learn about cells by reading, listening, and viewing diagrams and photographs. But we think it is the variety of hands-on activities that will help them internalize these concepts and begin to appreciate the fascination of life science.