Many middle school students believe that the defining characteristic of living things is that they move. When they see the “glue monsters” wiggle in the Petri dish, most will immediately assume they are alive. What is going on? Duco® cement is polymer mixed in a water soluble solvent. When the cement is exposed to air as it drops into the dish, a thin, solid polymer skin quickly forms around the liquid, solvent-polymer mixture. When the bead of cement is immersed in water, the solvent diffuses through the skin, causing the bead to shrink and the skin to rupture on one side of the bead. The solvent squirts out of the hole and the surface tension of the water on that side of the bead suddenly falls. Since the surface tension is now uneven, the bead will move away from the hole, towards the area with greater surface tension. The hole quickly repairs itself but the skin then bursts in another location. Thus, the bead appears to wiggle and twist as the surface tension changes depending on where the skin bursts.
Another way to demonstrate surface tension propulsion is to place a paper boat in a tub of water. Take a toothpick dipped in concentrated dish soap (which will lower the surface tension of water) and touch it to the water near the back of the boat. The boat with rush away from the toothpick.
As to the card sorting activity, students will struggle over many of the items. Do not expect students to correctly categorize items, even after a group discussion. The goal is to get students thinking and debating about the characteristics all living things share, not to get the “right” answer. Their classifications will also give you a good sense of their current state of understanding and sophistication. Keep a list of the items students disagree on or misclassified. Revisit these items at the end of the unit when students have mastered the major concepts. Keep in mind that there are some items that even scientists disagree on, such as viruses and prions. Some of the items (dirt and air) are mixtures in which some parts are alive and some are not. In addition, how you classify a part of a multicellular organism (like a single leaf, blood, or pollen) depends on your point of view. These ambiguous items provide opportunities to discuss the characteristics of life with your students.
Defining the characteristics of life is difficult and not completely clear cut. Although you will find different lists at different sources, most scientists agree that the following characteristics are shared by all living things:
In addition to these 6, some lists included 2 additional characterisitics:
The goal of these activities is not to force students to memorize the list above. Many are new, difficult concepts (like cells, metabolism, and organic molecules) that will develop over the course of the unit. Students should experience the process of creating the list themselves and revising it periodically as they learn new things. For instance, in the preliminary list, the items: “need nutrients”, “make wastes” and “need energy” may appear separately. After students learn about photosynthesis and respiration as metabolic processes, these 3 items can be combined under the umbrella of “living things metabolize”.