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:
- Living things are made of cells. The basic unit of life is the cell. All living things are composed of one or more cells.
- Living things grow larger over time.
- Living things reproduce. Each individual, given the right circumstances, has the potential to produce a new individual that resembles its parent.
- Living things respond to the environment. Sometimes this response takes the form of motion such as an animal running away from danger or a plant orienting towards the sun. Sometimes this response is more subtle such as closing certain membrane channels in response to changing salt concentrations.
- Living things metabolize, that is, they take in raw materials and convert them into energy and wastes. For instance, animals convert glucose and oxygen into energy and waste products (water and carbon dioxide) through the process of respiration.
- Living things evolve. Over many generations the traits of the species will change by natural selection to better fit the current environmental conditions. In other words, living things adapt to their environment. A basic assumption of evolution is heredity, the passing of traits from parent to offspring through genes. Thus it is also accurate to say that all living things inherit traits from their parents through some type of genetic material.
In addition to these 6, some lists included 2 additional characterisitics:
- Living things maintain homeostasis. That means that living things can maintain their internal environment relative to changes in the external environment. A good example is how our bodies maintain a constant body temperature – sweating to cool our bodies down and shivering to warm our bodies up when necessary.
- Living things are made of organic molecules. Organic molecules include proteins, lipids, carbohydrates (starches and sugars), and nucleic acids (like DNA and RNA).
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”.