Paper punch-out dots soaked in isopropyl alcohol will move about randomly when placed flat on the surface of water, superficially resembling swimming movements of single-celled protists. This exercise is a hands-on and dynamic activity to generate discussion on the generally accepted characteristics of living things.
In many general biology courses, one of the first topics discussed is the characteristics of living things. This topic is covered in the first chapter of many introductory biology textbooks (e.g., Krogh, 2005; Raven et al., 2005; Alters & Alters, 2006; Brooker et al., 2008; Campbell & Reece, 2008; Russell et al., 2008). The National Science Education Standards (National Research Council, 1996) include the characteristics of living things, either directly or indirectly, in the K–4, 5–8, and 9–12 standards, and college courses often include the topic as well.
Although a comprehensive definition of life has long eluded biologists (Greener, 2008; Tsokolov, 2009), it is generally agreed that living things possess certain characteristics in common that could be termed a description of life. Seven characteristics are generally listed as common to living things: (1) homeostasis; (2) complex, cellular organization; (3) ability to reproduce; (4) metabolism; (5) growth and development; (6) ability to adapt and evolve; and (7) ability to respond to stimuli (Raven et al., 2005; Brooker et al., 2008; Campbell & Reece, 2008). Some textbooks list alternative characteristics such as organization in a hierarchy or the presence of chemical (DNA) instructions (Krogh, 2005; Alters & Alters, 2006; Russell et al., 2008). When asked, students will invariably list movement as a characteristic of living things, even though movement is not one of the characteristics generally associated with life (Krogh, 2005; Raven et al., 2005; Alters & Alters, 2006; Brooker et al., 2008; Campbell & Reece, 2008; Russell et al., 2008). This naive inclusion of movement can be used as a lead-in to a discussion of the generally agreed-upon characteristics of living things.
We typically use a laboratory exercise to introduce our students to these "life"concepts. In the past, we would place liquid mercury in a watch glass and mix in a solution of dilute nitric acid and potassium dichromate. This mixture caused the mercury to move in a manner roughly resembling an Amoeba. We referred to this chemical combination as the "mercury monster" and argued that it showed some characteristics of living things. These characteristics included reproduction when bits of the gyrating liquid mercury would break into two, primitive metabolism with the reaction between the mercury and the acid plus potassium dichromate, and response to a stimulus in the movement caused by the addition of the acidified potassium dichromate to the mercury. These interpretations stimulate classroom discussions of living versus nonliving things. The mercury monster elegantly illustrated several points about the differences between living and nonliving things; however, mercury and mercury compounds are highly toxic. Therefore, safety considerations, including laboratory handling of mercury by students, cleaning up of mercury spills, and difficulty in finding suitable disposal for the hazardous waste material from the laboratory, rendered it impossible to continue using this exercise and required us to seek an alternative exercise.
The alternative exercise that we have developed is referred to as the "dancing dots." The dancing dots consist of paper dots from hole-punches soaked in an alcohol solution (Figure 1). When placed on the surface of water, these alcohol-soaked dots quickly move in random directions over the surface of the water for a few seconds. We then show a short movie of motile Euglena or Paramecium populations. Superficially, the dancing dots and the motile organisms move about in similar "random" pathways. We then ask the students questions about the natures of living versus nonliving things. The dancing-dots activity can be done as a demonstration by the instructor, or the students can complete this activity individually or in groups. This activity can be easily completed in about 20 to 30 minutes, followed by discussion.
Two plastic cups or beakers
Isopropyl (rubbing) alcohol
Hole-puncher and ordinary copier/printer paper or 15 to 20 dots hole-punched from ordinary copier/printer paper per experimental group
Chemical splash goggles for each student and instructor
Pen that can write on the plastic cups or beakers
Short movies of Euglena and/or Paramecium, available on http://www.youtube.com
10 or 25 mL graduated cylinder
A strainer for disposal (optional)
Protective goggles must be worn for the duration of the exercise.
Label a plastic cup "Water" and add tap water to fill it about 3/4 to 7/8 full.
Label another plastic cup "Rubbing alcohol" and add 10 mL of rubbing alcohol and 10 mL of tap water to make a 50% solution of rubbing alcohol.
Put five paper dots in the rubbing-alcohol cup. Make sure that the paper is well soaked in the solution for 1–2 minutes.
Using forceps, take a dot out of the alcohol solution and gently place the flat side of the dot on the surface of the water in the other plastic cup. If you do not place the dot carefully on the surface, it will sink and you will not see the phenomenon.
Observe, and repeat with the remaining dots. If a dot sinks, try another one.
Rubbing-alcohol solution and water can be poured down the sink. We have found that a strainer is useful for catching the paper dots when pouring the solutions into the drain.
Discard the plastic cups and paper dots in the trash. It is best to discard the cups so that they are not mistakenly used for drinking.
Some Sample Discussion Questions & Answers
Are the Paramecium and Euglena that you saw in the video alive? Why or why not?
We typically look for answers that state that the Paramecium and Euglena are alive. The main characteristic that students can observe from the videos is that the organisms are complex and cellular. They might observe feeding, which indicates a metabolism. Few of the other characteristics are readily observable, but they can be inferred with discussion. Movement will usually come up as a reason, and this can be used as a comparison with the dancing dots.
Are the dancing dots alive? What characteristics might make you think that the dots are living things? Are these characteristics normally associated with living things?
We look for answers that state that the dancing dots are not alive. Few students have trouble recognizing this. The characteristic most often cited as a reason to think the dots are alive is movement. At this point a discussion of living things that normally do not move (e.g., some bacteria, algae, and fungi) and nonliving things that do move (e.g., wind, sand in a storm, water in a river) can be addressed.
In what ways do living things differ from nonliving things?
At this point the discussion proceeds to the generally accepted characteristics of living things, as well as characteristics, such as movement or possession of a vital force, that are not generally used to describe life.
What Does It All Mean?
The paper dots move on the surface of the water because the mixing of the alcohol from the dot into the water weakens the surface tension of the water near the dot. This creates an adjacent area of stronger surface tension, which pulls on the dot and causes it to move. Once the alcohol has dissipated, the surface tension regains equilibrium and movement ceases. This is a completely physical phenomenon (Herbert and Ruchlis, 1968; Palewa, 2009).
There are numerous activities available in print and on the Internet that are useful for leading into a discussion of the characteristics of living things. This topic is often taught with descriptions of objects and accompanying questions that focus on whether the object is nonliving, alive, or formerly alive but now dead (e.g., Carlson, 2002; Foust, 2009). The dancing-dots activity could be used in addition to such exercises or alone and can be used at any level, K–4, 5–8, 9–12, or college. The discussions accompanying this exercise would, by necessity, have to be appropriate for the particular age level involved. We have used this exercise successfully in college laboratory settings both on-campus (Mickle, 1993) and in distance education (Mickle and Aune, 2008). The expense for supplies to do the dancing-dot exercise is minimal, and it does not require purchase of specialized equipment.
We have found the dancing-dots exercise useful as a good starting activity for a general biology course because it is dynamic and engaging for students. Like the mercury monster, it could be argued for pedagogic purposes that the dancing dots show some characteristics of life. These characteristics include energy transduction, thus showing a very primitive metabolism, and they respond to a stimulus by moving when placed on water. It is obvious that the dots are not alive, but the students are challenged to overcome the common misconception that movement is a characteristic of living things, which leads to further discussion. Safety considerations have rendered the mercury monster impossible to use in a biology teaching laboratory, even as a demonstration. In fact, some universities have banned the use of elemental mercury in all laboratories, even in thermometers (e.g., Woods, 2002). The dancing dots can be used safely by all students at any educational level. Like the mercury monster, it acts as a thought-provoking and fun springboard for discussion on the characteristics of living things. Because students enjoy and can participate in the dancing-dots activity, they often become less reticent to speak, which promotes lively exchanges of ideas. These exchanges, in turn, set the tone for a more interactive laboratory environment.
This work was developed, in part, under a DELTA grant from North Carolina State University. The authors thank Profs. Alton Banks and Charles Boss of the Department of Chemistry, NCSU, and James Duis of BASF Corp., Charlotte, NC, for assistance in preparing this paper. Kathryn A. FitzGerald is thanked for providing the photograph of the Dancing Dots setup.