Matter Transformation Problems

Understanding Organisms as Matter Transforming Systems

The matter problem set is composed of five tasks (A through E) and the sequencing of tasks is essential in preparing the students cognitively for the problem to create “need” or “motivation”. In Part A, the students are introduced to the EcoSphere and are asked to create an explanation and diagram for why this biological system was able to function for a long period of time (several years). After each group shares their diagram with the class, they are given Matter Problem Part B either in class or as homework. (Suggestions: 1 diagram per group of 3 or 4, using white boards or poster paper works well because diagram is large and becomes a shared space for ideas.)

In previous implementations of these initial two tasks, almost all groups create a circular diagram for Part A to account for the functioning of the EcoSphere system. The diagram reflects only the feeding and gas exchange relationships between the species. For Part B, students almost always conclude that shrimp, algae, and bacteria are all essential components in the sphere, and argue that the shrimp are necessary because they are the only organisms that will provide the carbon dioxide needed by the algae. Students rarely consider decomposers as participants in gas exchange in the EcoSphere.

These introductory problem tasks (Part A and B) serve several purposes. First, a norm can be established such that the students begin to understand that they are responsible for their solutions. To facilitate the passing down of the problem to the students, the instructor should refrain from giving the students any feedback as to whether or not their cycles are accurate representations; rather, the instructor should allow each group to discuss and create their own explanation. Second, by sharing aloud, students gain confidence in their own conclusions when they find that the other groups constructed diagrams and explanations similar to their own—that bacteria, algae, and shrimp are all essential to the functioning of the sphere. This provides the opportunity for Part C to create perturbation—brine shrimp are not in fact necessary for all other species’ survival. Third, these two problems allow the instructor to verify his/her initial model of students’ ways of understanding and thinking. Finally, the problems provide an opportunity for the students to become familiar with the species in the EcoSphere and hypothesize about their relationships. Without this familiarity, the students would not assimilate and be puzzled by Part C.

Part C: The circular diagrams students create for Part A and B reveal that they understand that biological systems function as a result of relationships between organisms; yet they do not usually understand that all of the organisms within the system are participants in the cycling of elements or the transformation of matter and energy. Based on findings from multiple interviews conducted prior to the development of the problem situations (Maskiewicz, 2006), this “circle of life” representation was anticipated and exploited when designing Part C. The objective was to create in the students a need or motivation to understand that all organisms are matter-transforming systems including bacteria, and to begin to consider respiration as an underlying process facilitating matter cycling in an ecosystem. Recall from Parts A and B that students argue that the shrimp are a necessary component for the functioning of the sphere. In Part C they are asked to explain how it was possible for the sphere to function without shrimp. The intent of the problem is to cause initial surprise and puzzlement, and then to create a need to understand how the algae and bacteria’s needs are met. See Table for Parts C and D.

Depending on the class, developing a solution to Part C can take up to two hours. Working in groups, the students deduce that without shrimp there is no NH₃, and therefore, the nitrifying bacteria die. Eventually the students conclude that the heterotrophic bacteria must provide the algae with its needs, and vice versa. Coming to this conclusion, however, takes time, and the students need to be encouraged that they can make sense of the data and develop an explanation that can be supported by the data. From previous implementations, a few important instructor moves have facilitated the solution process and are discussed below.

Appropriate Intervention by Instructor. There are always at least a few groups that conclude early on that the shrimp are not necessary as a source of carbon dioxide or nitrogen because these compounds are already in the water. If the students find this solution satisfying, then they would not have a need for bacteria to transform matter and, therefore, would not develop the intended way of understanding. The role of the instructor is important in guiding the learners to understand the insufficiency of their initial solution by helping the students to recognize the need to account for a source: If the ecosphere survives for eight years algae, will the algae be able to keep taking these compounds from the water that whole time? Students always realize from this question that there must be a source that they are not accounting for.

Refrain from “fixing” the cycle. Often times the intermediate cycle diagrams that students generate while trying to develop a solution are incorrect. However, the desire to create a cycle plays a critical role in their development of the desired understanding. It is this existing knowledge that plays a key role in helping the students develop an awareness and, subsequently, an understanding of organisms’ participation in the cycling of elements. Once the students recognize that both algae and bacteria require similar elements for survival, they begin to account for the exchange of these elements between the organisms. This is an important step in the process of developing an understanding of bacteria as matter converters. It is often the case that students will struggle to reconcile their current cyclical diagram with the data to account for the transfer of various elements between species, and the data should be the impetus for changing an unscientific cycle, not the teacher.

Sharing ideas. Usually after 30 to 40 minutes, a few groups in the class are considering the idea that the nitrifying bacteria die off. At this point I have each group share their current ideas with two other groups, organizing this sharing such that the hypothesis is “spread” among the groups. I do not tell some of the groups that they are on the right track and that they should share this “correct” knowledge with other groups, but rather, through sharing and challenging each other’s ideas, the groups come to the conclusion that this is a plausible explanation that accounts for all of the data. Comparing the data with this proposed explanation becomes the focus after sharing.

For example, in a previous implementation of these tasks, one group spent almost an hour trying to make sense of the relationships between the algae and the two kinds of bacteria. The various explanations they proposed accounted for only part of the data. For this reason, they were never satisfied with their account of why the EcoSphere system was able to function. The information sheet stated that nitrifying bacteria converted ammonia to nitrogen oxides to meet its energy needs; however, the data sheet for the ecosystem with no shrimp showed that the ammonia levels decreased to zero. The group created a cycle to show how nitrogen moved between the bacteria and algae, but their cycle included ammonia as one of the products. The fact that ammonia decreased to zero presented an obstacle that the group could not resolve. When another group visited this group and shared their solution that nitrifying bacteria die, the first group had been ready for a solution that allowed them to reconcile the ammonia and nitrifying bacteria issue. Prior to this sharing, the first group was not able to achieve their goal—to create a cycle to account for the conversion of elements between the bacteria and algae in the sphere—without conflicting with the data until hearing from another group. The cycle they eventually developed was an acceptable biological explanation that identified bacteria and algae as matter transforming systems.

Part D: The goal of this last Bacteria Problem is to provide the students an opportunity to apply their newly developed understanding of bacteria as matter transformers. It is important to point out here that Part D is fundamentally different from Part C because D does not necessitate a specific way of understanding for the students. Instead, Part D provides an opportunity for the students to solidify and appreciate the way of understanding they recently formed, as well as allow the instructor to formatively assess students’ understanding. In other words, Part C is designed to create in the student a need for a particular way of understanding not yet formed, while Part D provides the student an opportunity to apply his or her newly developed or refined way of understanding, which can facilitate the development of an interconnected way of thinking.

One more set of tasks with data was recently developed: Matter Problem Part E. The purpose of this task is to help students develop and apply an understanding that producers perform cellular respiration 24/7. This has turned out to be the only content knowledge that students have not improved on in pre- and post-tests conducted on the original Ecosphere problem sets. These new tasks were developed and tested by Josh White in his freshman high school biology class in 2013 (see White, 2013).

**If you use these problem tasks, please cite this website as the source.