5.1. Genesis of Alternative Conceptions
Due to these findings, we can state that students often hold non-scientific conceptions about plant nutrition (I). These conceptions derive from our embodied experience about the human diet as a food-taking process (II). Thus, they are based on the Container Schema
and the Source-Path-Goal Schema
. This leads to the observed phenomenon that plant nutrition is also conceptualized as a food-taking process. As a consequence, the van-Helmont experiment inevitably does not trigger learning processes (III) [41
]. Subsequently, students’ concepts are not changed but reinforced because they are proven appropriate. But how can we deal with the problem of teaching plant nutrition? We assume to teach photosynthesis from a different perspective.
The embodied conceptions students have about nutrition cannot be erased or changed. However, they have to be used as learning potentials [21
]. The challenge is both: to change the learner’s perspective that plants are thought as food-taking—in a way heterotrophic organisms—to see them as food-producing, autotrophic organisms and to take embodied conceptions into account.
5.2. Using Alternative Conceptions and Image Schemas for Conceptual Change
International curricula like in the US, Great Britain, or Germany suggest structuring the topic of plant nutrition in the following way: Assimilation-before-Dissimilation. In this context, assimilation is understood as the process of providing essential nutrients. Heterotrophic organisms realize this by digestion processes. Autotrophic organisms produce their own nutrients. Furthermore, the term dissimilation is defined as the usage of high-energy nutrients for energetic and constructive reasons. This leads, for example, to the topics of growth, cell division, and cellular respiration.
The curricular approach first introduces the process of photosynthesis to the students to convey the biological concept of producing organic matter (assimilation). Second, it points out the importance of the produced substances like glucose and starch for, for example, cellular respiration (dissimilation). In conclusion, photosynthesis as a necessary assimilation process is preliminary for cellular respiration, a proceeding dissimilation process. We claim that the described Assimilation-before-Dissimilation
structure is not fruitful to elicit conceptual reconstruction. Neither does this lesson plan take learners’ mental frameworks and embodied knowledge into account, nor does it effectively link human and plant nutrition by showing important similarities and differences [43
]. When it comes to plant nutrition, a change in thinking must take place: instead of conceiving nutrition as a process of food taking, the concept of food production has to be constructed.
Seen from a learner’s perspective, plant nutrition has to be thought from embodied conceptions about the human diet. Thus, the identified conceptions, frames, and underlying image schemas are to be seen as learning potentials. Therefore, we suggest reversing the Assimilation-before-Dissimilation structure to a Dissimilation-before-Assimilation approach.
Humans and plants alike need the same types of substances to stay alive. In a first step, it has to be elaborated on the usage of nutrients for energetic (cellular respiration) and constructive metabolism (growth, etc.). Nutrients are seen explicitly as high-energy substances such as carbohydrates, fats, and proteins. Questions like “What do humans and plants need in order to grow?
” have to be part of a constructive teaching unit (see Figure 2
). Thereby, embodied cognition about human nutrition is mapped onto plant nutrition. As a consequence, students are able to discover that plants and humans are similar by using the same low-energetic substances like water, minerals, and high-energy nutrients to stay alive (similarity: usage of nutrients). Here the comparison of plants and humans points out that both are conceptualized as objects that consist of different types of substances (Component-Integral-Object Schema
). An increase in numbers of these substances automatically leads to growth (Becoming-Bigger Schema
). Subsequently, they are experienced as living organisms of the same structure (dissimilation aspect
). For means of reduction, vitamins and fibers needed by heterotrophic organisms were left aside.
After addressing dissimilation processes by showing the similarities between humans’ and plants’ usage of nutrition, the inevitable question about the acquisition of these fundamental substances and, thereby, the differences between human and plant nutrition can be derived—“Where does the organism get water, minerals and nutrients from?” Talking about human nutrition (source domain), learners’ know that they have to consume necessary substances (Container Schema, Source-Path-Goal Schema). Because the only source that consists of high-energy nutrients comes with other (living) organisms, humans are called heterotrophic—obtaining their food by consuming other organisms. As far as plant nutrition is concerned (target domain), it is essential to point out that plants need to absorb minerals, water, and carbon dioxide (Container Schema, Source-Path-Goal Schema) to produce high-energy nutrients (frame Nutrition due to Nutrient Production). These nutrients are the building units used for plant growth (Transformation Schema, Component-Integral-Object Schema). For many students it remains unclear that humans as well as plants actually consist of the same types of substances leading to the genesis of alternative conceptions such as absorption of nutrients from the soil. The learning potential can be seen in pointing out the similarities both types of organisms have regarding their kind of nutrition: Plants need the same nutrients, but produce them themselves. They both need to absorb minerals and water (frame Nutrition due to Absorption). In this way, the topic of plant nutrition can be addressed by using embodied conceptions based on human nutrition.
In the second part of the teaching unit, it is now possible to confront students with their own conceptions using the van-Helmont experiment. The preconditions to create a conceptual conflict are provided [32
]. In this particular context, water and minerals do not suffice anymore to explain the plant’s growth. Consequently, there has to be a different way to acquire high-energy nutrients. Here the differences
between human and plant nutrition have to be pointed out. Whereas humans are heterotrophic by consuming other organisms, the idea of plants as food producers can be introduced. After the teaching unit, plants should be seen as food-taking organisms as far as water and mineral consumption is concerned and, more of importance, food-producing organisms in regard to high-energy nutrients (see Figure 2
By pointing out similarities and differences of human and plant nutrition, a different approach in teaching has been suggested in this paper. Our findings indicate that teaching dissimilation
processes before assimilation
processes makes it easier for learners to reconstruct sustainable knowledge based on embodied knowledge. This new approach may function as a door opener when it comes to teaching plant nutrition. First results, where students (N
= 16) were confronted with this alternative approach, seem very promising and indicate that conceptual reconstruction towards a more accurate scientific understanding takes place. However, changing learners’ conceptions is not only about cognitive variables. One has to take social and emotional parameters into account. Therefore, conceptual change does not merely happen on the basis of a singular intervention [44
]. Nevertheless, photosynthesis is a recurrent topic in science education. The learning pathways shown offer the opportunity to capture students with their individual learning stages. This makes it possible to create learning-friendly environments based on relevant frames and image schemas to trigger gradual and continuous learning processes. Consequently, further studies about the efficacy and sustainability of the Dissimilation-before-Assimilation
approach have to be done surveying long-term effects and focusing on the degree of understanding.