Chemical senses: a context-based approach to chemistry teaching for lower secondary school students

Activity 1: resolution of an initial survey

Students were asked to answer a printed Initial Survey during the initial 15 min of the first meeting, to let their individual previous knowledge emerge and be registered. The Initial Survey consisted of five triggering sentences that comprised subjects on general senses, taste and smell.

Students were asked to select, for each sentence, all the options (out of five) that they assumed to be right to correctly complete that sentence. Table 1–Table 3 present those five triggering sentences with their respective five options as well as the percentages of choice selected by G1 and G2.

Activity 2: acknowledgement of the diversity of choices

Once all the students completed the Initial Survey by selecting all the options that could properly complete each sentence (Table 1–Table 3), students had to become aware of the diversity of choices made by the group as a whole. To achieve this goal, the teacher read each question with its options and requested that those who had chosen each option raise their hands. A quick count generated students’ astonishment when they acknowledged the great variety of elections. Then, a brief justification of their choices was asked to those who had chosen, or not, each option. Thus, mental representations, ideas and/or previous learning were clearly put forward.

After this activity, all students of each group requested to know which was/were the correct option/s. This was a piece of evidence to illustrate their motivation to learn.

Activity 3: introduction of the scientific subjects with performance of simple hands-on experiments

After the exchange about the Initial Survey options selected by the students, the teacher provided information to start a discussion from each triggering sentence with scientific arguments that would sustain -or not- each option. The theoretical content was adapted to the level of each group.

Some of the theoretical contents were reinforced with hands-on experiments. Since these were simple and brief experiences (see Table 4), all participants had the opportunity to experiment with their own senses of taste and smell. This instance was highly attractive for students.

A scientific explanation for each sentence and their relationship with the hands- on experiment are further introduced in this work.

Altogether, this activity demanded two classes.

Activity 4: special project for students – an artistic video production

Once discussed the contents related to the chemical senses, both theoretically and experimentally, students were given, during the fourth class, the instructions for a special final project. The class was divided into groups of 4–5 people. They had to choose one out of eight possible given titles: Map of the tongue?, Olfactory adaptation, The five basic tastes, Taste buds, Perception vs. Sensation, Taste and flavor, The sense of taste and The sense of smell. They had to prepare a 3-min video accompanied by a written account of their outcome about the selected topic.

They were given a month to prepare their video and, after that time, each group presented its production to the rest of the class. After each presentation, students were encouraged to self-criticize their work and to express constructive opinions about the work of the other groups.

Students’ presentations and their discussions demanded two classes.

Activity 5: resolution of a post-test

Before the end of the last class, students were asked to briefly solve a Post-test (PT) in order to analyze if the main scientific ideas presented in the Initial Survey had been achieved throughout the activities. It consisted of six sentences to which they should assign the values of true or false. Table 5 shows the correlation between main scientific issues that should have been learnt from the Initial Survey, the performed experiments, and the Post-test sentences. Results are shown in Table 6 as percentages of correct answers achieved for G1 and G2, respectively.

Initial survey (IS)

The data collected from the Initial Survey are presented in Table 1–Table 3, expressed as percentages of choice by G1 and G2, respectively, for each option. The total sum of percentages of choice may exceed 100 % because participants could select more than one option. These figures are used for exclusively comparative purposes since their values are neither important nor generalizable to other populations, but pieces of evidence to analyze the development of students’ responses to the present approach.

Next, a brief description of scientific main issues involved in sentences of Table 1–Table 3 will be presented along with the corresponding discussion of the students’ answers.

The only correct option for this sentence is 1c. The senses, depending on their modality, are adapted to respond to the stimuli that occur in the environment thanks to specialized cells that are activated against a certain type of interaction (Foley & Matlin, 2010; Proctor & Proctor, 2006). This answer is correct but strictly incomplete because stimuli do not come only from the external world but also from the internal environment. Although 33 % of G1 and 56 % of G2 correctly chose this option, when carrying out the oral discussion it was evident that practically none of them knew that the information could also come from within their own body.

Option 1a is incorrect because, although smell, taste, touch, sight and hearing are the five traditional senses, today there is broad consensus that humans have many more, including thermoception, nociception, proprioception and equilibrium (Proske & Gandevia, 2012; , 2001). Unfortunately this information, which is well known in academic and medical fields, is not yet installed among teachers and continues to erroneously appear in school textbooks. An overwhelming majority of students chose option 1a as the correct one (93 % G1 and 90 % G2).

Option 1b is also incorrect because, though in colloquial terms, perception can be used as a synonym to sensation, both processes are physiologically different. While the sensory process receives simple isolated physical stimuli from the environment or the body itself, the perceptual process is an interpretation of that information provided by the sensory process. Thus, our senses do not perceive information but rather sense the stimuli (Foley & Matlin, 2010; Proctor & Proctor, 2006). Discussion with the students showed that the 33 % of G1 and 60 % of G2 who selected this option did not acknowledge a difference between these two meanings.

Option 1d is too general: it is not possible to assert that human beings have more or less developed senses than the rest of animals. It is not even possible to say that they are the same senses (Drake, 2011; Enjin et al., 2016; Wu & Dickman, 2012). Nine percent of G1 and 29 % of G2 % indicated this option as correct and their explanation was limited to the sense of smell in cats or dogs and the sight of some birds.

Regarding option 1e, among all the senses, only smell and taste are stimulated by the presence of chemical substances – molecules or ions – that interact with sensory cells (chemoreceptors). That is why they are often referred to as chemical senses (Buck, 2000a,Buck). Only 15 % of G2 and none of the students of G1 indicated this option but, during the oral discussion, it was clear that it was not because students knew that it was incorrect to generalize all sensory processes to interactions between ligands and receptors, but because they had not fully understood the sentence.

Table 1 also shows that older students (G2) felt more comfortable when choosing more than one option compared to G1.

When presented the theoretical content to the students, it was emphasized that many senses existed, beyond the five traditionally considered ones and, especially, that the chemical senses are the only ones mediated by chemoreceptors. This led to work on the concept of ligand-receptor interaction, i.e. the binding of a signaling molecule to its receiving macromolecule, as well as an introductory presentation of intermolecular forces. It was pointed out that receptors and ligands come in many forms, but they all have in common that the receptor is capable of recognizing just one (or a few) specific ligands, and a ligand is capable of binding to just one (or a few) target receptors. It was also highlighted that the binding of a ligand to a receptor changes its shape or activity, allowing it to transmit a signal or directly produce a change inside of the cell.

This idea was further explored in the theoretical discussion about the senses of taste and smell to introduce basic ideas about stereochemistry.

Regarding sentence 2, options 2a, 2b and 2c are correct. For sentence 3, options 3a, 3b, 3c and 3e are correct.

The human sense of taste is focused in the tongue, where the majority of the taste buds are located – although there are also taste receptors on the palate, the cheeks, the upper part of the throat and the epiglottis. Adults have between 2000 and 4000 taste buds and their distribution depends on each individual (Arvidson, 1979). While the ability to sense tastes is determined, in part, genetically, numerous non-genetic factors such as temperature (Verhagen & Engelen, 2006), saliva production (Matsuo, 2000), age (Mojet, Christ-Hazelhof, & Heidema, 2001; Stevens, Bartoshuk, & Cain, 1984), previous experiences (Small & Prescott, 2005) and non-sensory contextual factors have an impact (Duffy & Bartoshuk, 2000; Reed, 2008).

Human beings have specific receptors for only five basic tastes, that is, they do not result from the combination of others: sweet, salty, sour, bitter and umami (Chandrashekar, Hoon, Ryba, & Zuker, 2006; Ikeda, 2002). Less than half of the participants chose this option in both groups (2a, 47 % G1 and 46 % G2). During Activity 2, those who had not chosen it explained that, in their opinion, humans can sense much more than five tastes. This explanation may be due to the misconception of considering taste and flavor synonyms. (Rozin, 1982). Flavor is a more complex concept that results from different afferents including taste, smell and somatosensory fibers (Small & Prescott, 2005).

Options 2d and 2e are not correct although 2d was particularly highly selected. There is a very popular false belief that each basic taste is sensed in a different section of the tongue, a representation known as “map of the tongue”. Although the idea has been widely refuted, (Collings, 1974), 35 % of G1 and 50 % of G2 chose this option. These results could be due to the fact that it is still present in textbooks and through schooling progress, students would reinforce the misconceptions.

The discussion about these contents was complemented by three hands-on experiments: Dry tongue, Map of the tongue in doubt and Counting taste buds (Table 4).

During the theoretical discussion of these contents, the five basic tastes and representative ions and molecules capable of interacting with their specific receptors were presented. The existence of two different types of receptors was mentioned: ionotropic (for salty and acidic taste, i.e. Na⁺ or H⁺ and G protein-coupled receptors for bitter, sweet and umami tastes.

In the case of G2 students, as for the sweet taste receptor, the structure of different sugars (glucose, fructose, sucrose) was shown and from the values of its sweetening power, it was discussed how the shape of the molecule influences its interaction with the receptor (how it “fits”). The structures of some commonly used sugar substitutes (Aspartame, Acesulfame, etc.) were also shown.

The concept of solubility was explained based on the “Dry tongue” experiment; it is observed that for taste receptors to sense molecules or sapid ions they must be dissolved. We delved into this topic with different mental experiments (for example, what would happen if our saliva was not composed mostly of water but by other types of solvents? Could we sense all tastes in the same way? What do you think would change?).

Regarding sentence 4, only option 4d is correct. The functioning of the olfactory sensoperceptive process is complex in humans (Buck 2000a; 2000b; Firestein, 2001; Su, Menuz, & Carlson, 2009). Olfactory information is key when it comes to identifying flavor (Rozin, 1982). If the smell is absent, due to some physiological problem or a cold, we are not able to perceive the differential aroma that characterizes each food although we can still identify the basic tastes and other somatic sensations. It is necessary to know the difference between the concepts of flavor and taste. Clearly, this was not the case for G1 students since option 4d was the least chosen (19 %).

For sentence 5, options 5b and 5d are correct. After being a long time exposed to a stimulus, the senses of taste and smell decrease and the receptors do not continue to react at the same initial speed even when the stimulus retains its intensity. This reduction of the response to a constant and uniform stimulus is called adaptation and is believed to be an important functional mechanism that prevents excess neuronal activity and it allows us to remain alert and to obtain new information (Köster & de Wijk, 1991). Students’ prevalent choice was that common sense idea about “getting used to” aromas, without any real understanding of what that means (option 5c, 81 % G1 and 71 % G2).

To broaden these explanations, three hands-on experiments were carried out: Pinched nose, Odor Eraser and Olfactory confusion, detailed in Table 4.

When it came to the theoretical discussion of the sense of smell, the concept of ligand-receptor interaction was retaken, but in this case, to deepen the notion of chemical equilibrium as a dynamic process in which the ligands are not “fixed” to the receptor but a permanent exchange takes place. Therefore, we emphasized that the concept of saturation (that can be further related to enzymatic dinamics) does not refer to a static situation; the sensorial process continues and what is modified is the transmission of the nervous impulse (its velocity or its intensity) once this sensing has taken place.

The concept of volatility was also presented. The amount of volatile compounds was related to the temperature at which food is served (for example, when comparing the flavor of hot and cold meat). In the case of G2 students, the chemical structures of compounds such as limonene were shown for an incipient analysis of intermolecular forces and its relation to vapour pressure of a given substance.

Post-test (PT)

The Post-test was taken after completion of Activity 4. It consisted of six sentences to which students should assign the values of true or false. These sentences are presented in the right column of Table 5 as well as their direct correlation with the ones that had been presented in the Initial Survey and with the contents related to the hands-on experiments.

Table 6 shows the comparative results of percentage of correct answers between the Initial Survey (equivalent to a pre-test) and the Post-test for G1 and G2, respectively.

While percentages are simply for comparative purposes, it is notorious that most of the concepts explained and experienced during the classes were incorporated, specially items a, d, e and f, by G1, and a, c, d and e by G2 students.

It is striking that G1 and G2 got only 30 % of FALSE answer in item b. This result may be driven from imprecision in sentence b of the Post-test. Indeed, the brain is the organ that centralizes and processes all the information coming either from external or internal stimuli. However, during classes, a semantic difference was given between the verb “sense” and “perceive”. Results may indicate that students did not understand those differences. Anyway, the fact that 70 % would have qualified sentence (b) as true still should be considered a progress on knowledge because of a previous frequent misunderstanding, as emerged during the theoretical explanation, which did not correlate with the functioning of sensorial organs with the brain.

Another interesting result is that only 40 % of G1 students responded properly about the need of saliva to sense taste. This could be due to the fact that these students do not yet handle concepts such as dissolution, molecules and ions, necessary to understand the phenomenon in depth. So, despite having conducted an experiment on the subject, most students have failed to give it the appropriate meaning.

Strategies developed to promote students’ motivation for learning

Evidence of conceptual change and science-based ideas learning impact

Great cognitive advance could be perceived repeatedly after the discussion of each triggering sentence without the need of a formal evaluation. Students were aware of the adequacy or not of their ideas and previous knowledge, of their prejudices and how they had to modify them to correctly support concepts, to build their new knowledge. Some students could clearly express their astonishment at the arguments of other participants. The teacher took advantage and reinforced those moments, knowing that strong idiosyncratic mental representations act, very frequently, as obstacles in communication and/or learning (Garófalo, 2010). The possibility of “making mistakes” could be experienced as a stage of positive emotion, to be able to self-regulate and self-question, in opposition to the traditional negative sensation coming from an “external assessment”.

Furthermore, Post-test results show that many of the main scientific issues could be achieved.

Students’ metacognitive perceptions about their own learning processes

Immediately after completing the Post-test, students were asked to fill an anonymous written poll about their metacognitive perceptions on the whole experience in order to know their degree of self-commitment and their feelings towards the teaching approach. They were asked to choose from a list of words and emoticons those that best fitted their emotions during the activities considering what they had learnt, the experiments, the contents and the special project (artistic production) (Pérgola, 2014; Hugo, 2012; Sánchez Díaz et al., 2018).

The lists of words and emoticons are shown in Figure 1. This is a worthy instrument to evaluate students’ commitment because each number in the word list does not correlate with the number of an emoticon with the similar meaning.

Figure 1:

List of words and emoticons given to the students to self-evaluation and assessment after the didactic approach, considering the following items: “what you have learnt”, “the experiments”, “the theoretical explanation‟, “the special project” and “what you remember now”.

Table 7 shows a classification of those words and emoticons as positive, neutral or negative. This classification was known by the authors but not by the students. G1 and G2 choices of words and emoticons are shown in Figure 2 and Figure 3, respectively.

Figure 2:

G1 students’ opinions about the experience expressed (a) with words; (b) with emoticons.

Figure 3:

G2 students’ opinions about the experience expressed (a) with words; (b) with emoticons.

Figure 2 and Figure 3 show the results from the polls separated by age groups (G1 and G2). In all cases, a very positive assessment of the experience was verified. The results from emoticons or words did not show major differences between them, which confirms that students made a committed selection and not a random one.

Comparison between G1 and G2 responses (Figure 2 and Figure 3, respectively) show impressive similarities. Inside each group, 90 % of the students expressed their joy for doing experiments, and it is worthy to highlight that up to 80 % of students claimed that they had learnt and just 50 % admitted they had enjoyed the theoretical explanations.

The mixed results observed in “The theoretical explanation” category for G2 students (Figure 3) are considered to be due to two interrelated reasons: (i) issues of language and (ii) depth of the chemical content taught. On the one hand, words are clearer and more precise than emoticons to express emotions. For example, the negative assessment of the theoretical explanation by students (both G1 and G2) consisted, mainly, in the use of adjectives “complicated” and “difficult” with just a few “boring” while, in the case of emoticons, students used emoticons 6, 7, 8, 9, 16 (negative), 13 (neutral) and 15 (positive) indistinctly. It is this emoticon, number 15, which raises the controversy. In our categorization, emoticon 15 was considered positive but many students who valued the theoretical explanation as “difficult” or “complicated” chose this emoticon to represent it. We assume this is due to the stereotypes related to scientists (i.e. they are males; they wear glasses; they work on complicated subjects) (Finson, 2002). This would correspond to the second reason: the contents explained to G2 students were deeper from the chemical point of view, that is, more “scientific” and that possibly contributed to their choice of the stereotypically “scientific image" (emoticon 15).