Natural experiments: taking the lab outdoors Teach article

By assembling a ‘backpack laboratory’, you can break away from the lab bench and take tests for starch and glucose into the wild outdoors.

Many teachers like to take their classes on field trips. So why do we usually confine chemistry lessons to the laboratory or classroom? The benefits of outdoor teaching are well documented (Malone and Waite, 2016; Dillon et al., 2006), and include both cognitive and skills-based aspects. It’s also well known that, for most students, solving puzzles is more exciting than reading textbooks or listening to lectures. 

For these reasons, we decided to develop an enquiry-based approach to science lessons, where students act as ‘detectives’, first creating their own hypotheses in the classroom, then gathering information outdoors and applying their findings to the hypotheses.

In this article, we describe an activity focused on measuring the levels of glucose and starch found in plants and fungi. We equip the students with an investigation bag containing everything they need for the chemistry field work. At the end of the activity, the students return to the classroom to discuss their findings.

The activity is suitable for students from 16 years of age, due to the chemistry involved and the sources of error that need to be considered. The whole teaching sequence takes approximately 3–4 hours.

Testing for starch and glucose

To test for starch, we used the iodine test in the form of Lugol’s solution (aqueous potassium iodide and iodine), as this simple test is well suited to taking out of the classroom, and only needs one droplet to produce a result. 

Replacing the traditional test for glucose

Traditionally, Fehling’s reaction is used to test for glucose (and other reducing sugars). However, as some of the chemicals involved are environmentally toxic or corrosive, we wanted to replace this test.

Instead, we used glucose test strips designed for urine analysis. These give a semi-quantitative result, providing a more informative, as well as a safer and more convenient, test for field work. We tried out test strips from three different manufacturers, and we recommend using those that show glucose concentrations of up to 5% – for example, the Keto Diabur Test 5000 made by Roche.

Sources of error

Because the test strips are designed for use only with urine, there are some important sources of error that need to be considered when using the strips in other contexts, as they may reduce the accuracy of the readings obtained. These include: 

1. Colour: the strong colour of some fruits and vegetables can mask the colour of the test result, which has to be compared with colours on a scale to find the glucose concentration.
2. Chemistry: plant saps or juices are complex mixtures of molecules and ions, some of which can interfere with the chemical reactions involved in the test. For example, large amounts of ascorbic acid may lower the test result.
3. Viscosity: thick liquids can give misleading results as there is more liquid on the test stick, and the glucose molecules take longer to reach the stick as they have further to diffuse.
4. Practicality: some test kits have scales that are difficult to distinguish, and some kits measure more factors than glucose (such as ketones or proteins) with up to ten test squares on a strip, which can be confusing for students.
    

How do glucose test strips work?

In commercially available urine glucose tests, two enzymatic reactions are coupled.

First, glucose is oxidised in a reaction that produces gluconolactone (C₆H₁₀O₆) and hydrogen peroxide (H₂O₂):

C₆H₁₂O₆ + O₂       ^{glucose oxidase}    >  C₆H₁₀O₆ + H₂O₂

To make the reaction visible, the hydrogen peroxide is then reduced by another enzyme (a peroxidase), and in the process oxidises a chemical substance called a chromogen. The chromogen gradually changes its colour as it is oxidised, so the final colour depends on the amount of glucose originally present. 

Finally, to find the glucose concentration, the colour obtained is compared with a row of colour blocks provided in the kit (see figure 1 and figure 2).

 

 

Phase 1: Learning to use the tests

Working in groups, students learn to use the tests before leaving the classroom. This initial activity takes around 20–30 minutes. The investigation bags should be made up beforehand.

Materials

• 

  Glucose (dextrose) in tablet form

• Apple juice, or glucose solutions (e.g. 0.1% (0.1 g/100 ml) and 1% (1.0 g/100ml))
• Food containing lots of starch (e.g. potato, bread), or a starch suspension
• Investigation bags, each of which should contain the following (see figure 3):
  • Glucose test strips
  • Lugol’s solution (starch test)
  • Pestle and mortar (or other small dish) to extract juice from the samples
  • Knife (to cut branches or roots)
  • Wipes and distilled water (to clean the tools)
  • Box for litter
  • Magnifying glass
  • Cutting board (optional)Gloves and safety glasses

Procedure

1. Distribute the investigation bags, providing one bag per group of students.
2. Ask the students to read the instructions on the glucose test kits in preparation for conducting a test with the glucose solutions or apple juice. Remind students that the test square should be dipped into the sample liquid for just one second and excess liquid removed. They will need to wait (normally between 30 seconds and 2 minutes) before reading the result.
3. For starch testing, place one droplet of Lugol’s solution on the sample under test. The solution will change to black if starch is present, or remain orange-brown if the test is negative. 

 Phase 2: Deciding on the hypotheses to test

Allow about 20–30 minutes for this discussion phase. 

Procedure

1. Remind students of what they already know about glucose as the central molecule in cellular respiration, and also as the product of photosynthesis in plants (and thus the basis of the energy flow from producers to primary consumers in ecosystems). In organic chemistry, glucose is the building block of many carbohydrates.
2. Set students the task of working as ‘molecule detectives’ who search for answers by detecting glucose and starch in the natural environment. Ask the students, who should work in groups of 3–5, to formulate hypotheses about starch and glucose that they can then investigate. Table 1 below shows some examples of hypotheses, with the reasons why students may propose them ­– which may be false or invalid.
3. Collect and discuss the hypotheses as a class, and task each group with investigating at least one hypothesis. Ideally, each group should have one investigation that will result in a high glucose reading, and one that will result in a low or no glucose reading.

 Phase 3: Field work

We suggest 30–60 minutes for the outdoor phase of the activity; the time needed depends partly on weather conditions. Students will need appropriate outdoor clothing, and taking a first-aid kit is a sensible precaution.

Before the field work begins, check the location where the students will carry out the investigation, and consider the season. Is the location safe? Are there enough plants, berries and other materials to test, and do your students have permission to pick them? 

Materials

Each group of students will need:

• Investigation bag (as for phase 1)
• Cameras, smartphones and/or notebooks, to record the species investigated and results obtained
• Optional: field guide about common plants in the area

Procedure ​

1. Go with your students to the location, keeping the groups gathered together so that you can answer any initial questions.
2. Working in groups, students gather samples to test their hypotheses. Samples may include any of the following:

• Berries
• Leaves
• Roots
• Stems
• Flowers
• Seeds
• Tree bark
• Wood (from tree branches and twigs)

3. Students test their samples for glucose and starch using the equipment in the investigation bag. When testing for starch, use the magnifying glass to see details – for example, in grass seeds.
4. Students collect all their belongings and any litter before returning to the classroom.

Phase 4: Analysis and discussion

Allow at least 45–60 minutes for analysis and discussion back in the classroom. At the start, advise the students not to worry if their results seem inaccurate, as it is just as important to think about the sources of error in the measurements (outlined above) to account for the results they obtained. 

Procedure

1. First, students write up their field notes and results from the outdoor phase. In their groups, they discuss their results and draw conclusions about their initial hypotheses in a way that can be presented to the class.
2. Students then use the internet to find out scientific reference values for the items they tested outdoors. It is quite difficult to find glucose levels for specific plant sources online; see the resources section at the end of this article for suggested websites. Remind students to take extra care when converting units, so that they can compare values from the test sticks to sources on the internet or in books. For some inedible items, students can try thinking of a similar species that is edible and find those values in a nutrient database, as we did with dandelion leaves and lettuce (see table 2). 

The examples here give a realistic idea of the results you can expect: sometimes the values are nearly identical, and in other cases there are large differences.

Discussion

The whole class should discuss the actual results obtained by each group, their conclusions about their hypotheses, and the comparisons with the databases. 

Topics for discussion of the results could include the following:

• Why is there more glucose in berries than in leaves?
• Why is there no starch in berries, but a lot in roots?

Questions relating to the data comparison could include:

• Why are the test results and the database values different?
• How much of this difference is due to measurement errors in the test itself?
• How much of the difference is due to natural variation?

Extension activity… and beyond

As a further research project, students can follow up the discussion about starch to dig deeper into the different carbohydrates produced by plants.

We have extended this outdoor teaching approach to other topics in chemistry, for example: finding different metals and metal ions in the rural or urban vicinity; investigating campfire chemistry; studying the phase transitions of water; and measuring carbon dioxide levels. What other opportunities might there be in your location?

Acknowledgement

This teaching unit is part of a project called ‘The mobile chemist’, which received financial support from the Department of Education at The Arctic University of Norway. More information about the project is available from the author, who can be contacted at jan.hoper@uit.no

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