A coupled inquiry lesson
explores the catalytic
activity of amylase on starch
G re g o r y T. R u s h t o n ,
M i c h a e l D i a s , a n d G ra n t
McDurmon
T
he manner in which enzymes function as biological catalysts is a fundamental concept taught in high
school biology courses, usually during the study of cellular structure
and function (NRC 1996, p. 184).
We have found it worthwhile to
engage students in inquiry experiences involving the enzyme amylase as it digests the polysaccharide
starch. This experimental system
is simple and relevant. Amylase is
present in human saliva, and the
substrate, cornstarch, serves as a
clear example of a macromolecule
that humans digest.
In this article, we describe a
two-phase inquiry lesson in which
students explore the catalytic
activity of amylase on starch
(Rungruangsa and Panijpan 1979).
In the first phase, students’ prior
knowledge about the reaction is
assessed through a set of directed
prompts and small-group discussion, then challenged or reinforced
as students carry out a laboratory
investigation.
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September 2008
60
During the second phase, students design and carry out
an experiment to further explore the phenomenon, choosing to study the effect of pH, temperature, or substrate
concentration on the kinetics of the starch hydrolysis.
The instructional design follows the 7E learning cycle
(Eisenkraft 2003) and draws upon several features of classroom inquiry (NRC 2000, p. 29). As shown in Figure 1,
the learning cycle consists of seven phases: elicit, engage,
explore, explain, elaborate, evaluate, and extend.
Elicit and Engage
We first elicit students’ prior knowledge relating to the
topic and engage them in the lesson by asking them to predict the composition of a packing peanut used for shock
protection of shipped items. We also ask students to think
about why the peanuts are made out of that material before they respond. Most students propose that the peanuts
are composed of plastic or Styrofoam and that companies
use the plastic because it is inexpensive, light, and good
for protecting objects against breakage. Because we have
found students rarely predict that the peanut is made from
starch, we pause at this point in the lesson to introduce
environmentally friendly, biodegradable packing peanuts
made from cornstarch. We show this by shaking one or
two of the packing peanuts in 1 L of warm water for a few
seconds to create a dilute starch solution.
We continue the elicit and engage phase of the learning
cycle by asking students to predict the outcome and explain
their reasoning for what they expect to happen when a few
drops of light brown Lugol’s iodine solution is added to the
starch solution. Lugol’s iodine is typically prepared with
5% iodine and 10% potassium iodide in distilled water or
can be purchased directly from most chemical and biological suppliers. More dilute solutions can be used without a
loss of color formation. We use a 0.002 M solution with
good results. We ask students to write down their predictions and explanations in their science journals because
we find that this improves the engagement of all students
in the class discussion—more outgoing students tend to
dominate the discussion otherwise. Although it may seem
more expedient to merely pose this question for discussion,
eliciting responses from each student in writing allows us
to formatively assess their prior knowledge.
Before being asked to share their ideas in front of the
whole class, students are directed to discuss their thoughts
with one or two classmates nearby, and then gain a consensus
as a small group with respect to what will happen. While students are thinking, writing, and discussing their ideas with
each other, we give each student one test tube full of the iodine solution and another containing the starch solution. The
two most common responses to this prompt are that some
type of chemical reaction will occur between the starch and
the iodine (e.g., “bubbles,” “heat,” or a “color change” will be
observed), and, alternatively, that “nothing will happen” (because iodine and starch do not react with each other).
FIGURE 1
Enzyme lesson using the 7E learning
cycle (Eisenkraft 2003).
Elicit prior knowledge and engage students
u
Ask, “What happens (and why) when saliva is added
to the starch-iodine solution?”
Explore the phenomenon
u
Guide collaborative student groups through an
initial investigation of the basic reaction of amylase
on a starch-iodine substrate. This guided inquiry
builds on prior knowledge to establish conceptual
understandings that will support further inquiry.
Explain based on evidence
u
Have students reason through the reaction they observe by explaining what they think happened based
on the evidence collected during their experiment.
Elaborate
u
Perform a new set of experiments to test three
different factors; have students generate research
questions, hypotheses, and procedures to carry out
a novel experiment.
Evaluate
u
Have students make presentations based on their
experimental results and follow-up research.
Extend
u
Investigate additional questions and issues related
to enzyme function.
After we clarify and summarize their preconceptions (usually by writing them on the board), we direct students to explore by adding a few drops of iodine solution to their starch
solutions. Students are asked to write down as many observations as they can about the changes that take place as they
consider what is happening in the test tube. After students are
finished writing, we allow a few minutes for student groups
to describe the observed changes. Students usually write
about how the starch solution was a bit “milky” or “cloudy,”
and how it turned “blue” or “blue-black” after adding the
iodine solution. The most students can explain at this point is
that “some type of reaction took place.” We explain to them
that the starch and iodine bind together as a “complex” and
the blue-black color is the indication that the specific complex
between the amylose in the starch and the iodine has formed
(Cesaro, Benegas, and Ripoll 1986).
Explore
At this point, students are ready to explore the action of
amylase on starch. As modeled earlier in the lesson, we
first elicit students’ prior knowledge and engage them
in the activity. We prompt students to predict the effect
of adding 2 mL of “synthetic saliva” to the test
tube containing the blue-black colored solution.
(Safety note: We recommend the use of a prepared
September 2008
61
Explain
FIGURE 2
Student experiments.
Photo courtesy of the author
Students investigate the effect of increased temperature on
the speed of the reaction.
solution containing amylase rather than using human
saliva, given the potential hazards associated with using
bodily fluids in the classroom.) We found an inexpensive
and simple method to prepare the synthetic saliva solution by dissolving 1 g of lyophilized (freeze-dried) a-amylase in 1 L of pH-7 buffer just prior to use. The enzyme
can be obtained inexpensively (about $2/g) from most
chemical suppliers. The reagent bottle should be stored in
a freezer to reduce loss of enzymatic activity.
At room temperature, the reaction usually takes 1–5 minutes before a visual change can be observed. Before asking
students to share their predictions with the whole class, we
direct them to draw up approximately 2 mL of the prepared
saliva into the pipette and deliver it to the tube containing
the starch and iodine, then set the tube aside. We elicit student responses by asking them to read their responses to the
class. At this point, we clarify these ideas as necessary, but
do not validate or critique their predictions. The idea is to
determine students’ understanding of enzyme function so
their knowledge gaps and alternate conceptions can be addressed during postlab discussion. We make mental notes of
the areas of student confusion or misunderstanding so that
we can address them sometime before the lesson’s end. For
example, some students predict that if enough saliva is added
to the starch solution, it will turn the solution colorless again,
but if not, the solution will remain blue. A response such as
this highlights a misconception about the fact that enzymes
are catalysts and do not behave the same way as reactants in
chemical reactions.
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The Science Teacher
By the time students have shared their predictions and
explanations, the starch digestion has usually progressed
significantly so that an obvious visual change (i.e., the blueblack color fades) is apparent. Students are prompted to individually write down observations about the appearance of
the solution and to develop compare-and-contrast statements
regarding the solution’s condition before and after adding
the saliva. In small groups, students are asked to construct a
scientific explanation that accounts for the observations they
have just made, and present these arguments to the class for
discussion (the explain element in the 7E learning cycle).
This informal “peer review” process is exciting to watch
and can be quite powerful for driving concept formation,
as student interactions lead to the preference for certain
presented ideas over others (NRC 1996, pp. 175–176). For
example, some students suggest that the saliva “neutralizes” the iodine, while others reason that the starch is being
“digested” by the enzymes in their saliva. Most students
prefer the latter view after it has been presented. After
some time, the class usually arrives at one or two reasonable
hypotheses, which we address through a minilecture. At
this point, the gaps and misconceptions identified through
students’ earlier predictions can also be discussed.
In addition, a few other formative assessment items
are administered (either verbally or in writing) to elaborate on the activity such as:
u What would you expect to see if more iodine solution was added to your reaction tube after the
blue-black color had disappeared? (No change,
because the starch needed to complex with the iodine
has already been hydrolyzed.)
u What would happen in the test tube if additional
starch solution was added to your reaction tube,
after the blue-black color had disappeared?
(Blue-black color should return since the amylase
has not yet broken it down into simpler sugars.)
u What would have happened in your reaction tube
if you had added a starch solution containing 5–10
dissolved peanuts instead of the solution containing
only 1–2 peanuts? (The changes would have been the
same as the original experiment, but the color would
persist longer since the concentration of starch is higher
and requires a greater period of time to be broken down
by the amylase.)
We bring closure to this phase by providing explanation
in response to student questions about this phenomenon.
The big ideas, from our perspective, include the following:
u Enzymes are biological catalysts whose role is to
accelerate chemical reactions in living systems.
u As catalysts, enzymes are not consumed during
chemical reactions and therefore, in theory, can
be used indefinitely (or repeatedly).
u It is essentially impossible to predict how fast a
Enzyme Inquiry
catalyzed reaction will proceed without carrying
it out experimentally.
u Enzymes are substrate-specific—they will only
catalyze reactions between specific reactants.
u The enzyme amylase in human saliva catalyzes
the hydrolysis of amylose (a polysaccharide that
forms helical structures and binds triiodide ions
to form a dark-blue complex) into maltose (a disaccharide that cannot bind triiodide effectively),
hence the disappearance of the blue color.
We find that at this point students are fairly comfortable
with this experimental system and with the function of enzymes as biological catalysts. They also are ready to extend
this developing knowledge by conducting a more open-inquiry investigation aimed at answering the question, “What
variables can we investigate that may have an effect on the
rate of starch digestion?” Instructional sequences such as
these, in which an initial teacher-guided investigation lays
the foundation for a subsequent inquiry that is more open
or student-directed, have been referred to as “coupled inquiry” (Eick, Meadows, and Balkcom 2005; Martin-Hansen
2002). In our experiences with high school biology classes,
students usually suggest temperature and amount of catalyst present, and less frequently mention the solution pH.
Students are then asked to design an experiment to answer
a question about the reaction kinetics (Figure 2). In pairs,
students are asked to complete the following:
u Formulate a research question (e.g., “What is the
effect of pH on the rate of hydrolysis?”).
u Devise a procedure for carrying out an experiment to help answer the question posed (e.g., “Do
the experiment at different pH levels and measure
the time it takes the blue color to disappear.”).
u Determine the data that needs to be collected
(e.g., “the time in seconds”).
u Suggest a tentative hypothesis and your reasoning (e.g., “The reaction goes faster at higher pHs
because the enzyme works better under those
conditions.”).
FIGURE 3
Amylase presentation rubric.
Criteria
Experimental
method
Exemplary
(4)
u
Research question is
u
clearly stated
Variables controlled
uProcedure is explained
u
and consistent with
the research question
Analysis and
conclusions
Proficient
(3)
u
Appropriate data is
collected
u
Multiple trials are
Results are analyzed
thoroughly and
completely
u
Clarity and
presentation
Conclusions are based
on evidence and
consistent with data
uData is clearly
displayed in at least
two forms (e.g.,
graphs, tables, charts,
diagrams)
u
Presentation uses
u
clearly stated
u
Some variables
u
Procedure is somewhat
consistent with
research question
uAppropriate data is
collected
u
Multiple trials may
have been conducted
u
Results are analyzed
incompletely
u
Conclusions are
somewhat based on
evidence and fairly
consistent with data
uData is clearly
displayed in at least
two forms
u
Presentation lacks
some accuracy or
clarity
Limited
(1)
Research question is
u
stated
u
controlled
conducted
u
Research question is
Developing
(2)
and procedure are
not clearly defined or
consistent with one
another
Some variables
controlled
u
Procedure does not
address research
question
uData is collected
u
Data is collected
uResults are not well
u
At least one trial is
conducted
u
analyzed
Results are analyzed
u
incompletely
u
Research question
Conclusions are not
based on evidence or
experimental data
Some conclusions are
not based on evidence
or consistent with the
experimental data
u
Data is displayed in at
u
least one form
u
Presentation lacks
some accuracy or
clarity
Data is displayed in at
least one form
u
Presentation lacks
accuracy and clarity
correct terminology
and is clearly spoken
September 2008
63
Enzyme Inquiry
FIGURE 4
Lessons learned.
What worked:
u
Using two class periods (50 minutes each) to complete
the lesson, and letting students work on their presentations at home before presenting them to the class.
u Refrigerating the amylase (saliva) solution prior to
using it and preparing a new solution if the lesson
would be taught over multiple days.
u Insisting on students writing their answers down
before discussing them with others.
What did not work:
u
Using a concentrated starch solution. The amylase
takes too long to digest the starch and students
have to wait far too long to see the color change.
One peanut per liter of water works well.
u Using a concentrated iodine solution. A very dilute
solution (~0.002 M) in water gave a pleasant darkblue complex with starch; more concentrated iodine
solutions took on a dark gray or black tinge.
u Letting students go to the lab to start their openinquiry investigations without approving their
procedures with the teacher. Most groups needed
some help with making sure they collected the appropriate data and controlled variables.
u
Carry out the procedure (with multiple trials,
if possible).
u Present your findings to the class after experimentation, further reading, and research on the
topic using textbooks, websites, or other reference materials.
Evaluate
To complete the lesson, we evaluate student learning through
a short (3–5 minute) class presentation in which each group
presents its research question, procedure, results, and conclusions to the class and opens the floor for questions (Figure 3,
p. 63). In a typical class of 24–28 students, we structure the
group work such that each experimental variable is tested by
at least two different lab groups. This increases the opportunity to seek patterns in the data. We grade students on three
elements for their final score:
1. Experimental method (consistency, methodology,
appropriate data collected)
2. Conclusions and discussion based on the data collected, their analysis, and further research on the subject
3. Clarity of communication in their presentation
and visual representation of their data
Extend
Through the class presentations, students learn how to
defend their scientific claims using evidence from their experiments, and negotiate understanding in a community of
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The Science Teacher
learners. We believe this practice of guiding class dialogue focused on interpreting
evidence to be consistent with teaching
recommendations in the National Science
Keyword: Enzymes
Education Standards and are encouraged by
the fact that students find this work chal- at www.scilinks.org
lenging, relevant, and meaningful (NRC Enter code: TST090802
1996, pp. 173–176). See Figure 4 for lessons learned. (Note: For a complete lesson plan and teacher’s notes on the inquiry lesson described in this article, visit
www.nsta.org/highschool/connections.aspx.)
The lesson can also be extended by asking learners
to research questions related to, but different from, the
specific action of amylase on starch. Questions we have
found to be thought-provoking and useful for students
include, “Why can we not digest paper the way a termite
can?” and “Why are some people unable to digest dairy
products while others can?”
The responses students give, especially as they relate to
what they have learned through the 7E learning cycle approach to this lesson, can offer insight into how well they
have internalized the concepts and are able to make appropriate connections between related science topics. Students
go beyond rote memorization toward developing deep,
conceptual understanding when they are able to construct
their own science knowledge and make it explicit prior to
a formal presentation by the teacher. For this reason, we
have found the learning cycle and coupled inquiry to be of
great benefit when directing students in the study of enzymes, and invite you to share insights on this lesson based
on experiences with your own students. ■
Gregory T. Rushton ([email protected]) is an assistant professor of chemistry and Michael Dias ([email protected]) is an
assistant professor of science education, both at Kennesaw State
University in Kennesaw, Georgia; Grant McDurmon ([email protected]) is a science teacher at North Cobb High School
in Acworth, Georgia.
References
Cesaro, A., J. Benegas, and D. Ripoll. 1986. Molecular model of the
cooperative amylose-iodine-triiodide complex. Journal of Physical
Chemistry 90(12): 2787–2791.
Eick, C., L. Meadows, and R. Balkcom. 2005. Breaking into inquiry.
The Science Teacher 72(10): 49–53.
Eisenkraft, A. 2003. Expanding the 5E model. The Science Teacher
70(6): 56–59.
Martin-Hansen, L. 2002. Defining inquiry. The Science Teacher 69(2):
34–37.
National Research Council (NRC). 1996. National science education
standards. Washington, DC: National Academy Press.
National Research Council (NRC). 2000. Inquiry and the national science
education standards. Washington, DC: National Academy Press.
Rungruangsa, K., and B. Panijpan. 1979. The mechanism of action
of salivary amylase. Journal of Chemical Education 56(6): 423–424.