Enzyme Activity: An Inquiry Based Approach
Brookfield High School
The "cookbook " style approach to studying biochemical reactions mediated by enzymes is changed to an inquiry approach. Cooperative teams are each given two questions about the types of living materials that contain the enzymes that break down hydrogen peroxide, and under what conditions the enzyme works best. They write hypotheses and design and conduct experiments to test these hypotheses. The activity provides students the opportunity to engage in meaningful scientific inquiry because they must truly understand the problem in order to attempt to solve it, to construct meaning in performing experiments, to practice observational skills, and to communicate in writing and in reporting orally to the class on their group results.
STRATEGY AND RATIONALE
Our biology curriculum has for many years included the lab activity "A Study of Biochemical Reactions" from the BSCS green version (also updated in the BSCS blue molecular biology version). This activity has been traditionally used as the focal point of our unit on enzymes since it demonstrates the difference between an inorganic catalyst and an enzyme, shows that enzymes are not "used up" in a reaction, and vividly shows the conditions under which enzymes work best or don't work at all.
However, every year after having students carefully follow the "cookbook" approach to this lab--"arrange the test tubes in a rack, number from 1 to 10, pour 2 ml of hydrogen peroxide....etc.", I found that when it came time to draw some conclusions, they had no idea what the different degrees of activity meant, what substances contained the enzyme, or what substance was the substrate. They loved watching the "fizzing" and dutifully recorded observations and measurements of the reactions, but the conceptual understandings about enzymes were missing. This was true in both general and honors biology classes, and other teachers who taught the same course had the same experience.
This is one example of a lab activity which students approach as simply a "recipe" where the results are already known, and they have to follow directions, all in the same way, to get to the endpoint: the correct answer. But this does not require them to construct meaning, to think logically, to use the science they have learned to solve problems. In following a recipe they are not really "doing" science or using the scientific method. This has bothered me about our traditional lab activities. So last year I modified this lab and other activities so that students have more of an opportunity to construct their own knowledge; I am in the process of changing other lab activities to use an inquiry approach.
Students read about catalysts and enzymes in the textbook, we discuss in class the "lock-and-key" model for enzyme/substrate, and the chemical makeup and structure of enzymes. Students are given the information that hydrogen peroxide is a toxic byproduct of metabolism in cells and that the formula for hydrogen peroxide is H2O2.
During class discussion, students are asked to examine the formula and decide how this compound can be changed to become non toxic. They usually suggest that H2O2 can be changed to water and oxygen, or to hydrogen and oxygen. After some inquiry, they realize that if hydrogen peroxide is to be chemically changed, an enzyme must be involved. That is the background information they have available as they proceed to the lab to answer some questions about the enzyme.
- The students are divided into cooperative groups. Each team selects a leader and recorder.
- All groups must answer the first question: "Do inorganics(sand and manganese dioxide) contain enzymes or catalysts that break down H2O2? "Each group must come up with an "if....then" hypothesis, an experimental design, perform their experiment, record results, and come to a conclusion which answers the question.
- Then each group is given a separate question to research as a team.
The questions are:
- Do both plants and animals have an enzyme that breaks down H2O2?
- How can enzyme activity be sped up?
- How does temperature affect enzyme activity?
- How does pH affect enzyme activity?
- How do you know that the organic materials contain an enzyme and not manganese dioxide (the catalyst)?
- Can enzymes be reused?"
MATERIALS AND SAFETY CONSIDERATIONS
Materials to have on hand for students to examine and consider using include raw liver of any type, potatoes, and other varieties of raw meat and vegetables. Also, hydrogen peroxide(3%), test tubes, hot plates, incubator, ice or freezer/refrigerator, beakers for hot plates, thermometers, test tube holders, mortar and pestle, knives, forceps etc.
I suggest that some of these item should not be openly displayed. For example, the mortar and pestle. Instead, let students decide on a hypothesis and test by brainstorming and using their own previous knowledge and creativity. You'll be amazed when they think of valid experiments you've never thought of!
Have students check their proposed experiment with you before proceeding. Provide them with materials they request as long as they are available and as long as there are no safety problems.
I have found that the inquiry approach , allowing students to construct their own experiment, takes more time and is "messier" because it often takes several trials before they can answer the question. I must stand back and act as a guide, not a dispenser of "correct answers". I learn along with them. Some groups come up with modifications that I may not have tried before, answering the basic questions and then changing variables to come up with new questions and new answers.
The group discussions show me that they are doing science, thinking about controlled experiments, suggesting ways of testing variables, and arguing with one another about which method may be best. They satisfy arguments about procedure not by asking me if one or the other is right, but by trying each idea in an actual experiment. They are eager to test their hypotheses because these are their hypotheses and their experiments, not the textbook's recipe. And they find out that all answers are correct, because everything they do leads to some information about enzyme activity.
The group responsible for answering the question "How can the enzyme activity be sped up?" struggled with that question for 2 days. They came up with 2 ideas: increase the temperature (their logic was that this would add energy), and chop up the liver or the potato with a scalpel (they explained that this would increase the surface area for reactions). They tried both ideas, got some increased activity with warm temperatures, and much more activity with the chopped liver. I encouraged them to look around the room and see if they could find something that would further "chop up" the liver. They tried the mortar and pestle, and found even greater activity. After some thinking, they realized that since enzymes are contained in cells, grinding must open up the cells and the enzymes are then free to react with the substrate.
My students use their lab notebooks and journals to record their thinking while doing lab activities and later to respond to questions about what they have learned. This is how I evaluate their learning. Compared to the years when my students followed the traditional step by step lab procedure, I have found that students using the inquiry approach to discovery not only understand more about enzymes after doing this activity but they also use more skills and tap into more of their scientific knowledge than they would if they just followed directions without thinking.
This lab was successful for these reasons and also because it has made my students more comfortable in the lab, less anxious about getting the wrong answer, and paved the way for further creative challenges in scientific problem solving. I now look at all textbook labs as an opportunity to use the inquiry approach. Except for skill building labs, like using the microscope, just about any lab you can find in a manual or textbook can be modified to make it more open-ended. Try It!