Chemical Reactions:The Catalase Reaction An enzyme-catalyzed reaction between two molecules of hydrogen peroxide. Nome Baker, Ph.D. © 2009 Gnomus @ IMSLI
Bridge No. 1: A video review of the students’ experiments involving catalase • Hyperlink to a demo video that will show students doing simple experiments that all students should do before beginning this activity. [Video to be taped. For now, see the earlier PD demo by Nome Baker; it needs editing.] [ The experimental session, in brief: The students test their hypotheses: 1) Do plant materials have an enzyme, catalase, that helps convert hydrogen peroxide to O2 and H2O? 2) Does the O2 formed behave as CO2did earlier in this system?]
Bridge No. 2: Web research. How many uses of hydrogen peroxide in humans, plants, & animals can you find? For many years hydrogen peroxide (H2O2) was used on cuts to kill bacteria. Why isn’t it used for that purpose anymore? Oxygen bubbles appear when hydrogen peroxide comes into contact with blood cells. You will find out more about how this happens starting here.
Find the card……. • Find the card that shows the reaction that took place in your experiment: 2 H2O2 → 2 H2O + O2 • Hint: Models of the molecules are shown using special symbols for each element. Key for the molecular models
Key to symbols used: • Each Answer Card shows molecular structures and uses different symbols for the various atoms in each molecule. • This is the key to the elements shown: Next slide
Does this card show thereactant molecules in this reaction?2 H2O2 → 2 H2O + O2 Key Yes No
Not quite. Look again. • The reactants are two molecules of hydrogen peroxide (H2O2): 2 H2O2 → 2 H2O + O2 These molecules are the products of the reaction. Try again!
Your right! . • Neither molecule shown is hydrogen peroxide (H2O2). One kind of molecule is H2O; the other is a gas you breathe (O2). Which molecule is the gas and which one water? Go to the next challenge.
Does this card show hydrogen peroxide (H2O2) as the product of a catalyzed reaction? Reactants Product Yes No
Your right! • Good! You know the difference now between the reactants and the products of a reaction. The metal catalyst (Pd or Au) greatly increases how fast H2 reacts with O2 in this reaction. Reactants Go to the next challenge. Product
Careful! Look out! • In this reaction (H2 + O2 → H2O2), hydrogen peroxide is the product of a reaction catalyzed by a metal. • In the main reaction that you are learning about, hydrogen peroxide molecules are the reactants in an enzyme-catalyzed reaction. Go back; look again! Be sure that you know the difference between the reactants and the products of a reaction.
Find a card showing two hydrogen peroxide molecules reacting to form water and oxygen:2 H2O2 → 2 H2O + O2 Click here to see if this is the right card.
Does this card show this reaction:2 H2O2 → 2 H2O + O2 ? Reactants Products Yes No
How could Card A2 be right?You’re looking for a card with this reaction: 2H2O2 → 2 H2O + O2 • Compare the reactants:You’re looking for 2 H2O2; Card A2 shows H2O2 + RH2. • 2) Compare the products:You’re looking for2 H2O + O2; Card A2 shows2H2O + R. Reactants Products Try again
Your right. That’s wrong!You’re looking for a card with this reaction: 2H2O2 → 2 H2O + O2 • Compare the reactants:You’re looking for 2 H2O2; Card A2 shows H2O2 + RH2. • 2) Compare the products:You’re looking for2 H2O + O2; Card A2 shows2H2O + R. Reactants Products Try another card.
Does Card A3 show this reaction:2H2O2 → 2 H2O + O2? Reactants Products Yes No
Your right. You’ve found the card with this reaction: 2H2O2 → 2 H2O + O2 Before learning more about catalase, the enzyme that catalyzes this reaction, look at examples of two other enzyme-catalyzed reactions involving hydrogen peroxide. Reactants Products Next card
Take another look!2H2O2 → 2 H2O + O2 • You are looking for two H2O2 (= HOOH) molecules reacting to form two H2O (= HOH) molecules and one oxygen O2(= OO) molecule. • Make sure that you can find them on this card before you go on to the next card. Reactants Products Next card
Compare these two different enzyme-catalyzed reactions: Reactants Products Next slide
Cards A2 and A4 show similar reactions. In each one an O atom from H2O2 removes two H atoms from a molecule to form two molecules of water. Reactants Products Model the reactions
Build models of these reactions. • Use beads or beans or candy or cards (representing different atoms and molecules) to make models of these two enzyme-catalyzed reactions: 2H2O2 → 2 H2O + O2 H2O2 + RH2 → 2 H2O + R How do enzymes work?
How the enzyme catalase works See a molecular model of catalase.] The next cards (A5 to A8) show four different stages of the enzymatic reaction that you have been studying: Try to put the cards in their correct order. [We will add a hyperlink to a url showing the structure of catalase.] Next slide
Click on the card that shows the enzyme ready to catalyze the reaction. Hint: Before the reaction starts, none of the reactants and none of the products is attached to the enzyme. After the reaction the enzyme is free of reactants and product, ready for new reactants.
You’re right! This represents the enzyme before any reactants are attached to it and also how the enzyme might look after all the products have left it free to be used again. So what happens next?
True, this is an early stage in the reaction, but try to find a card that shows an even earlier stage. Try again
This is the first step in the reaction. A hydrogen peroxide molecule is shown binding to the enzyme, catalase.2H2O2 → 2 H2O + O2 The equation tells you what molecule still has to be attached before the reaction happens. Do you know what happens next?
Two molecules of hydrogen peroxide have to bind to the enzyme for the reaction to occur. Last step?
At last! The reaction has taken place and the enzyme will be ready to act again as soon as the products of the reaction leave their binding sites. And finally…
This is the first and last step in the sequence. The enzyme, catalase, is ready to act again. Check the sequence.
Reviewing the reaction: You’ve now put your model in the correct order: A6 – A8 – A7- A5 (and back to A6).
Now can you explain why bubbles form when you pour hydrogen peroxide on a cut? First you need to know: • Blood forms at the site of the wound. • Blood contains red blood cells. • Red blood cells contain the enzyme catalase. • Use the knowledge that you have gained in this lesson to explain: • What happens when you pour hydrogen peroxide on a cut. • What kinds of molecules form the bubbles?
Acknowledgment • Special thanks to the invaluable assistance of Ms. Nicole Ng and Ms. Asya Grigorieva in designing this unit. • We are indebted to Dr. Ruth Dusenbery and to Dr. George W. Gagnon for their thoughtful criticisms and suggestions. • This work was greatly influenced by the input from various middle school teachers, most notably by that of Mr. Alan Lee.
An brief addendum follows • You may end the slide show now. The next three slides are incomplete and are planned for the use of teachers.
Student’s summary: [Link to the Learning Record. Response may be written and/or verbal as a presentation to the class. ] Review the catalase reaction using Cards A5 – A8 in the “correct” order to describe the catalase reaction in your own words. Try to include the following: What is the equation (write it) that describes the reaction? What are the reactants? ….the products of the reaction? What is the gas that forms when you add hydrogen peroxide to a fresh wound (e.g., a cut)? What is an enzyme and what does it do? Are any atoms lost during the reaction? If so, which?
Review As a further review and as reinforcement of the concepts being taught, look at the video (insert link) of the experiment that you (the student) did in preparation for the lesson. You may also do the experiment [insert link] again--or for the first time if you’ve never done it. Review the “wrong” Answer Cards. What lessons can be learned from those (e.g., balanced equations, catalysts). Review the “Learning Aims”. (Insert Link) Insert link to an interactive test (concepts learned). [Test to be constructed.]
Lesson Aims and Major Concepts; Understand the following: • In a chemical reaction all of the atoms in the reactants appear in the products of the reaction. • Chemical formulas and equations are used to describe a chemical reaction . • A specific chemical equation describes your earlier, hands-on experiments in which oxygen molecules were the product you could observe. [Reminder: Students should perform hands-on experiments involving hydrogen peroxide and catalase-containing tissues before and/or after this lesson.][Insert link to lesson plans for suggested experiments.]