1. Chemical Reactions and Enzymes
Read the lesson title aloud to students.

2. Learning Objectives
Explain how chemical reactions affect chemical bonds.
Describe how energy changes affect how easily a chemical reaction will occur.
Explain why enzymes are important to living things.
Click to reveal each of the learning objectives.
Ask students if they have ever put together a jigsaw puzzle. Call on a volunteer to draw a simple sketch that shows how two adjacent puzzle pieces might look.
Ask: How can you tell that the two puzzle pieces fit together?
Answer: They have complementary shapes.
Tell them that certain proteins called enzymes fit together with other molecules in a similar way.
Alternatively, set up stations where students can quickly compare enzyme activity in common plant and animal tissues. Provide room-temperature samples of 1-cm cubes of carrot, potato, apple, and calf liver at each station. Instruct students to use forceps to place a sample into a Petri dish, add 5 drops of hydrogen peroxide (3 percent is available at drug stores) to the sample. Students should observe bubbling, which indicates enzyme activity.

3. Chemical Reactions
A chemical reaction changes one set of chemicals into another.
Define and discuss the importance of chemical reactions.
Tell students that an example of an important chemical reaction is the one that helps carry carbon dioxide from cells to the lungs so that it can be removed. Use the diagram to explain this process:
CO2 moves into the bloodstream from body tissue.
The first chemical reaction binds CO2 to water to form carbonic acid.
Carbonic acid travels through the bloodstream into the lungs and is broken down into CO2 and water.
Define the terms reactants and products.
Ask a volunteer to come to the board and circle and label the reactants and products of the first chemical reaction shown.
Click to reveal the answers.
Click to remove the circles and labels.
Ask a volunteer to come to the board and circle and label the reactants and products of the second chemical reaction shown.
reactants
product
reactant
products

4. Energy in Reactions
Chemical reactions that release energy often occur spontaneously.
Chemical reactions that absorb energy require a source of energy.
Which reaction can occur spontaneously?
Describe the graphs and orient students to the information presented on the graphs.
Define and discuss energy-absorbing reactions and energy-releasing reactions.
Explain that organisms need to carry out reactions that require energy and must have a source of energy.
Ask: What happens during a chemical reaction when products contain more energy than reactants?
Answer: Energy is absorbed.
Ask: What happens during a chemical reaction when products contain less energy than reactants?
Answer: Energy is released.
Ask: Which graph could represent a reaction in which food is broken down for energy?
Answer: the energy-releasing reaction
Click to reveal the question, “Which reaction can occur spontaneously?”
Ask students to identify which reaction can occur spontaneously either verbally or by drawing an arrow on the board.
Click to reveal the answer.
Students may have the misconception that spontaneous reactions are always rapid. Use the example of diamonds spontaneously decaying into graphite; this takes millions of years.
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Define activation energy.
Click to reveal the activation energy for the energy-absorbing reaction.
Ask a volunteer to show the activation energy of the second reaction by drawing an arrow on the board.
Activation Energy
Activation Energy

5. Energy in Reactions Review
Energy-absorbing
Can occur spontaneously
Requires more energy to get started
Reactants
Has a lower activation energy
Ask: Are these reactions energy-absorbing or energy-releasing?
Click to reveal the answer: energy-absorbing
Ask: Do these reactions occur spontaneously?
Click to reveal the answer: Occur spontaneously.
Click to reveal the statement: Requires more energy to get started
Ask: Which of these reactions requires more input of energy to get started?
Click to reveal the answer: blue
Click to reveal the statement: Has a lower activation energy
Ask: Which of these reactions has a lower activation energy?
Click to reveal the answer: red
Click to reveal the statement: Will proceed more quickly
Ask: Which of these reactions will proceed more quickly?
Introduce the term catalyst.
Will proceed more quickly

6. Enzymes
Enzymes lower activation energy and speed up chemical reactions in cells.
Explain that enzymes are catalysts. Discuss the importance of enzymes and the way enzymes work. Explain how lowering the activation energy speeds up the reaction by allowing many more molecules to react.
Point out to students that enzymes change the activation energy but not the overall change in energy between reactants and products.

7. Enzymes in the Body
Carbonic anhydrase speeds up this reaction by a factor of 10 million.
Remind students that the reactions shown here are essential to remove carbon dioxide from the body.
Explain that enzymes are necessary for these reactions to occur in a reasonable timeframe.
Discuss the specificity of enzymes.
Ask a volunteer to identify which arrows indicate reactions that would be catalyzed by enzymes. The volunteer can either answer verbally or circle the arrows on the board.
Click to reveal the answers.

8. The Enzyme-Substrate Complex
Substrates
Product
Explain that this diagram shows how a sample enzyme works.
Click to highlight the sample enzyme, carbonic anhydrase.
Click again to remove the highlight
Click to highlight the enzyme bound to carbon dioxide and water.
Define the term substrate.
Ask students to come up to the board and circle the substrates.
Click to reveal the answer.
Click again to remove the answer.
Define enzyme-substrate complex and explain that the enzyme converts the substrate into products.
Click to highlight the products.
Draw students’ attention to how the appearance of the chemicals within the active site has changed.
Ask students to come up to the board and circle the product.
Click again to remove all the circles.
Ask: Why is a cycle diagram an appropriate way to illustrate how an enzyme works?
Answer: because enzymes can be used over and over again, which allows the process to keep repeating
Define the term active site.
Click to highlight the active site.
Introduce enzyme specificity.

9. Active Site
The active site of the enzyme and the substrate have complimentary shapes.
Substrate
Enzyme
Explain that this is a space-filling image of an enzyme and substrate.
Ask students to identify the red and blue structures in the image as the enzyme or the substrate.
Click to reveal blanks and have volunteers identify the structures verbally or by writing on the board.
Click to reveal the answers.

10. Active Site
The active site of the enzyme and the substrate have complimentary shapes.
Discuss the lock-and-key analogy with the class.
Have students identify what the lock, key, and keyhole represent in the analogy.
Click to reveal the answers in order: lock—enzyme; key—substrates; keyhole—active site
Tell students that the analogy is a simplified representation of what happens when substrates bind to the active site of an enzyme. For example, rather than being rigid like a keyhole, the active site may actually change shape when substrates bind to it.
Explain that temperature, pH, and regulatory molecules can affect the activity of enzymes.

11. Enzyme! The Play
Develop a short play or demonstration that illustrates how enzymes catalyze reactions.
Group students and have each group develop and share a short skit that shows how enzymes catalyze reactions. Tell students to include the terms enzyme, active site, substrate, and products.
Click to reveal the cycle diagram to help struggling students or just prior to the demonstrations.