Enzymes and pH Lab

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Objective

Students will examine how pH of a substrate affects the rate of enzyme activity.

Big Idea

The rate of enzymatic activity is affected by pH.

Warm-Up

5 minutes

Warm-Up: You are a parent of a six month old baby and you suddenly notice that your baby has a temperature of 105?  You are extremely worried. How does this relate to your understanding of enzymes?

This question challenges students to activate prior knowledge about enzymes from lessons Enzymes, part 1 and part 2 using a real world application.  Giving students questions that apply the classroom knowledge to real world scenarios builds relevance for scientific topics.

Allow students to talk in small groups before allowing a 2-4 students to share their thoughts.  Require students to explain and defend their thinking with evidence. Because literacy development does not focus only on reading but speaking, as well; be sure to  encourage students to use the content specific vocabulary when they share.

Review of Concepts

15 minutes

Give groups of 2-3 students one question from a set of questions:

  • What is a catalyst?
  • Why do we need enzymes?
  • What is an active site?
  • What is a substrate?
  • What is denaturation?
  • How do enzymes work?
  • What can denature an enzyme?

Ask groups to prepare a 2 sentence response to share with the class.  I call this activity, 2 in 2 Review.  Inform students that they will have 2 minutes to create their responses. Set the timer for 2 minutes and allow students to work with a seat partner to complete the task. Walk around the room as students are working to note comments in the group that suggest clear understanding or misconceptions.  Make a note to clarify any misconceptions that are heard while walking around.

At the end of the time, allow each group to give up to a 1 minute response to the question.  Instruct the class to listen for use of content specific vocabulary and accuracy of the response in user-friendly language.  Emphasize that definitions are not what is expected in the response.  When the groups’ share, let the students in the audience know that if they feels that a significant point was not stated in the response, then they should raise their hands and add to what has been said.

This student-led review helps reinforce key concepts from Enzymes, Lesson 1 and Enzymes, Lesson 2 before students conduct the enzyme and pH lab.             

Guided Practice

15 minutes

Explain that students will be investigating whether changes in pH can change the function of hydrogen peroxide.  

Distribute copies of the Investigating enzymes lab Instructions.   Display a copy of the lab using a LCD projector.  Ask for 2-3 readers to read parts of the introduction aloud to the class.

Summarize the procedure, stopping to review independent variables, dependent variables and controls.  Also review the characteristics of acids and bases. 

Note: Potato catalase is easily made by pureeing potato in a blender with a small amount of water.

Model how to perform the lab procedures correctly.  Make sure that students understand that a drop of water is not the same thing as a stream of water.  Also, model how to accurately measure the height of the foam using a measuring tape (mm).

Emphasize the need for students to collect accurate measurement.  As a quick review, ask students to recall the difference between accuracy and precision, which was taught in the lesson, 1-2-3-4, I Declare War.

Display the Investigating enzymes data sheet.  Go over each section of the data sheet and explain what students are expected to do and know in each section. 

Point out that the lab requires students to complete a graph of the enzyme activity.  Make sure that students have correctly identified the dependent and independent variables.

Inform students that the lab will be conducted in groups of four, with each student completing his/her own lab analysis and conclusions.  Decide beforehand if you will assign the student groups or whether you will allow students to select their own groups.  Because this lab involves several steps, it might be best to assign the work groups to ensure that students form productive work groups.

Review the 4 lab roles: technician, data collector and materials managers:

Technician

–       Performs the lab actions, making sure to follow the steps of the lab as written

Data Collector

–       Measures the height of the foam in each test tube

Materials Managers (2 people)

–       Collects lab materials

–       Returns materials to designated area

–        Cleans lab supplies

Independent Practice

20 minutes

Instruct students to take 5-7 minutes to perform a close read of the lab.  Instruct them to write down any questions that they have as they read the lab.  The close read strategy allows students to read with a purpose; which is to learn what the lab will require them to do and know. At the end of the allotted time, give students an opportunity to ask their questions so that you can provide clarification and eliminate confusion about the procedure.

Release student groups in small numbers to collect the lab materials.  Be explicit about how the movement should look when students go the designated area to collect materials.  Remind students of the safety rules for the classroom.

Walk around while students are working.  Make sure students are using the metric side of the measuring tape and  performing correct measuring procedures.  Also watch for appropriate safety procedure (wearing goggles, no horseplay, etc…). and observe students use of the pipet for adding drops of substrate to the catalase. Students who are following the lab procedure correctly should see catalyzed reactions.

Listen to student comments and redirect their thinking with inquiry questions, as needed.

Remind students to complete the data tables and analysis questions.  The student work  that is attached shows that students were able to complete the data table, graph and analysis questions.  

Close

5 minutes

Ask students to raise their hand to indicate their agreement with one of the two statements:

  • Potato catalase work best at a higher pH.
  • Potato catalase works best at a lower pH.

This question is a good way to quickly assess before class ends how many students performed the lab correctly.  Make sure students know that the results should show that catalase works best in a pH of 7.  This type of clarification allows errors of thinking to be cleared up before students leave.