Looking For Lipids

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SWBAT construct a model of a lipid molecule and test unknown substances to determine if they are composed of lipids.

Big Idea

How can we identify one organic molecule from the next? Students will investigate the physical properties of lipids through a laboratory activity.

Lesson Introduction: Differentiating Lipids From Other Molecules

1 minutes

This video clip will serve as an introduction for today's lesson focusing on the structure and function of lipids, as well as the NGSS Life Science Standard 1-6:

Hook - Linking Lipid Structure and Function

10 minutes

Students will watch the video clip below and take notes regarding the structure and function of lipids. As the students observe the clip, they need to draw a model of a triglyceride molecule and explain the necessary role of lipids in the human body. 

The class will participate in a whole group discussion at the conclusion of the video to determine if fats are as bad as the media makes them out to be.  The student responses should focus on the essential role of fats in our body's physiology and the possible dangers of too many fats in our diet.

Independent Practice - Building Our Lipids Piece By Piece

5 minutes

Students will receive their lipid building blocks handout when they walk into the classroom at the beginning of the lesson.  Students will use the colored pencils to color the glycerol head green and the three fatty acid tails yellow.  Students will then cut out their four puzzle pieces and use the tape to construct their fatty acid model.  

Students will reflect on the following questions:

  1. What process must occur for the gylcerol and the three fatty acids to form chemical bonds?  
  2.  What is the general shape of the trigylceride molecule that you have created? 
    • Answer - a capital letter "E"
  3. In your opinion, how does the structure of the trigylceride model determine its function in the human body?  
    • Answer - answers will vary but reference the video clip to talk about saturated and unsaturated fatty acids

Here is a sample a student's lipid molecule model and their response to how lipids function in the human body:

Looking For Lipids: Sample of Student Response - Students were able to successfully respond to these basic introductory questions that review the structure of lipid molecules.  

Image of Lipid Paper Model - Although paper models are very basic, these rudamentary models are able to give a generalized structure of a lipid molecule for students to use as a point of reference throughout this activity.  More sophisticated models can be purchased from science supply catalogs, but my experience has been that the paper models accomplish the task of student understanding for the organic molecule structures.

Guided Practice: Lipid Investigation

30 minutes

Keeping in mind the video clip that explained the molecular structure of lipids and their function in the human body, students will follow the procedure described in the Lipid Identification Lab to determine if these common household substances are lipids.  

 Students will use a sheet of brown paper bag that has been pre-cut large enough so that the students can draw six circles the size of a quarter evenly spread out on one side of the their test paper.  Students will label each circle with the name of the test substance and then place a small amount of the substance in the middle of the circle.  

Students will need to wait 15 minutes for the test data to be observed.  

While students are waiting, they will watch their teacher conduct a demonstration to test each substance with the Sudan IV solution.  A volunteer lab group of students will prepare their well plate with samples of each of the six test substances.  Students must make sure to copy the diagram of the well plate on their paper so they are able to identify each of their test substances.  As a class demonstration, the teacher will place two drops of the Sudan IV indicator in each well and record the observation.  If the substance turns a red color then the test substance is has a positive lipid identity.  If there is no color change observed, then the substance is not lipid.  

Safety Precaution: Sudan IV is flammable and hazardous solution that must be handled with care.  You can order the Sudan IV indicator through your science supply catalog, but please be careful with this substance and follow all precautions as listed on the shipping directions.  Due to these safety concerns, the Sudan IV indicator portion of this laboratory investigation is completed as a teacher demonstration for the students to observe.  Always place safety as the top priority in your lab/classroom!

After students have recorded their observations and data for the Sudan IV indicator based on the teacher demonstration, they can go back and check their brown paper bag test.  The students need to lift up their test paper and view from the bottom.  If a grease mark is present then the substance has a positive lipid identity.

Students need to record their brown paper test data on their student lab sheet and review all of the data.  Once the student lab partners have completed their data analysis, each student needs to independently formulate their own conclusion statement.  

These statements will be shared in small groups and followed up with a discussion of the similarities and differences in the data collected by the lab groups. Students will work to determine which substances contained lipids and which of the unknown substances did not contain lipids.

The class will then cast their votes in a class poll to determine the identity, lipid or not a lipid, of each of the test substances.  

Students Working on Lipid Investigation:  Students are determining if the unknown substances will dissolve in lighter fluid.

Collaborative Group Work - Students work together to determine if the unknown substances are lipids.

Unknown Lipid Image - These substances will tested through the student investigation.

Student Lab Image - Students have prepared their known and unknown materials to test which substances are lipids and which substances are not lipids.


Close - Will The Real Lipid Please Stand Up?

10 minutes

The class will conclude this activity with a vote.  The teacher will prompt the students to raise their hand if they believe Substance #1 was a lipid.  A tally will be kept on the front board and after all votes have been cast the teacher will reveal the identity of the mystery substance.  The process of voting, tallying student input, and identifying will be repeated until all 6 substances have been reviewed.  

Lipid-containing Substances: lard, cream cheese, mayonnaise, and ranch salad dressing 

Non-lipid Substances: marshmallow fluff and the non-fat whipped cream

As a follow-up to today's lesson, students will write a paragraph that highlights their understanding of the molecular composition of lipids, the vital role of lipids in the human body, as well as the risk of too many triglycerides to our overall health.

Sample of Student Response Paragraph: Most students were able to apply the information presented and reinforced in this lesson to discuss the structure and function of lipids as they relate to the human body.  Lipids are discussed in the news, in the students' Health class, and in Biology so students are able to gain cross-content references and apply prior knowledge to develop a more detailed understanding of the curriculum.