SAM Teachers Guide  Lipids and Carbohydrates (short version) 

Overview  This activity focuses on the basic structure and function of both lipids and carbohydrates, two of the four major macromolecule families in biological systems. Students will look specifically at the polarity of the molecules and how polarity affects their solubility and behavior in different environments. Students will apply their understanding of intermolecular attractions, three‑dimensional structures of molecules, and electronegativity.  

Note: This activity assumes you have done the Introduction to Macromolecules activity first, a short (three page) activity that is intended to be paired with either the Lipids and Carbohydrates activity or the Nucleic Acids and Proteins activity. There are long versions of both of those activities that eliminate the need for the Intro to Macromolecules activity. 

Learning Objectives  Students will be able to:  

• Define lipids as molecules that do not dissolve in water.  • Recognize what makes lipids non‑polar and carbohydrates polar, and determine 

how polarity affects the solubility of the molecule.  • Explore the structure and function of various polysaccharides.  

 Possible Student Pre/Misconceptions  

• Dissolving involves the breakdown of molecules into smaller molecules, as opposed to the separation of intact molecules in solvent.  

• All fats are bad for health (rather than being needed by the cell). • The only kind of sugar is table sugar. • Table sugar and salt have similar chemical structures.   

 Models to Highlight and Possible Discussion Questions  Page 1 – Introducing Lipids  

Models: Substances in Water 

• Before using the models, remind students of the concept of dissolving. On this page, the ChemLink ʺWater‑Water Attractionsʺ provides a review using a model from a SAM chemistry activity.   

• After running the model, ask students to explain why water molecules work their way between the larger molecules in one model but not in the other. The theme of interaction with water will be prominent in later SAM biology activities.   

• Students may need to be reminded that dissolving is not a chemical reaction.   

• Link to other SAM activities: Solubility (in chemistry series).               Possible Discussion Questions 

• Why is interaction with water an important consideration in biology? 

 

Page 2 ‑ Lipids in Water  Model: Fatty Acids in Water 

• To prepare students for interpreting the behavior of a non‑polar lipid tail in water, you can use the ʺPolarityʺ ChemLink on the first half of the page.  

• Point out that that the tails do not attract each other with anything more than van der Waals and induced‑dipole attractions. The tails cluster in water because they cannot compete with water‑water attractions.  

 Possible Discussion Questions  

• What role do water molecules (not seen explicitly) play in forming this structure? Ask students to draw a picture of how the water molecules influence the formation of the micelle structures. 

• What would happen if the water in the model were replaced with oil?  

Page 3 ‑ Phospholipids Build Biological Membranes  Model: Lipid Bilayer 

• Point out that lipid tails are packed closely together in a membrane.  

• You may wish to use images that show the tails in a space‑filling representation to emphasize that water usually does not penetrate into the lipid‑crowded space.  

 Possible Discussion Questions  

• Why do people say, “Oil and water don’t mix”?  Explain this phrase using what you now know about water and oil molecules.  

• Why do you think cells evolved to have a lipid bilayer? Why is this type of protective coating particularly advantageous?  

 Page 5: Carbohydrates in Water 

Model: Polarity and Separation of Charge • Make sure students compare the charge surface distributions, looking for 

causes of the charge separation.  • Link to other SAM activities: Intermolecular Attractions. Highlight how 

polarity affects the strength of intermolecular attractions.  Possible Discussion Questions  

• What is the relationship between the surface charge shown in the 3D models and the solubility models on the first half of the page? 

• How does the presence of oxygen affect molecules made primarily of carbon and hydrogen?  

• What are some structural differences between lipids and carbohydrates? How might these differences affect their function?  

 Page 6 ‑ Linear vs. Branched Polysaccharides  

Model: Linear vs. Branched Polysaccharides • Students may need help to understand the representation of polymers in 

this model. Highlight the graphic that shows the equivalence of the gray ball monomers to the sugars in the 3D model above. 

 Possible Discussion Questions:  

• What about the structure of polysaccharides makes them good “building materials” in living systems?  

Connections to Other SAM Activities  

 

Connections  The Lipids and Carbohydrates activity is supported by many earlier SAM physics and chemistry activities. The Introduction to Macromolecules introduces several basic concepts relevant to all macromolecules: size, shape, composition, monomers, and polymerization. The Electrostatics activity helps students learn about charges in atoms; Chemical Bonds explores the different types of bonding patterns that result from different patterns of sharing electrons; and Intermolecular Attractions helps students to predict and understand what happens between molecules once these chemical bonds have formed. These activities help students understand the polar nature 

of carbohydrates and the non‑polar nature of lipids. The Solubility activity helps students relate the properties of the molecules to the environment in which they are found. Finally, Molecular Geometry supports learning about the structure of both carbohydrates and lipids, and how structure relates to function.  

Lipids and Carbohydrates in turn supports Diffusion and Osmosis and Protein Partnering and Function. Knowledge of the basic structure and function of lipids is helpful in understanding lipid membrane systems, and an understanding of how polar and non‑polar entities behave in water is essential for understanding how proteins adopt their three‑dimensional shapes.  

Activity Answer Guide   Page 1: 1. To dissolve in water a substance must be polar. In the space below, place a snapshot of the substance that is NOT polar. Use the annotation tools to explain how you know this substance is not polar. 2.  The molecules that ARE dissolved     (a) and (e)  

Page 2:  

1. Which end of the fatty acid do you think would be better at attracting water molecules? Explain your thinking. (You may wish to return to Page 1 to review water attractions.) 

The polar end would be better able to compete with the water‑to‑water attractions.  

2. The molecules that ARE dissolved (Check all that apply.) (a) (e) Page 2: 1. Which end of the fatty acid do you think would be better at attracting water molecules? Explain your thinking. (You may wish to return to Page 1 to review water attractions). The head would be better at attracting water because it is polar, and water is polar. So the positive and negative parts of each can attract each other. 2. What makes the tail of a fatty acid non-polar? (c)

 3. In the space below, place a snapshot of the structure (a micelle) formed by the fatty acids in water.

 

4. Explain why the lipid head faces outward in a micelle. Make sure your explanation includes the polar and non-polar parts of the lipid, as well as water. The polar lipid heads are more attracted to water than the non-polar tails, so the heads stay in contact with water more.  Page 3:  1. In the space below, place a snapshot of the phospholipid. Use the annotation tools to indicate the head, tails, polar areas, and non-

polar areas.

2. Which statement about membrane phospholipids is NOT true? (e) 3. After you use the model above, choose one thing in the model that can cross the membrane, and one thing that can't. For BOTH of your choices, explain why it can or cannot cross a bilayer membrane. CO2 can cross the membrane because it is non-polar, so it is not very attracted to water. Sugar does not cross the membrane because it is polar, so it is more attracted to water than to the lipid tails. Page 4: 1. What elements are carbohydrates made of? (Check ALL that apply.) (c) 2. What element is present in large quantities in carbohydrates, but is only a small part of lipid molecules? Oxygen  Page 5: 1. Insert a snapshot of glucose dissolved in water, and use the annotation tools to explain how you know the glucose is dissolved.

 

2. Stop the model and drag a water molecule so that it is close to the different parts of a lipid and a sugar molecule. Which lipid or sugar atoms tend to form hydrogen bonds with water? (b) (c) (d)  3. What does the intensity of color indicate on the surface of the molecule? (Check all that apply.) (a) (b) (c)  4. In the first half of this activity you explored lipids and saw that they are largely non-polar. Explain why the chemical makeup of sugars causes them to be very polar molecules. Sugars have many oxygen atoms, so they have polar bonds. Around the oxygen atoms, there is more negative charge because they have a stronger pull on electrons than carbon or hydrogen. Page 6: 1. Why are fibers formed from long linear polysaccharides very strong? (a) 2. Branched polysaccharides can be more easily separated into individual chains than

linear polysaccharides, such as cellulose. This is explained by: (c) Page 7: 1. In the model to the left, three molecules tagged as "A," "B" and "C" are placed in water. Click the "Run" button to run the model and observe what happens. Based on your observation, can you tell which molecule is the most polar? (a) 2. Compare two molecules made primarily of carbon and hydrogen, one with some oxygen atoms and one without any. Which of the following statements are true? (Check all that apply.) (a) (c) 3. Which statement below about membrane phospholipids is NOT true? (d) 4. Cellulose is made of multiple long polysaccharide chains that line up side by side in parallel. What keeps the cellulose molecules stuck together? The linear chains are held together with many hydrogen bonds. 5. Explain how the presence of oxygen-hydrogen bonds affects the solubility of glucose. The more oxygen-hydrogen bonds in a substance, the more it can attract water by forming hydrogen bonds to it. Since it can attract water as well as water attracts itself, the water molecules will surround and dissolve the sugars.

   

   

SAM HOMEWORK QUESTIONS Lipids and Carbohydrates  

   

Directions: After completing the unit, answer the following questions to review.    

1. What is the difference between a polar and a non‑polar molecule? 

   

    

2. Write a caption that explains what is happening to the fatty acids in the picture shown below.   

  

 

 

 

    Caption:                                                                                        

 

3. Phospholipids form the cell membrane. In the space below, draw the way the phospholipids arrange themselves in the cell membrane. Then use what you know about the polar parts of the molecules to explain why the phospholipids are arranged this way. Be sure to label your drawing.  

 

4. Explain why oxygenʹs electronegativity (ability to attract electrons) helps sugar dissolve in water. 

 

 

 

 

 

5. How do intermolecular attractions affect the strength of fibers formed by polysaccharides? Be sure to compare branched and unbranched polysaccharides. 

 

 

 

 

6. Career connection: There are several ways scientists working in labs and industry use computer models to understand lipids and carbohydrates. The membrane of every cell and organelle within each cell is made from lipids. Understanding how they interact to form the membrane is crucial for understanding cellular function. Find the name of one company or university that is doing cell membrane research, and give a brief summary of what they are studying.

SAM HOMEWORK QUESTIONS 

Lipids and Carbohydrates – With Suggested Answers for Teachers    

Directions: After completing the unit, answer the following questions to review.

1. What is the difference between a polar and a non‑polar molecule? 

Polar molecules do not share electrons evenly among atoms, while non-polar molecules do share electrons evenly. This is because polar molecules have some atoms that are a lot more electronegative than other atoms, and the electrons are pulled to the more electronegative atoms. This creates separated areas of positive and negative charge on the molecule.

 2. Write a caption that explains what is happening to the fatty acids in the picture shown below.  

Caption: Water molecules are attracting each other more than the non-polar fatty acid tails, so the tails get pushed away and excluded from the water. The fatty acid heads are polar, so they attract water molecules.  

 

  

     

 

3. Phospholipids form the cell membrane. In the space below, draw how the phospholipids arrange themselves in the cell membrane. Then use what you know about the polar parts of the molecules to explain why the phospholipids are arranged this way. Be sure to label your drawing. 

 

In the membrane, the polar lipid heads attract water, so they face outward. The hydrogen bonds with water. The non-polar tails don’t attract water very well and are inside the membrane. Because water is polar, it attracts the polar heads more than the non-polar tails.

 

 

4. Explain why oxygenʹs electronegativity (ability to attract electrons) helps sugar dissolve in water. 

The oxygen in sugar is more electronegative than the hydrogen is, so it makes the sugar’s bonds polar by attracting electrons away from hydrogen. This gives the oxygen atoms a partly negative charge and hydrogen atoms a partly positive charge, so hydrogen bonds can form between water and sugar molecules.  

 

5. How do intermolecular attractions affect the strength of fibers formed by polysaccharides? Be sure to compare branched and unbranched polysaccharides.  

Polysaccharides are chains of sugar molecules. Sugars are polar, and their hydrogen atoms can form hydrogen bonds. The longer and straighter the chain is, the more hydrogen bonds can form between them. This is why straight chains form stronger fibers than branching chains.

6. Career connection: Cell membrane research occurs at many pharmaceutical companies and universities. An internet search for Cell Membrane Research or more specifically Cell Membrane Dynamics to better focus results on computer modeling.