SAM Teachers Guide Lipids and Carbohydrates

Transcription

1 SAM Teachers Guide Lipids and Carbohydrates Overview Students will explore the structure and function of two of the four major macromolecules, lipids and carbohydrates. They 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 to these specific molecules. Learning Objectives Students will be able to: Explore organic compounds and identify the carbon backbone. Recognize what makes carbohydrates polar and determine how polarity affects the solubility of the molecule. Define lipids as molecules that dissolve in fats. Compare and contrast the properties of lipids and carbohydrates. Explore the structure and function of various polysaccharides. Possible Student Pre/Misconceptions Lipids and carbohydrates are giant and confusing molecules. Lipids and carbohydrates are taken in the body to give energy and then leave. Muscles turn into fat if you don t continue to use them. Fat is bad for you. Models to Highlight and Possible Discussion Questions After completion of Part 1 of the activity: Models to Highlight: Page 3 The Importance of Polarity o After running the model, review with students why ethylene boils first. Use this as an opportunity to review the terms polarity, electronegativity, and intermolecular attractions. o Link to other SAM activities: Intermolecular Attractions. Students can discuss how the strength of intermolecular attractions relates to boiling point. Page 5 Changing Solvent

2 o Highlight the fact that the lipid remains clustered together when placed in water as a solvent. Yet, when placed in oil, the lipid particles spread out. Review the terms hydrophobic and hydrophilic and how they relate to solubility. Page 6 Lipid Bilayers o Review what types of materials can pass through the cell membrane and why. Highlight why the bilayer is arranged as it is, referring to the heads and tails. o Link to other SAM activities: Diffusion, Osmosis, and Active Transport. Students can discuss what they remember about how particles move and why. They can also discuss the fate of molecules that cannot pass through membranes passively. Possible Discussion Questions: How does the presence of oxygen in a hydrocarbon change its properties? How does oxygen affect the polarity of a molecule? Why? Explain the difference between the terms hydrophobic and hydrophilic. If one molecule has both hydrophobic and hydrophilic regions, how might this affect its behavior in different solvents? Why do people say, oil and water don t mix? Explain this phrase using what you now know. Why do you think cells evolved to have a lipid bilayer? Why is this type of protective coating particularly advantageous? After completion of Part 2 of the activity: Models to Highlight: Page 7 Comparing the Solubility of Sugars and Hydrocarbons o After students have run the model, highlight the link between polarity and solubility in water. o Link to other SAM activities: Solubility. Highlight how electrostatic distribution of charge is related to polarity and a molecule s ability to dissolve. Page 8 The More Hydrogen Bonds, The Stronger the Polysaccharides o Students can discuss why it was difficult to pull apart the two chains of polysaccharides. o Link to other SAM activities: Intermolecular Attractions. Highlight how hydrogen bonding is optimal when the chains are linear and how the shape of the two molecules allows them to line up close together. Page 10 Energy in Polysaccharides

3 o Highlight the difference in energy released between the branched and unbranched molecules (starch vs. glycogen). o Link to other SAM activities: Chemical Bonds and Molecular Geometry. Use this as a chance to review the energy stored in chemical bonds and how the shape of molecules influences their behavior. Possible Discussion Questions: Structurally, what are some differences between lipids and carbohydrates? Will these differences affect their function? Why or why not? What about the structure of polysaccharides makes them good building materials in living systems? What is the difference between linear and branched polysaccharides in how they behave at different temperatures? Why? Why would plants be satisfied storing energy in starch molecules while animal cells use glycogen for short term storage? Refer to what you remember from the model on page 10. What does branching have to do with the amount of chemical energy released as enzymes attack?

4 Connections to Other SAM Activities The focus of this activity is on the basic structure and function of both lipids and carbohydrates in biological systems. The Lipids and Carbohydrates activity is supported by many other SAM activities. 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. This activity supports both Diffusion, Osmosis, and Active Transport and Molecular recognition. In both of these, knowledge of the basic structure and function of lipids is helpful in understanding lipid membrane systems.

5 Activity Answer Guide Page 1: Introduction, no questions. Page 2: 1. Hydrocarbons are referred to as organic molecules with a "backbone." Take a snapshot of the hydrocarbon you created and drag the snapshot into the box below. Use the arrow tool to point to the carbon backbone. The oxygen atoms make ketene a polar molecule, increasing the attraction between partial positive and partial negative ends of molecules. Therefore, it requires more energy, in the form of heat, to break the molecules apart and cause the liquid to boil. Page 4: 1. Which of the following are true? (a) (b) (c) 2. Take a snapshot of a lipid that is fully dissolved (evenly spread in the solvent). Sample snapshot: Lipids evenly spread in the solvent. Sample snapshot: Hydrocarbon molecule created. 2. How many bonds does every carbon atom have? Four (4) Page 3: 1. Why do you think the charge arranges itself for ketene in this way? The negatively charged electrons are attracted to the oxygen atom. Since the oxygen end of the molecule attracts more of the negative charge, the remainder of the molecule is left partially positive. 2. Which compound boils first? (a) 3. How does the presence of the oxygen atoms in the molecule account for the difference? 3. Hydrophilic means something is attracted to water and hydrophobic means something is excluded from water because water is more attracted to other water molecules than to it. Are the lipids hydrophobic or hydrophilic? Explain your answer. The lipid molecule is hydrophobic. This is apparent because the lipid molecule is not dissolving in water. The water molecules are more attracted to each other than they are to the fat. This results in the lipid remaining together as a clump. Page 5: 1. What can you tell about the properties of the fatty acid from observing it? Check all that apply. (a) (c) (f) 2. Compare the structure and the polarity of the fatty acid to the hydrocarbon. The hydrocarbon is non-polar (neutral) while the fatty acid is polar. The carboxyl head on the fatty acid has oxygen atoms that attract more negative charge.

6 3. Explain why the tails are located in the oil and the heads in the water. The tail of the molecule is a long hydrocarbon chain that is hydrophobic. The head is a polar carboxyl group that makes it hydrophilic. So water molecules attract the heads, but not the tails. 4. Why are fatty acids compared to the twofaced deity Janus? (c) 5. Take a snapshot of the lipid structure in water. Drag the snapshot into the box below. The oil forms clusters on the surface of the soup because they are not soluble in the water-based soup. The fat molecules are excluded from the water. Page 6: 1. Which statement about membrane phospholipids is NOT true? (e) 2. Membranes can keep chemicals inside and outside of the cell. What would happen if a cell membrane breaks? If the cell membrane breaks there would be no regulation of what can enter and leave the cell. Page 7: 1. How is the lipid molecule different from the sugar molecule? (Check all that apply.) (b) (c) 2. An increase in the ratio of oxygen atoms to hydrogen and carbon... (Check all that are true.) (a) (c) Sample snapshot: Fatty acids in water. 6. Take a snapshot of the lipid structure in oil. Drag the snapshot into the box below. 3. Select a snapshot image that shows benzene molecules do not mix well with water molecules. Sample snapshot: Fatty acids in oil. 7. Have you ever observed beads of oil on the surface of soup? Explain how this model can help explain the occurrence of those beads. Sample snapshot: Benzene with water molecules. 4. Fill in a snapshot image that shows glucose molecules mix better with water molecules.

7 apart more easily whereas wood s linear nature makes so many more attractions between molecules, and they do not break apart Easily. Page 10: 1. Which substance produces more chemical energy, starch or glycogen, when enzymes work on it? Why? Glycogen produces more chemical energy according to the bar graph. Because it is more highly branched, it has more "ends" for the enzymes to attack. Sample snapshot: Glucose with water molecules. 5. Explain how the presence of six OH groups affects the solubility and properties of glucose. The presence of six OH groups makes glucose a polar molecule. The uneven distribution of charge increases its solubility in water, which is also polar. Page 8: 1. Cellulose is made of long polysaccharide chains that line up in parallel. How does this structure relate to its function? When aligned together with strong bonds between them, the molecules can act as structure and protective coating of plants. 2. Long linear polysaccharides make strong fibers because: (d) Page 9: 1. Branched polysaccharides can be more easily separated into individual chains then linear polysaccharides like cellulose. This is explained by: (d) 2. Based on your observations of the polysaccharides in the models, explain why starch is edible and wood is not. 2. Which substance would produce more chemical energy if enzymes could attack in the middle of the chain rather than just at the ends? There would be less of a difference in the chemical energy released if the enzyme could attack anywhere in the molecule. Page 11: 1. In the model to the left, three molecules tagged as "A", "B" and "C" are placed in water. Click the " " 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 hydrocarbon chains: one that includes oxygen atoms and one without. How does the presence of oxygen affect the properties of a hydrocarbon chain? (a) 3. What makes cellulose so strong? (a) 4. Describe why lipids are so important to cellular evolution. Scientists believe that the first step in cellular evolution was the emergence of lipids. These lipids are thought to have become more complicated. Then, the lipids formed membranes that created an interior space, separating it from an outside environment. Starch, a branched polysaccharide, has many fewer intermolecular attractions because the polymers cannot fit together tightly. They break

8 5. Table sugar and wood are both made of glucose. Why can you serve cubes of sugar with tea, but not cubes of wood (without offending your guests)? The amount of branching in polysaccharides impacts their structure, particularly at different temperatures. Cubes of sugar will dissolve when heated by being placed in hot tea while cubes of wood will maintain their solid structure. This is due to the increased attraction (hydrogen bonding) in wood, which is linear as opposed to sugar, which is branched. 6. Which statement about membrane phospholipids is NOT true? (d)

9 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 non-polar molecule? How does this relate to the term electronegativity? 2. What property affects a molecule s solubility in specific solvents? 3. What will happen if you try to dissolve a lipid in water? If you change the solvent to oil, instead of water, what will happen to the lipid? Why? 4. Phospholipids form the cell membrane. In the space below, draw how the phospholipids arrange themselves in the cell membrane. Then, explain why this is the case given what you know about the hydrophobic and hydrophilic parts of the molecules. Be sure to label your drawing. 5. Write captions that explain what is happening to the phospholipids in the two pictures seen below. Note: They are in water on the left and in oil on the right. Caption: Caption: 6. How do intermolecular attractions play a role in the behavior of polysaccharides?

10 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 non-polar molecule? How does this relate to the term electronegativity? Polar molecules do not share electrons evenly among atoms while non-polar molecules do. The term electronegativity refers to how well an atom can attract electrons from another atom. Highly electronegative atoms affect the polarity of molecules. 2. What property affects a molecule s solubility in specific solvents? Polarity. This goes back to the concept of like dissolves like when referring to solubility of macromolecules. 3. What will happen if you try to dissolve a lipid in water? If you change the solvent to oil, instead of water, what will happen to the lipid? Why? Following the rule that like dissolves like, a lipid will not dissolve in water, but will dissolve in oil. This has to do with the intermolecular attractions between non-polar lipids and different solvents. Water is a polar solvent while oil is a non-polar solvent. 4. Phospholipids form the cell membrane. In the space below, draw how the phospholipids arrange themselves in the cell membrane. Then, explain why this is the case given what you know about the hydrophobic and hydrophilic parts of the molecules. Be sure to label your drawing. In the bilayer, the polar, hydrophilic heads face out while the non-polar, hydrophobic tails face inward, towards each other due to the intermolecular attractions. 5. Write captions that explain what is happening to the phospholipids in the two pictures seen below. Note: They are in water on the left and in oil on the right. Caption: Hydrophobic tails do not dissolve in water. Caption: Hydrophilic heads do not dissolve in oil. *The tails of the molecules are long hydrocarbon chains that are hydrophobic. The heads are polar carboxyl groups that make them hydrophilic. So, water molecules attract the heads, but not the tails. 6. How do intermolecular attractions play a role in the behavior of polysaccharides?

11 Polysaccharides are formed by joining sugar molecules. They are held together by hydrogen bonds. The strength of these bonds and the shape of the molecules (the degree of branching) play a role in their multitude of functions.