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
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Before using the models, remind students of the concept of dissolving. On
this page, the ChemLink "Water-Water Attractions" 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 "Polarity" 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.
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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.
3. In the space below, place a snapshot of
the structure (a micelle) formed by the fatty
acids in water.
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.
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.
2. The molecules that ARE dissolved (Check
all that apply.)
(a) (e)
Page 3:
Page 2:
1. In the space below, place a snapshot of
the phospholipid. Use the annotation tools to
indicate the head, tails, polar areas, and non-
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 nonpolar?
(c)
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 nonpolar, 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 nonpolar. 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 oxygenhydrogen 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 oxygen's 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 oxygen's 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.