AP Biology
Modeling with C, H and O
In this exercise, you will explore the structures that can be constructed using only carbon,
hydrogen and oxygen. These are the elements needed to build carbohydrates and lipids, and they
are essential components of proteins and nucleic acids, along with nitrogen, phosphorus and a bit
of sulfur. Make sketches of what your draw in your lab notebook and add notes about the
molecule based on the questions below.
Conventions:
Black = C
red = O
white = H; one section of white straw = single covalent bond
For ring structures, atoms that stick out toward you should be drawn as heading up from the
carbon, while those that point away from you (towards the desk top) should be drawn as sticking
down from the carbon.
Carbohydrates
1. Start with 6 C, 12 H and 6 O, and connect these to form a linear or chain structure. You
must use ALL of the atoms, but you are free to consider double bonds in addition to single.
Rearrange the atoms to form two more structures. Sketch each structure on your answer
sheet.
2. These structures all have the same chemical formula, but the atoms are arranged in different
ways. How are these different shapes related? What are they called? Must you break a bond
in order to rearrange the atoms from one form to another, or can you rotate atoms around a
carbon?
3. Consult the reference sheet and try to identify each molecule. There are other possibilities,
but the ones shown are the most common and most likely to be biologically active.
4. Build a linear molecule of glucose. Recall that in water, in other words in living things, the
monosaccharides tend to form ring structures. Change your linear model into a biologically
active ring form as shown below. Note that only atoms attached to carbons 1 and 5 should be
involved.
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5. In the linear form, what functional group was visible? Where is it in the ring form?
6. The transition to a ring may produce two different forms of glucose,  and  as shown
below. How are they different? The difference is subtle, but it has major implications.
7. Time to create a disaccharide! Build two molecules of glucose. These can be joined
through a condensation reaction involving the hydroxyl groups attached to carbons 1 and 4.
This is a 1,4 glycosidic linkage. Draw the products of this reaction on your answer sheet.
What is this disaccharide called?
8. On to polysaccharides! Join your disaccharides to others in your class. As the chain
elongates, what form does it take? Describe it. You are creating starch.
9. Now hydrolyze the linkages to recover your monomers.
10. Turn one of your -glucose forms into the  form. Join these together using the 1, 4 linkage.
You can do it, but it requires some thinking. Explain on your answer sheet.
11. Now join your disaccharide with others in the class to create cellulose. How does the
structure of this chain differ from the starch?
12. Hydrolyze the bonds to recover your starting glucose molecules and then dismantle the
structures. It’s time to repurpose your atoms as we consider lipids.
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Lipids
1. The most common basic form of lipids that you should know is the triglyceride. It consists
of one glycerol molecule bonded to three fatty acids. This is a macromolecule, but not a
polymer. Explain why.
2. Build a glycerol: C3H8O3. It should contain three hydroxyl groups. Sketch it on your
answer sheet.
3. Now build a short fatty acid. It must have a carboxyl group at one end and otherwise should
only consist of a chain of 5 additional carbons and enough hydrogens to fill the carbons.
Everything should be single bonded. Write the formula for this molecule. Is it hydrophobic
or hydrophilic? Explain.
4. Join the fatty acid to the glycerol molecule using a condensation reaction. The carboxyl of
the fatty acid should interact with a hydroxyl of the glycerol. Share with another group or
two to create a full triglyceride. What byproduct has also been produced from the formation
of each bond?
5. Now hydrolyze the bonds and take back your original molecules. Use the carbons and
hydrogens from the glycerol to extend your hydrocarbon chain. The carbons are saturated
with hydrogens. Modify this molecule so that is polyunsaturated – two double bonds should
be enough. How has the structure of the fatty acid chain changed relative to when it was
saturated?
6. Which form, saturated or unsaturated, would you be able to pack more densely? How does
your answer relate to dietary fats? We consider lipids that are solid at room temperature to
be fats, and those that remain liquid at room temperature to be oils. Which form would likely
contain more unsaturated bonds?
7. If you wanted to use fatty acids as a component of a flexible barrier - like maybe a cell
membrane – would saturated or unsaturated chains be a better bet? Explain.
8. Disassemble you model and put all the pieces back in the original plastic bags.
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Modeling Carbohydrates and Lipids
Carbohydrates
1. Sketch each of your three linear structures
2. These structures all have the same chemical formula, but the atoms are arranged in different
ways. How are these different shapes related? What are they called? Must you break a bond
in order to rearrange the atoms from one form to another, or can you rotate atoms around a
carbon?
3. Consult the reference sheet and try to identify each molecule. Add these labels to the
molecule sketches above.
5. In the linear form, what functional group was visible? Where is it in the ring form?
6. Draw the products of this reaction. What is this disaccharide called?
7. On to polysaccharides! As the chain elongates, what form does it take? Describe it.
Name
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9. Turn one of your -glucose forms into the  form. Join these together using the 1, 4 linkage.
You can do it, but it requires some thinking. Explain on your answer sheet.
10. Now join your disaccharide with others in the class to create cellulose. How does the
structure of this chain differ from the starch?
Lipids
1. This is a macromolecule, but not a polymer. Explain why.
2. Build a glycerol: C3H8O3. It should contain three hydroxyl groups. Sketch it below.
3. Write the formula for this molecule. Is it hydrophobic or hydrophilic? Explain.
4. What byproduct has also been produced from the formation of each bond?
5. How has the structure of the fatty acid chain changed relative to when it was saturated?
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6. Which form, saturated or unsaturated, would you be able to pack more densely? How does
your answer relate to dietary fats? We consider lipids that are solid at room temperature to
be fats, and those that remain liquid at room temperature to be oils. Which form would likely
contain more unsaturated bonds?
7. If you wanted to use fatty acids as a component of a flexible barrier - like maybe a cell
membrane – would saturated or unsaturated chains be a better bet? Explain.