Download Lipids and Membranes, Fall 13--Worksheet Crowe

Lipids and Membranes, Fall 13--Worksheet
Adapted from a POGIL exercise developed by J. Loertscher & V. Minderhout
Crowe
Work in groups of two or three to complete this worksheet. Your understanding will be
checked using clicker questions periodically throughout the activity.
Membranes are some of the most important macrostructures of biological systems. Membranes
define the organism and separate it from its environment. All biological membranes contain
lipids as major components. Understanding how these lipids contribute to the structure of
membranes and interact with membrane proteins will help you understand many cellular
processes including cell signaling, cell motility and cell respiration.
1. Based on what you know about hydrophobic interactions, draw phospholipids indicating
possible ways that phospholipid molecules would be expected to interact (use circles to
represent the polar head groups and wavy lines to indicate hydrophobic tails)
2. Based on what you know about the structure of a bi-lipid membrane, compare and contrast
how a plastic seal surrounding the cell would be similar and how it would be different from a
cell membrane.
3. Based on your answer to Q2 and what you have already learned about cell organelles, list at
least 2 possible purposes of membranes.
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Table 1: Membrane Passage
Molecule
Molecule Structure
Name
# of molecules
that pass
through a lipid
bilayer at 25 °C
# of molecules
that pass
through a lipid
bilayer at 42 °C
Indole
30,000,000
34,500,000
# of molecules
that pass
through a lipid
bilayer with
additional
unsaturated lipid
tails at 25 °C
33,900,000
Adenosine
Triphosphate
(ATP)
2
2.3
2.5
18
45,600
27
51,300
25.5
52,000
Methane
190,000,000,000
290,000,000,000
240,000,000,000
Octane
99,800
129,200
112,300
Glycine
?
85,000
?
Ethanol
4,090,000
?
?
Potassium
Fructose
K+
4. Use Table 1 to answer the following questions
1) Which molecule is the least likely to pass through a membrane at a given temperature?
2) Which molecule is the most likely to pass through a membrane at a given temperature?
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3) What are two molecular characteristics that account for the rate of membrane passage? In
other words, what is it about the molecules that makes them pass through differently?
(Answer each in a single complete sentence.)
4) Which characteristic from #3 above helps to explain the data you see for the Potassium? If
neither of your characteristics helps explain this data, then you may need to go back and
change your answers for #3.
5) Which characteristic from #3 above helps to explain the data you see for the differences
between Methane and Octane? Again, if you don’t have an explaining factor then you may
need to change your answer for #3.
6) Make an educated guess for the rate of Ethanol passage through a membrane at 42°C. There
are many possible correct answers, but this answer should make sense based on what you see
in the rest of the table.
7) What is happening to the molecules in the lipid bilayer with increasing temperature? In a
single sentence, develop a hypothesis that would explain your rate of Ethanol passage at 42°C.
[Challenge questions – to be completed if you have time]
8) Fill in the table with reasonable rates of passage that agree with the rest of the data.
9) Changing the lipid tails in the membrane can change the passage of molecules through that
membrane. Can you make a confident prediction about the 3rd rate for glycine? If not, why
not?
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Information: Integral membrane proteins often contain helical segments of appropriate length
to span the lipid bilayer. In a protein that has a single segment that spans the membrane, the
helix usually only contains hydrophobic residues and is called a single-span membrane
protein. In transmembrane proteins with multiple segments that span the membrane,
hydrophilic residues are often found in sequences of the helices.
5. Why are only hydrophobic amino acids likely to be found in single-span membrane
proteins?
6. From Fig. 1, what is the role of Protein A?
7. You discover a mutant form of protein A that has
a Lysine (basic amino acid) in place of a Valine (nonpolar amino acid) in one of the membrane spanning
regions. What effect do you predict this change to
have on the function of protein A?
.
Fig. 1. Diagram of a cell membrane
either containing Protein A (top) with
multiple or without Protein A (bottom).
OUT = outside the cell; IN = inside the
cell. Protein A has multiple
transmembrane segments.
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On your own:
1. The molecule shown to the right is a hormone that is
important for sending signals from the pituitary to other
organs in the body. In order to receive the signal, cells
within these organs contain receptors. Where in the cell
would you expect to find a receptor for this hormone?
Explain your reasoning.
2. The structure of cholesterol is
shown in Fig. 2. What effect do you
predict the presence of cholesterol in
a cell membrane to have on
membrane fluidity.
Fig. 2. Structure of cholesterol
3. Cell membranes are described
l mosaic model. Does this model fully describe the
using a fluid mosaic model. Define the fluid
reality of a cell membrane? Explain why or why not.
4. Why is it important for determining the function of a membrane protein to know if it spans
the bilayer or appears only on one face of the membrane? Provide examples of different
proteins’ functions to support your answer.
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