Download Lipids and Membranes, Fall 12—Worksheet - KEY

Lipids and Membranes, Fall 12—Worksheet - KEY
Adapted from a POGIL exercise developed by J. Loertscher & V. Minderhout
Crowe
Work in groups of 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.
Similar: keeps things separated
Different: not permeable, so doesn’t allow for certain molecules to cross
3. Based on your answer to Q2 and what you have already learned about cell organelles, list at
least 2 possible purposes of membranes.
Possible answers::
1.Concentrates reactants – resulting in increased frequency of chemical reactions
2. Forms selective barrier - allows for different environments inside and outside the cell
3. Allows different environments/gradients to exist inside and outside a cell
4. Forms internal compartments within a cell
B
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+
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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?
ATP
2) Which molecule is the most likely to pass through a membrane at a given temperature?
Methane
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Q4 Continued
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.)
a. Size: Smaller molecules are more likely to pass through a membrane than large bulky molecules
b. Hydrophobicity: Hydrophobic molecules are more likely to pass through a membrane than
hydrophilic molecules
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.
Potassium is a hydrophilic molecule due to its positive charge
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.
Octane is larger than methane, so does not travel through the membrane as easily, but both are
hydrophobic so can pass through relatively easily compared to hydrophilic molecules.
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.
~5,000,000 . Should be greater than the number that pass through at 25°C. Other molecules shown
have an increased rate of about 1.2-1.3-fold at 42°C compared to 25°C.
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.
Increased fluidity of the membrane at 42°C allows ethanol to pass more easily between the nonpolar tails
in the lipid bilayer resulting in increased passage at the higher temperature.
[Challenge questions]
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?
Cannot make a confident prediction. The effect of more unsaturated lipids on a molecule’s passage is
molecule-specific, so must be tested for each molecule
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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 hydrophobic residues favored in single-span membrane proteins?
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6. From Fig. 2, what is the role of Protein A?
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Fig. 2. Diagram of a cell membrane
either without Protein A (top) or with
Protein A (bottom). OUT = outside the
cell; IN = inside the cell.
Protein A enables the passage of Cl- across the cell
membrane, down the Cl- ion concentration gradient.
7. What effect do you predict the presence of a
hydrophilic amino acid in the membrane-spanning
portion of protein A to have on its function?
This could be beneficial, if the hydrophilic residue is facing
into the channel and can interact with chloride ions being
transported through the channel.
It could also disrupt the structure of Protein A if the
hydrophilic residue is facing outward to the hydrophobic
environment of the lipid membrane.
.
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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.
In the cytosol or nucleus, because this steroid can cross them
membrane, so the receptor does not need to be located on the cell surface.
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.
Proteins floating about in a fluid membrane (definition of the fluid mosaic model)
In reality, there are a lot of interactions between transmembrane proteins and intracellular proteins that
impact fluidity of proteins in the membrane. Location of membrane proteins, like receptors, are also
influenced by extracellular signals (like the chemical cues from bacteria!) causing them to group in on
region of the membrane. So the reality is more complex.
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.
Only proteins that span the membrane would be able to function as channels or active transport
molecules to ferry other molecules across the membrane. Proteins whose function is to interact with
other cells would need to be located on the outer face of the cell membrane, whereas proteins whose
function was to transmit signals to the cytoplasm from a cell receptor would need to be located on the
inner face of the cell membrane.
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