Lecture 3.2 Study Guide – Biological Membranes
Assigned Reading: Chapter 6.1; 6.3-6.4
Learning Objectives – At the conclusion of this lecture, you should be able to:
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List & explain the functions of biological membranes.
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Differentiate between the types of movement that components can exhibit within a membrane.
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Describe the fluid mosaic model for biological membranes.
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Explain the experimental data that provided evidence for the fluid mosaic model.
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Define transition temperature.
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List and describe the four factors that affect membrane fluidity.
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Predict what changes an organism would make to its membrane composition in response to to a
given temperature change.
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List and describe the three types of membrane proteins.
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Differentiate between passive and active transport mechanisms.
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Differentiate between channels, carriers, and pumps.
Key Terms You Should Be Able to Use & Apply –
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Active transport
Amphipathic
Antiporter
Aquaporin
Carrier protein
Cell fusion
Channel protein
Cholesterol
Concentration gradient
Diffusion
Electrochemical gradient
Equilibrium
Facilitated diffusion
Flip-flop movement
Fluid mosaic model
Gated channel
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Heterokaryon
Hypertonic
Hypotonic
Integral membrane protein
Ion channel
Isotonic
Lateral movement
Lipid bilayer
Lipid-anchored protein
Osmosis
Passive transport
Peripheral membrane protein
Phospholipid
Primary active transport
Pump
Rotational movement
Lecture 3.2 Study Guide
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Secondary active transport
Selective permeability
Simple diffusion
Symporter
Transition temperature (Tm)
Uniporter
Key Concepts You Should Be Able to Use & Apply –
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All life consists of cells bounded by membranes composed of lipid bilayers containing
amphipathic phospholipids, membrane proteins, and cholesterol.
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According to the fluid mosaic model, the plasma membrane is a mosaic of lipids and proteins,
which (unless restrained) are free to move laterally within the plane of the membrane.
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Cell fusion experiments by Frye & Edidin revealed the mobility of membrane proteins.
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Membrane fluidity is affected by temperature, the saturation and length of hydrocarbon tails,
and cholesterol.
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The main types of membrane proteins are integral membrane proteins, peripheral membrane
proteins, and lipid anchored proteins, which differ in their affinity for the hydrophobic interior
of the membrane. Transmembrane proteins serve a variety of cellular functions.
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Selective permeability allows the membrane to determine what substances enter or leave a cell
or organelle.
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Substances cross biological membranes by passive or active transport processes. Passive
transport is diffusion of a substance across a membrane that may or may not rely on the
assistance of a transmembrane protein. Protein pumps perform active transport using energy.
Examples of Skills You Should Be Able to Demonstrate –
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Predict how changes in environment and/or lipid composition will affect membrane fluidity.
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Describe how cell fusion can be used to study the movement of a protein, and be able to make
predictions, interpret the data and make conclusions from such an experiment.
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Identify which type of membrane protein a given protein is based on its amino acid composition
and/or orientation within a membrane.
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Predict whether a given molecule would be permeable across a selectively permeable
membrane.
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Predict which way a certain substance will diffuse, given its concentration on either side of a
selectively permeable membrane.
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Lecture 3.2 Study Guide
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Given concentration gradient(s) across a membrane, identify whether an ion/molecule will
move by passive or active transport and what type of protein will be used to facilitate
movement.
Recommended Chapter Questions –
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5.1 Recap: Question 2.
6.1 Recap: Questions 1-4 (NOTE: ignore the freeze-fracture portion of question 3).
6.3 Recap: Questions 1, 3.
6.4 Recap: Questions 1-2, 4.
Figure Questions: Figure 6.1.
Additional Practice Problems –
1. Complete each statement with the correct Key Term:
A. ____________________ comprise the major lipid component of biological membranes, where
membrane formation is a consequence of the ____________________ nature of the molecules.
B. The ____________________ is a molecular model for the structure of biological membranes
consisting of a fluid phospholipid bilayer in which suspended proteins are free to move in the
plane of the bilayer.
C. ____________________ are at least partially embedded in the plasma membrane, where
____________________ are associated with but not embedded within the plasma membrane.
D. ____________________ is the process of random movement toward a state of equilibrium.
E. There are two fundamentally different processes by which substances cross biological
membranes: ____________________ processes do not require the input of chemical energy to
drive them, where ____________________ processes are driven by chemical energy.
F. ____________________ involves a transport system coupled to an exergonic chemical reaction
(most commonly the hydrolysis of ATP). ____________________ involves the coupled transport
of two substances: one down its concentration gradient and the other against its concentration
gradient.
2. Describe the energetic driving force for the formation of a phospholipid bilayer.
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Lecture 3.2 Study Guide
3. Why does membrane fluidity matter? What happens if a membrane becomes too rigid? … too fluid?
4. It is very rare for a phospholipid in the outer leaflet of a bilayer to spontaneously flip to the inner
leaflet of a bilayer. Why? What is a positive consequence of this lack of flip-flop movement for the
cell’s membrane?
5. Consider a plant cell’s phospholipid bilayer membrane. Determine the ways that the plant cell’s
membranes might accommodate for the following changes in temperature by assigning each
outcome (a-e) to a corresponding temperature change.
Increase in environmental temperature
a.
b.
c.
d.
e.
Decrease in environmental temperature
Increase the quantity of unsaturated fatty acids in the membrane.
Increase the quantity of saturated fatty acids in the membrane.
Convert the unsaturated fatty acid tails from cis to trans conformations.
Lengthen the hydrocarbon tails.
Shorten the hydrocarbon tails.
6. Saline solution is a 0.9% solution of NaCl in water, and is injected directly into the bloodstream to
treat dehydration. Why is pure water not injected into the bloodstream to treat dehydration?
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Lecture 3.2 Study Guide
7. Transport mechanisms for the movement of materials across a membrane are depicted below.
Match each label to the correct term. A term may be used more than once.
A
L
K
M
Movement along the concentration gradient
J
H
F
C
B
G
D
I
E
_____ Active transport
_____ Against
_____ Carrier
_____ Channel
_____ Down
_____ Energy
_____ Facilitated diffusion
_____ High concentration
_____ Low concentration
_____ Passive transport
_____ Pump
_____ Simple diffusion
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Lecture 3.2 Study Guide
8. You have prepared two different sets of liposomes, which are small, spherical phospholipid-bilayer
enclosed structures with an aqueous interior. Set A lacks transmembrane proteins altogether, while
set B contains a variety of transporters and ion channels. You prepare these liposomes such that the
interior of the liposome is topologically equivalent to the extracellular space, and the outside is
topologically equivalent to the cytoplasm.
A. What general types of molecules can pass into each type of liposome?
B. Liposomes in set B contain the K+ channel, which transports K+ down its concentration gradient.
You can monitor movement into and out of the liposomes using a radioactive isotope of K+. You
can also adjust the K+ concentrations inside and outside of the liposome. Under what conditions
will you see radioactive K+ flow into the liposomes? …out of the liposomes?
9. The figure below shows the concentrations of three ions on the inside and outside of a cell, as well
as the direction of transport of each through a membrane protein. Which ion(s) is/are moved by
active transport and why?
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Lecture 3.2 Study Guide
Lecture 3.2 – Biological Membranes
Lecture 3.2.1
Reading: Chapter 6.1; 6.3-6.4
The Fluid Mosaic Model
Learning Objectives:
• List & explain the functions of biological membranes.
• Differentiate between the types of movement that components can exhibit
within a membrane.
• Describe the fluid mosaic model for biological membranes
• Explain the experimental data that provided evidence for the fluid mosaic
model.
Kathryn Gardner, Ph.D.
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Cells contain each of the four macromolecules
image from: H.T. Jansen, et al. Communications Biology 2:336 (2019)
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image from: P. Raven, et al. Biology, 10th ed. (2014)
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All life consists of cells bounded by membranes composed of lipid bilayers
Fibers of extracellular
matrix (ECM)
Biological membranes have many functions
Carbohydrate
Lipids
Glycoprotein
Cholesterol
Peripheral
protein
Integral
protein
Microfilaments
of cytoskeleton
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adapted from FIGURE 6.1
All life consists of cells bounded by membranes composed of lipid bilayers
image from: G. Karp. Cell and Molecular Biology: Concepts & Experiments, 7th ed. (2013)
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Phospholipids are the most common lipid found in biological membranes
Example of a PHOSPHOLIPID –
choline*
Fibers of extracellular
matrix (ECM)
Carbohydrate
phosphate
glycerol
Lipids
Glycoprotein
Cholesterol
Peripheral
protein
Integral
protein
Microfilaments
of cytoskeleton
fatty acids
adapted from FIGURE 6.1
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image from: J.B. Reece, et al. Campbell Biology, 10th ed. (2013)
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A lipid bilayer spontaneously assembles in an aqueous environment
to minimize the energy of the system
According to the FLUID MOSAIC MODEL, a membrane is a fluid bilayer of lipids
with a mosaic of associated proteins
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FIGURE 6.2
Membranes must be fluid to work properly
FIGURE 6.9
image from: J. Hardin & G.P. Bertoni. Becker’s World of the Cell, 9th ed. (2015)
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Membranes components (unless restrained) can move within the membrane
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image from: J. Hardin & G.P. Bertoni. Becker’s World of the Cell, 9th ed. (2015)
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Cell fusion experiments by Frye & Edidin revealed the mobility of membrane proteins
Cell fusion experiments by Frye & Edidin revealed the mobility of membrane proteins
H-2
Fluorescent dye
time
Antibody
Heterokaryon
Draw what you would predict to see in the
microscope if proteins laterally diffuse.
Human
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image from: L.D. Frye & M. Edidin. J. Cell Science 7:319-335 (1970)
Mouse
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TOPHAT L3.2 Q1
Frye & Edidin followed the movement of proteins at the cell’s equator
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Lecture 3.2.2
Membrane Fluidity
Learning Objectives:
• Define transition temperature.
• List and describe the four factors that affect membrane fluidity.
• Predict what changes an organism would make to its membrane
composition in response to a given temperature change.
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Fluidity affects membrane permeability and the ability of membrane proteins to move
to where their function is needed
In-Text Art, p. 111
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Membrane fluidity changes with temperature
image from: J. Hardin & G.P. Bertoni. Becker’s World of the Cell, 9th ed. (2015)
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Chain length and saturation affect the Tm of a membrane
image from: J. Hardin & G.P. Bertoni. Becker’s World of the Cell, 9th ed. (2015)
CHOLESTEROL acts as a fluidity buffer
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image from: J. Hardin & G.P. Bertoni. Becker’s World of the Cell, 9th ed. (2015)
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Let’s Think About That!
Let’s Think About That!
To begin, first complete LaunchPad Activity 6.2/Lipid Bilayer Composition Simulation
(www.life11e.com/ac6.2).
How would the cell membrane composition of each of the following animals compare
to that of a counterpart animal @ 25°C?
A. A hibernating hedgehog whose environment is 5°C.
More saturated fatty acids.
More unsaturated fatty acids.
More short-tail fatty acids.
More long-tail fatty acids.
More cholesterol.
Less cholesterol.
B. A warm-blooded tropical fish whose environment is 35°C.
More saturated fatty acids.
More unsaturated fatty acids.
More short-tail fatty acids.
More long-tail fatty acids.
More cholesterol.
Less cholesterol.
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All life consists of cells bounded by membranes composed of lipid bilayers
Lecture 3.2.3
Transmembrane Proteins
Fibers of extracellular
matrix (ECM)
Carbohydrate
Learning Objectives:
Lipids
Glycoprotein
• List and describe the three types of membrane proteins.
Cholesterol
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Integral
protein
Microfilaments
of cytoskeleton
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adapted from FIGURE 6.1
Membrane proteins differ in their affinity for the hydrophobic interior of the
membrane and therefore the extent to which they interact with the lipid bilayer
image from: J. Hardin & G.P. Bertoni. Becker’s World of the Cell, 9th ed. (2015)
Peripheral
protein
Integral membrane proteins are amphipathic
FIGURE 6.16
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Functions of transmembrane proteins
Signaling molecule
Enzymes
ATP
Receptor
Signal transduction
Glycoprotein
image adapted from: J.B. Reece, et al. Campbell Biology, 10th ed. (2013)
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Biological membranes have many functions
Lecture 3.2.4
Transport Across Membranes
Learning Objectives:
• Differentiate between passive and active transport mechanisms.
• Differentiate between channels, carriers, and pumps.
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Selective permeability allows the membrane to determine what
substances enter or leave a cell or organelle
TABLE 6.1
image from: G. Karp. Cell and Molecular Biology: Concepts & Experiments, 7th ed. (2013)
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DIFFUSION is the process of random movement toward a state of equilibrium
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In-Text Art, p. 118
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PASSIVE TRANSPORT is diffusion of a substance across a membrane
Osmosis is the diffusion of water across membranes
lipid
bilayer
image from: S. Freeman, et al. Biological Science, 5th ed. (2013)
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PASSIVE TRANSPORT can involve either of two types of diffusion
image from: J. Hardin & G.P. Bertoni. Becker’s World of the Cell, 9th ed. (2015)
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FIGURE 6.10
An AQUAPORIN is a channel protein through which water can move
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image from: A.S. Verkman, M.O. Anderson & M.C. Papadopoulos. Nature Reviews Drug Discovery 13:259-277 (2014).
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GATED CHANNELS are proteins that open/close in response to a specific stimulus,
allowing for the flow of ions and small molecules to be carefully regulated
Unlike channel proteins, CARRIER PROTEINS do not form a pore but instead undergo
structural changes to move substances during the process of facilitated diffusion
GLUT-1
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FIGURE 6.11
FIGURE 6.12
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ACTIVE TRANSPORT is the movement of ions or molecules across a membrane against
an electrochemical gradient
Animation 6.1
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image from: J. Hardin & G.P. Bertoni. Becker’s World of the Cell, 9th ed. (2015)
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Pumps (i.e., transport ATPases) perform active transport
image from: J. Hardin & G.P. Bertoni. Becker’s World of the Cell, 9th ed. (2015)
Active transport is directional
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PRIMARY ACTIVE TRANSPORT: Energy from ATP is used to move Na+ and K+ against
their concentration gradients by the sodium-potassium pump (Na+/K+-ATPase)
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FIGURE 6.13
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SECONDARY ACTIVE TRANSPORT: The movement of Na+ down its concentration
gradient drives the transport of glucose against its concentration gradient
FIGURE 6.15
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Animation 6.2
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