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Cellular Membranes
5
Membrane Composition and Structure
• Cell membranes are bilayered, dynamic
structures that:
 Perform vital physiological roles
 Form boundaries between cells and their
environments
 Regulate movement of molecules into and out
of cells
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Membrane Composition and Structure
• Lipids are like the water of a lake in which
proteins “float.”
• This general design is called the fluid mosaic
model.
Figure 5.1 The Fluid Mosaic Model
Figure 5.2 A Phospholipid Bilayer Separates Two Aqueous Regions
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Membrane Composition and Structure
• Although all biological membranes are structurally
similar, some have quite different compositions of
lipids and proteins.
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Membrane Composition and Structure
• All biological membranes contain proteins.
• The ratio of protein to phospholipid molecules
varies depending on membrane function.
• The association of protein molecules with lipid
molecules is not covalent.
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Membrane Composition and Structure
• Integral membrane
proteins have
hydrophobic regions of
amino acids that penetrate
or entirely cross the
phospholipid bilayer.
 Transmembrane
proteins show
different “faces” on the
two sides of the
membrane.
• Peripheral membrane proteins lack hydrophobic regions
and are not embedded in the bilayer.
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Membrane Composition and Structure
• Some of the proteins and lipids can move around
in the membrane.
• Some proteins are restricted in movement.
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Membrane Composition and Structure
glycolipid
• Carbohydrate-bound lipid is
called glycolipid.
• Most of the carbohydrate in
the membrane is covalently
bonded to proteins, forming
glycoproteins.
glycoprotein
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Cell Recognition and Adhesion
• Cells are able to arrange themselves into groups
by two processes:
 Cell recognition
 Cell adhesion
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Cell Recognition and Adhesion
 Homotypic binding
occurs when both
cells possess the
same type of cell
surface receptor and
their interaction
causes them to stick
together.
 Heterotypic binding
occurs between two
different but
complementary
proteins and
resembles a plug and
socket.
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• Specialized cell
junctions form
between cells in a
tissue.
• Animals have
three types of cell
junctions: tight
junctions,
desmosomes, and
gap junctions.
Cell Recognition and Adhesion
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Cell Recognition and Adhesion
• Tight junctions link adjacent epithelial cells to function to:
 Restrict the migration of membrane proteins and
phospholipids from one region of the cell to another
 Prevent substances from moving through the intercellular
space
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Cell Recognition and Adhesion
• Desmosomes act like spot welds on adjacent cells,
holding them together.
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Cell Recognition and Adhesion
• Gap junctions are connections that facilitate communication
between cells.
• Gap junctions are made up of specialized protein channels
called connexons.
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Passive Processes of Membrane Transport
• Biological membranes are selectively permeable:
allow some substances to pass, while others are
restricted.
• Some substances can move by
 Simple Diffusion: movement from high
concentration to low concentration
 Facilitated Diffusion: passive movement of
substances via a protein
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Passive Processes of Membrane Transport
• Diffusion is the process of random movement toward the
state of equilibrium.
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Passive Processes of Membrane Transport
• Diffusion rates are determined by:
 temperature
 size of the molecule
 electrical charge of the molecule
 concentration gradient.
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Passive Processes of Membrane Transport
• Small molecules can move across the lipid bilayer
by simple diffusion.
• The more lipid-soluble the molecule, the more
rapidly it diffuses.
• Polar and charged molecules such as amino
acids, sugars, and ions do not pass readily across
the lipid bilayer.
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Passive Processes of Membrane Transport
• Osmosis is the diffusion of water across
membranes.
• Water will diffuse from a region of its higher
concentration (low concentration of solutes) to a
region of its lower concentration (higher
concentration of solutes).
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Passive Processes of Membrane Transport
• Isotonic solutions
have equal solute
concentrations.
• Hypertonic solutions
have a greater solute
concentration than the
solution to which it is
compared.
• Hypotonic solutions
have a lower solute
concentration than the
solution to which it is
compared.
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Passive Processes of Membrane Transport
• One way for polar and charged substances to
enter cells is through the process of facilitated
diffusion.
Figure 5.9 A Gate Channel Protein Opens in Response to a Stimulus
Figure 5.11 A Carrier Protein Facilitates Diffusion (Part 1)
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Active Transport
• In contrast to diffusion, active transport requires
the expenditure of energy.
• Ions or molecules are moved across the
membrane against the concentration gradient.
• ATP is the energy currency used either directly or
indirectly to achieve active transport.
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Active Transport
• Three different protein-driven systems are
involved in active transport:
 Uniport
 Symport
 Antiport
Figure 5.12 Three Types of Proteins for Active Transport
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Active Transport
• If ATP is used directly for the pumping system, as in the
sodium–potassium pump, the system is a primary active
transport system.
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Active Transport
• Secondary active transport systems use
established gradients to move substances.
• This form of transport uses ATP indirectly.
Figure 5.14 Secondary Active Transport
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Endocytosis and Exocytosis
• Endocytosis
 Brings macromolecules, large particles, small
molecules, and even other cells into the eukaryotic cell.
• Exocytosis
 Process by which materials packaged in vesicles are
secreted from the cell.
Figure 5.15 Endocytosis and Exocytosis
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Endocytosis and Exocytosis
• Three types of endocytosis:
• Phagocytosis = cell eating: involves the largest
vesicles (entire cells can be engulfed).
• Pinocytosis = cell drinking: involves smaller
vesicles; dissolved substances and fluids are
brought into the cell.
• Receptor-mediated endocytosis: similar to
pinocytosis, but highly specific; receptor proteins
are exposed on the outside of the cell in regions
called coated pits.
Figure 5.16 Formation of a Coated Vesicle (Part 1)
Clathrin molecules
form the “coat” of
the pits.
Figure 5.16 Formation of a Coated Vesicle (Part 2)
Figure 5.17 More Membrane Functions (Part 1)
Membranes have many functions
Figure 5.17 More Membrane Functions (Part 2)
Membranes have many functions