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5
The Dynamic Cell Membrane
5 The Dynamic Cell Membrane
• 5.1 What Is the Structure of a Biological Membrane?
• 5.2 How Is the Plasma Membrane Involved in Cell
Adhesion and Recognition?
• 5.3 What Are the Passive Processes of Membrane
Transport?
• 5.4 How Do Substances Cross Membranes Against
a Concentration Gradient?
• 5.5 How Do Large Molecules Enter and Leave a
Cell?
• 5.6 What Are Some Other Functions of
Membranes?
5.1 What Is the Structure of a Biological Membrane?
The general structure of membranes is
know as the fluid mosaic model.
The phospholipid bilayer is like a “lake” in
which a variety of proteins “float.”
Figure 5.1 The Fluid Mosaic Model
Figure 3.20 Phospholipids (A)
Repeat Fig 3.20A here
Figure 5.2 A Phospholipid Bilayer Separates Two Aqueous Regions
5.1 What Is the Structure of a Biological Membrane?
Artificial bilayers can be made in the
laboratory.
Lipids maintain a bilayer organization
spontaneously—helps membranes fuse
during phagocytosis, vesicle formation,
etc.
5.1 What Is the Structure of a Biological Membrane?
Membranes may vary in lipid composition
Phospholipids vary—fatty acid chain
length, degree of saturation, phosphate
groups
Membranes may be up to 25 percent
cholesterol
5.1 What Is the Structure of a Biological Membrane?
Phospholipid bilayer is flexible, and the
interior is fluid, allowing lateral
movement of molecules.
Fluidity depends on temperature and lipid
composition.
5.1 What Is the Structure of a Biological Membrane?
Membranes contain proteins, the number
of proteins varies with cell function
Some membrane proteins extend across
the lipid bilayer—with hydrophobic and
hydrophilic regions or domains.
Figure 5.3 Membrane Proteins Revealed by the Freeze-Fracture Technique
5.1 What Is the Structure of a Biological Membrane?
The proteins and lipids in the membrane
are independent and only interact
noncovalently.
5.1 What Is the Structure of a Biological Membrane?
Two types of membrane proteins:
• Integral membrane proteins span the
bilayer, hydrophilic ends protrude on
either side.
• Peripheral membrane proteins do not
penetrate the bilayer.
Figure 5.4 Interactions of Integral Membrane Proteins
5.1 What Is the Structure of a Biological Membrane?
Transmembrane proteins may have
different domains on either side of the
membrane.
The two sides of the membrane can have
very different properties.
5.1 What Is the Structure of a Biological Membrane?
Some membrane proteins can move
freely within the bilayer, while some are
anchored to a specific region.
Some can be anchored by cytoskeleton
elements, or lipid rafts—lipids in
semisolid state.
5.1 What Is the Structure of a Biological Membrane?
Membranes are dynamic and are
constantly forming, transforming, fusing,
and breaking down.
Figure 5.5 Dynamic Continuity of Membranes
5.1 What Is the Structure of a Biological Membrane?
Membranes have carbohydrates on the
outer surface that serve as recognition
sites for other cells and molecules.
Glycolipids
Glycoproteins
Figure 5.1 The Fluid Mosaic Model
5.2 How Is the Plasma Membrane Involved In Cell Adhesion and
Recognition?
Cells arrange themselves in groups by
cell recognition and cell adhesion.
These processes can be studied in
sponge cells—the cells are easily
separated and will come back together
again.
Figure 5.6 Cell Recognition and Adhesion (A)
5.2 How Is the Plasma Membrane Involved In Cell Adhesion and
Recognition?
Binding of cells is usually homotypic: the
same molecule sticks out from both
cells and forms a bond.
Some binding is heterotypic: the cells
have different proteins.
Figure 5.6 Cell Recognition and Adhesion (B)
5.2 How Is the Plasma Membrane Involved In Cell Adhesion and
Recognition?
Cell junctions are specialized structures
that hold cells together:
• Tight junctions
• Desmosomes
• Gap junctions
Figure 5.7 Junctions Link Animal Cells Together (A)
Tight junctions help ensure directional movement of materials.
Figure 5.7 Junctions Link Animal Cells Together (B)
Desmosomes are like “spot welds”
Figure 5.7 Junctions Link Animal Cells Together (C)
Gap junctions allow communication
5.3 What Are the Passive Processes of Membrane Transport?
Membranes have selective
permeability—some substances can
pass through, but not others
Passive transport—no outside energy
required—diffusion
Active transport—energy required
5.3 What Are the Passive Processes of Membrane Transport?
Diffusion: the process of random
movement toward equilibrium
Equilibrium—particles continue to move,
but there is no net change in distribution
Figure 5.8 Diffusion Leads to Uniform Distribution of Solutes
5.3 What Are the Passive Processes of Membrane Transport?
Net movement is directional until
equilibrium is reached.
Diffusion is net movement from regions of
greater concentration to regions of
lesser concentration.
5.3 What Are the Passive Processes of Membrane Transport?
Diffusion rate depends on:
• Diameter of the molecules or ions
• Temperature of the solution
• Electric charges
• Concentration gradient
5.3 What Are the Passive Processes of Membrane Transport?
Diffusion works very well over short
distances.
Membrane properties affect the diffusion
of solutes.
The membrane is permeable to solutes
that move easily across it; impermeable
to those that can’t.
5.3 What Are the Passive Processes of Membrane Transport?
Simple diffusion: small molecules pass
through the lipid bilayer.
Lipid soluble molecules can diffuse
across the membrane, as can water.
Electrically charged and polar molecules
can not pass through easily.
5.3 What Are the Passive Processes of Membrane Transport?
Osmosis: the diffusion of water
Osmosis depends on the number of
solute particles present, not the type of
particles.
Figure 5.9 Osmosis Can Modify the Shapes of Cells
5.3 What Are the Passive Processes of Membrane Transport?
If two solutions are separated by a
membrane that allows water, but not
solutes to pass through, water will
diffuse from the region of higher water
concentration (lower solute
concentration) to the region of lower
water concentration (higher solute
concentration).
5.3 What Are the Passive Processes of Membrane Transport?
Isotonic solution: equal solute
concentration (and equal water
concentration)
Hypertonic solution: higher solute
concentration
Hypotonic solution: lower solute
concentration
5.3 What Are the Passive Processes of Membrane Transport?
Water will diffuse (net movement) from a
hypotonic solution across a membrane
to a hypertonic solution.
Animal cells may burst when placed in a
hypotonic solution.
Plant cells with rigid cell walls build up
internal pressure that keeps more water
from entering—turgor pressure.
5.3 What Are the Passive Processes of Membrane Transport?
Facilitated diffusion (passive):
• Polar molecules can cross the
membrane through channel proteins
and carrier proteins.
• Channel proteins have a central pore
lined with polar amino acids.
5.3 What Are the Passive Processes of Membrane Transport?
Ion channels: important channel proteins
Most are gated—can be closed or open to
ion passage
Gate opens when protein is stimulated to
change its shape. Stimulus can be a
molecule (ligand-gated) or electrical charge
resulting from many ions (voltage-gated).
Figure 5.10 A Gated Channel Protein Opens in Response to a Stimulus
5.3 What Are the Passive Processes of Membrane Transport?
Gradients can be a concentration
gradient of ions, or an electrochemical
gradient resulting from a charge
imbalance across the membrane.
5.3 What Are the Passive Processes of Membrane Transport?
Membrane potential is a charge
imbalance across a membrane.
Measured membrane potential of animal
cells: –70 mV—lots of potential energy!
5.3 What Are the Passive Processes of Membrane Transport?
Nernst equation:
[ K ]o
RT
EK  2.3
log
zF
[ K ]i
[ K ]o
E K  58 log
[ K ]i
Figure 5.11 The Potassium Channel (A, B)
5.3 What Are the Passive Processes of Membrane Transport?
Water may pass through the membrane
by hydrating ions that pass through a
channel.
Water also enters cells through special
water channels called aquaporins.
5.3 What Are the Passive Processes of Membrane Transport?
Carrier proteins transport polar
molecules such as glucose across
membranes.
Glucose binds to the protein, which
causes it to change shape.
Figure 5.12 A Carrier Protein Facilitates Diffusion (Part 1)
Figure 5.12 A Carrier Protein Facilitates Diffusion (Part 2)
5.4 How Do Substances Cross Membranes against a
Concentration Gradient?
Active transport: moves substances
against a concentration gradient—
requires energy.
5.4 How Do Substances Cross Membranes against a
Concentration Gradient?
Active transport involves three kinds of
proteins:
• Uniports
• Symports
• Antiports
Figure 5.13 Three Types of Proteins for Active Transport
5.4 How Do Substances Cross Membranes against a
Concentration Gradient?
Primary active transport requires direct
participation of ATP.
Secondary active transport: energy
comes from an ion concentration
gradient that is established by primary
active transport.
5.4 How Do Substances Cross Membranes against a
Concentration Gradient?
The sodium–potassium pump (Na+–K+)
is primary active transport.
Found in all animal cells
The “pump” is an integral membrane
glycoprotein. It is an antiport.
Figure 5.14 Primary Active Transport: The Sodium–Potassium Pump
5.4 How Do Substances Cross Membranes against a
Concentration Gradient?
Energy can be “regained” by letting ions
move across a membrane with the
concentration gradient—secondary
active transport.
• Aids in uptake of amino acids and
sugars
• Uses symports and antiports
Figure 5.15 Secondary Active Transport
5.5 How Do Large Molecules Enter and Leave a Cell?
Macromolecules (proteins,
polysaccharides, nucleic acids) are too
large to cross the membrane.
They can be taken in or excreted by
means of vesicles.
5.5 How Do Large Molecules Enter and Leave a Cell?
Endocytosis: processes that bring
molecules and cells into a eukaryotic cell.
The plasma membrane folds in or
invaginates around the material, forming a
vesicle.
Figure 5.16 Endocytosis and Exocytosis (A)
5.5 How Do Large Molecules Enter and Leave a Cell?
Phagocytosis: molecules or entire cells
are engulfed. Some protists feed in this
way. Some white blood cells engulf
foreign substances.
A food vacuole or a phagosome forms,
which fuses with a lysosome.
5.5 How Do Large Molecules Enter and Leave a Cell?
Pinocytosis: a vesicle forms to bring
small dissolved substances or fluids into
a cell. Vesicles are much smaller than in
phagocytosis.
Pinocytosis is constant in endothelial
(capillary) cells.
5.5 How Do Large Molecules Enter and Leave a Cell?
Receptor mediated endocytosis: highly
specific
Depends on receptor proteins—integral
membrane proteins—to bind to specific
substances
Sites are called coated pits—coated with
other proteins such as clathrin
Figure 5.17 Formation of a Coated Vesicle (Part 1)
Figure 5.17 Formation of a Coated Vesicle (Part 2)
5.5 How Do Large Molecules Enter and Leave a Cell?
Mammalian cells take in cholesterol by
receptor-mediated endocytosis.
Lipids are packaged by the liver into
lipoproteins—secrete to bloodstream.
Liver must take up low-density
lipoproteins (LDLs) for recycling. The
LDLs bind to specific receptor proteins.
5.5 How Do Large Molecules Enter and Leave a Cell?
Exocytosis: material in vesicles is expelled
from a cell
Indigestible materials are expelled.
Other materials leave cells such as
digestive enzymes and neurotransmitters.
Figure 5.16 Endocytosis and Exocytosis (B)
5.6 What Are Some Other Functions of Membranes?
Keeping different materials separated:
• Endoplasmic reticulum segregates
newly-formed proteins.
5.6 What Are Some Other Functions of Membranes?
Electrically excitable membranes—
• The plasma membrane of neurons
conducts nerve impulses.
5.6 What Are Some Other Functions of Membranes?
Membranes help transform energy:
• Inner mitochondrial membranes—
energy from fuel molecules is
transformed to ATP
• Thylakoid membranes of chloroplasts
transform light energy to chemical
bonds.
Figure 5.18 More Membrane Functions (A)
5.6 What Are Some Other Functions of Membranes?
Membrane proteins can organize
chemical reactions.
Many cellular processes involve a series
of enzyme-catalyzed reactions—all the
molecules must come together for these
to occur. Forms an “assembly line” of
enzymes.
Figure 5.18 More Membrane Functions (B)
5.6 What Are Some Other Functions of Membranes?
Membrane proteins process information.
Binding of a specific ligand can initiate,
stop, or change cell functions.
Figure 5.18 More Membrane Functions (C)
5.6 What Are Some Other Functions of Membranes?
The cholera toxin:
One subunit binds to a cell surface receptor—the
toxin molecule changes shape and allows the
other subunit to enter the cell.
The subunit acts as an enzyme to modify a
peripheral protein—this opens chloride channels
in the membrane.
Cl− and Na+ accumulate in the intestines, followed
by osmotic loss of water.