Download Lecture 6 - The Plasma Membrane

Lecture 6 - Membranes
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In this lecture…
• The fluid mosaic model of the plasma
membrane
– Components of the membrane
• Active vs. passive transport
• Diffusion and osmosis
• Endocytosis and exocytosis
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The plasma membrane
• The boundary that separates the living cell from its
surroundings
• The plasma membrane exhibits selective
permeability, allowing some substances to cross it
more easily than others
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However…
• The membrane isn’t just a phospholipid bilayer
• It is composed of a huge array of
phospholipids, regular lipids, proteins, and
other molecules
• This is called the fluid mosaic model
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Fluid Mosaic Model
• The fluid mosaic model states that a membrane is a
fluid structure with a “mosaic” of various proteins
embedded in it
• Phospholipids and some proteins can drift laterally
– Very rarely does someone “flip”
• How do proteins stay embedded?
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Figure 7.5
Fibers of extracellular matrix (ECM)
Glycoprotein
Carbohydrate
Glycolipid
EXTRACELLULAR
SIDE OF
MEMBRANE
Cholesterol
Microfilaments
of cytoskeleton
Peripheral
proteins
Integral
protein
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CYTOPLASMIC SIDE
OF MEMBRANE
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The components of the membrane
•
•
•
•
Phospholipids
Proteins
Cholesterol
Cytoskeletal support
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What are membranes made of?
Phospholipids!
• Phospholipids are
amphipathic molecules,
containing hydrophobic
and hydrophilic regions
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Movement within the membrane
An enzyme called
“flipase” (really)
can catalyse the
flipping of lipids
How was membrane movement
demonstrated?
RESULTS
Membrane proteins
Mouse cell
Mixed proteins
after 1 hour
Human cell
Hybrid cell
Some different types of phospholipids
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What factors influence fluidity?
• Membranes start solidifying at cool temps
• The temperature at which a membrane solidifies
depends on the degree of saturation in the fatty acid
tails
– Membranes rich in unsaturated fatty acids are more fluid
than those rich in saturated fatty acids
• Membranes must be fluid to work properly; they are
usually about as fluid as salad oil
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Cholesterol
• Cholesterol is a “fluidity buffer”
– Restrains phospholipid movement at body temps
– Also hinder close packing, so lowers the temp required for
membranes to solidify
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Membrane Proteins
• A membrane’s function is defined by the
proteins embedded in it
• Two types of membrane proteins:
– Integral protein
• Penetrate the hydrophobic interior; can stick out of the
surface
• Integral proteins that span the membrane are called
transmembrane proteins
– Peripheral protein
• Stick to the surface of the membrane
Figure 7.9
Structure of a membrane protein
EXTRACELLULAR
SIDE
Integral or peripheral?
N-terminus
Hydrophobic
R groups on
the  helices
of the
interior
Hydrophilic
R groups on
the waterfacing sides
 helix
C-terminus
CYTOPLASMIC
SIDE
This protein has secondary
and tertiary structure, but no
quaternary structure
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Six Abilities of Membrane Proteins
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Many membrane proteins are
glycoproteins/proteoglycans
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Membrane carbohydrates in cell-cell recognition
• Cells recognize each other by binding to surface
molecules, often containing carbohydrates, on
the extracellular surface of the plasma membrane
• Membrane carbohydrates may be covalently
bonded to lipids (forming glycolipids) or more
commonly to proteins (forming glycoproteins)
• Carbohydrates on the external side of the plasma
membrane vary among species, individuals, and
even cell types in an individual
Membrane proteins in cell-cell recognition
• Membrane proteins help HIV invade immune system cells
• HIV’s gp120 protein binds to human’s CD4 glycoprotein +
CCR5 coreceptor
CCR5-Δ32 Mutation
• The CCR5-Δ32 mutation deletes the CCR5
coreceptor, preventing HIV from infecting cells
• Present in 10% of people from Northern Europe
• Marrow transplantation from an immune donor
confers immunity
– Immune system cells come from bone marrow
– Clinical trials underway to treat HIV+ people with their
own genetically engineered marrow
• Those without CCR5 are more susceptible to
West Nile Virus
What is a receptor? A coreceptor? What are
they made of? How are they part of the cell
membrane?
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The ABO blood group
• Your genes determine
what kind of
carbohydrate you get on
the surface of your red
blood cells
• Three types of
glycoproteins: A, B, O
• Your body recognizes a
“not you” group, gets
mad, and destroys the
cell bearing it
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The internal cytoskeleton supporting
the membrane
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Selective permeability
• Nonpolar molecules can dissolve in the lipid
bilayer and pass through indiscriminately
– Carbon dioxide, oxygen, hydrocarbons
• Hydrophobic membrane interior prevents
polar molecules from easily crossing
• Transport proteins help the helpless
– Allow hydrophilic substances to pass through
• So…selective permeability depends both on
the lipid bilayer and on the transport proteins
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What is the fluid mosaic model? Describe how
the following components fit into the fluid
mosaic model:
• Phospholipid saturation
• Cholesterol
• Membrane proteins
• Glycoproteins and proteoglycans
• Cytoskeleton
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Properties of the plasma membrane
• Selective permeability
• Passive transport
– Diffusion
– Osmosis
– Facilitated diffusion
• Active transport
– Exocytosis
– Endocytosis
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Passive vs. active transport across the
membrane
• In some cases molecules will spontaneously
diffuse across the membrane
– Passive transport
• Other molecules require a protein or other
force to bring them across the membrane
– Active transport
Diffusion
• Passive transport
• Molecules tend to diffuse out evenly into
available space due to random movements
• A substance will diffuse where it is more
concentrated to where it is less concentrated
– Diffuse down its diffusion gradient
– “The region along which the density of a chemical
substance increases or decreases”
– Spontaneous, no energy input required
Figure 7.13
Molecules of dye
Membrane (cross section)
WATER
Net diffusion
Net diffusion
Equilibrium
Net diffusion
Net diffusion
Equilibrium
Net diffusion
Net diffusion
Equilibrium
(a) Diffusion of one solute
(b) Diffusion of two solutes
Diffusion of water - Osmosis
• Osmosis is the diffusion of water across a selectively
permeable membrane
• Water diffuses across a membrane from the region
of lower solute concentration to the region of higher
solute concentration until the solute concentration is
equal on both sides
Figure 7.14
Lower
concentration
of solute (sugar)
Higher
concentration
of solute
Sugar
molecule
H2O
Selectively
permeable
membrane
Osmosis
Same concentration
of solute
What happens in cells without walls?
Hypertonic or hypotonic environments create osmotic problems for organisms
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…and cells with walls?
• Cell walls help maintain water balance
• A plant cell in a hypotonic solution swells until it hits
the wall; the cell is now turgid (firm)
• If a plant cell and its surroundings are isotonic, there
is no net movement of water into the cell; the cell
becomes flaccid (limp), and the plant may wilt
• In a hypertonic environment, plant cells lose water;
the membrane pulls away from the wall, a usually
lethal effect called plasmolysis
• (Lysed means the plasma membrane has broken)
If you don’t water your plant…(or yourself)
• Water osmoses out of the plant cells and
evaporates, turning the entire plant limp
Reverse osmosis: turning seawater drinkable
• Drawing salt out of seawater is called
desalination
• Pressure applied to seawater forces it through
a semipermeable membrane
• The membrane allows water through, but not
solutes
Osmosis Vocabulary
• Tonicity is the ability of a surrounding solution to
cause a cell to gain or lose water
• Isotonic solution: Solute concentration is the same as
that inside the cell; no net water movement across
the plasma membrane
• Hypertonic solution: Solute concentration is greater
than that inside the cell; cell loses water
• Hypotonic solution: Solute concentration is less than
that inside the cell; cell gains water
Isotonic
solution
Hypotonic
solution
Hypertonic
solution
(a) Animal cell
H2O
Lysed
(b) Plant cell
H2O
Cell wall
Turgid (normal)
H2O
H2O
H2O
Normal
H2O
Shriveled
H2O
Flaccid
The protist Paramecium, which is hypertonic to its pond water
environment, has a contractile vacuole that acts as a pump
H2O
Plasmolyzed
Facilitated Diffusion
• Transport proteins speed the passive movement of
molecules across the plasma membrane
• Channel proteins provide corridors that allow a specific
molecule or ion to cross the membrane.
• Channel proteins include
– Aquaporins, for facilitated diffusion of water
• 3 billion water molecules/second
– Ion channels that open or close in response to a
stimulus (AKA ion-gated channels)
• Carrier proteins undergo a conformational change that
brings the molecule from one side to the other
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Active transport
• Uses energy to move molecules against their gradients
• All carrier proteins
• Includes ion pumps and endocytosis/exocytosis
The electrochemical gradient and ion pumps
• All cells have voltages across their membranes
– Voltage is electrical potential energy – a separation of
two opposite charges
– Called a membrane potential
– Inside of the cell is -50 to -200millivolts because of a
higher proportion of anions:cations
– “The cytoplasmic side is negative relative to the
outside because of unequal distribution of cations and
anions”
• The membrane potential favors cations passively
diffusing across
– Anions must use active transport
• So two forces actually drive the diffusion of ions
across a membrane: the concentration gradient
and electrical force
– Does not apply to solutes like glucose, only charged
ions
• Combination of the two forces is called the
electrochemical gradient
• Cells create an electrochemical gradient by
pumping ions inside or outside the cell
• The electrochemical gradient can be taken
advantage of to perform chemical reactions
– Convert electrical potential energy to chemical
potential energy
Channel
protein
Cotransport: changing between
different types of potential energy
• A substance that has been pumped across the
membrane can do work as it moves back
across by diffusion
– Water pumped uphill that spins a turbine as it
flows back down
• A cotransport protein can couple the
“downhill” passive diffusion to a second
“uphill” active transport of a different
substance
Final result: expending ATP to bring in sucrose
What a cotransporter actually
looks like (the Na-K-Cl transporter)
Cotransporters in diarrhea
• Bodily fluid high in salt is pumped into the
intestine to help digestion
• Normally sodium in waste is reabsorbed in
the colon
• With diarrhea, waste is expelled so rapidly
sodium cannot be reabsorbed fast enough
and levels fall to dangerous levels
• A salt-glucose solution is administered,
which is taken up by sodium-glucose
cotransporters
• Mortality rate of cholera (causes massive
diarrhea) dropped from 70% to 30% after
we figured out to administer this
hypertonic solution
Type of movement ‘Helping’ agent
across the
membrane
Diffusion
Diffusion gradient
Facilitated Diffusion Diffusion gradient +
transport proteins
Active transport
Transport proteins
Requires energy?
No
Sometimes, in the
form of ATP
Yes, in the form of
ATP
Endocytosis and Exocytosis: other
forms of active transport
Endocytosis: Engulfing
food/foreign material
Exocytosis: Expelling waste
products/unneeded material
Ex: nerve cell releasing neurotransmitters
Exocytosis in nerve cells
Endocytosis and Exocytosis
Pinocytosis: “Cellular drinking”
A type of endocytosis
Phagocytosis: “Cellular eating”
Another type of endocytosis
Pinocytosis in muscle cells
Phagocytosis in white blood cells
Phagocytosis in amoeba
Receptor-mediated endocytosis:
Targeted swallowing of material
Receptors on the cell surface membrane
only bind to certain things, triggering
endocytosis of that specific thing
A summary of the various -cytoses
Type of Cell Action
Description of action
Exocytosis
Endocytosis Pinocytosis
Endocytosis Phagocytosis*
Endocytosis Receptor-mediated
endocytosis
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Receptor-mediated endocytosis and HIV
• Viruses infect our cells by attaching to cellsurface receptors meant for receptormediated endocytosis
– They ‘fool’ the receptor by mimicking the natural
protein that would usually bind
– HIV’s gp120 protein binds to a white blood cell’s
CD4 and CCR5 protein
• Triggers endocytosis of the HIV virus
Gp120 is a
glycoprotein
Structure of gp120
protein blocking the binding
of gp120 to CCR5,
preventing HIV entry
(CCR5)
Vocabulary
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Fluid mosaic model
Selective permeability
Glycolipid, glycoprotein, proteoglycan
Transmembrane proteins
Cholesterol
Active transport, passive transport, facilitated diffusion
Diffusion gradient
Osmosis
Ion channels
Cotransport
Endocytosis, exocytosis
– Phagocytosis
– Receptor-mediated endocytosis