Chapter 7: Membrane
Structure & Function
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Phospholipids
 An amphipathic
Phosphate
“attracted to water”
molecule
 Phosphate head

hydrophilic
 Fatty acid tails

hydrophobic
 Arranged as a bilayer
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Fatty acid
“repelled by water”
Cell membrane defines cell
 Cell membrane separates living cell from
aqueous environment
 Selectively Permeable

allows some substances to cross more
easily than others
 Hydrophilic vs. hydrophobic substances
 Fluid Mosaic of lipids & proteins

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Contains embedded proteins
Membrane is a collage of proteins & other molecules
embedded in the fluid matrix of the lipid bilayer
Glycoprotein
Extracellular fluid
Glycolipid
Phospholipids
Cholesterol
Peripheral
protein
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Cytoplasm
Transmembrane
proteins
Filaments of
cytoskeleton
1972, S.J. Singer & G. Nicolson proposed Fluid Mosaic Model
Freeze Fracture
 Freeze fracture studies supports the
fluid mosaic model of the membrane
structure
 A cell is frozen and fractured with a knife
 Fracture plan follows the hydrophobic
interior of the membrane
 The membrane proteins go wholly with one of the
layers
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Fluid Mosaic Model
 A membrane is a fluid structure with a

“mosaic” of various proteins embedded
in it
Phospholipids & proteins can move

Laterally & horizontally
 making the membrane fluid
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Lateral movement
(~107 times per second)
Flip-flop
(~ once per month)
Membrane Fluidity
 The type of hydrocarbon tails in the
phospholipids affects the fluidity of the
plasma membrane.

Saturated vs. unsaturated
Fluid
Unsaturated hydrocarbon
tails with kinks
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Viscous
Saturated hydroCarbon tails
Membrane Fluidity
 The steroid cholesterol affects
membrane fluidity at different
temperatures in animal cells
Known as the “temperature buffer”
 Hinders solidification

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Cholesterol
Membrane Proteins
 Proteins determine membrane’s specific functions
 Classes of membrane proteins:

peripheral proteins
 loosely bound to surface of membrane
 attachment to cytoskeleton for support
 cell surface identity marker (antigens)

integral proteins
 penetrate lipid bilayer, usually across whole membrane
 transmembrane protein
 transport proteins
 channels, pumps
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Membrane Permeability
 A cell must exchange molecules with
its surroundings

Hydrophobic molecules
 Are lipid soluble and can pass through
rapidly

Hydrophilic molecules
 Do not cross the membrane rapidly
 What substances have difficulty
Impedes movement of polar molecules/ions
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
passing through?

Polar molecules

Water, glucose

Ions (charged)
 salts, ammonia
What substances can NOT get
through directly?

large molecules
 starches, proteins
Integral Proteins Create Channels
 Specific protein channels transport hydrophilic
molecules
sugar
H 2O
salt
polar
hydrophilic
heads
nonpolar
hydrophobic
tails
polar
hydrophilic
heads
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lipids
Proteins domains anchor molecule
 Within membrane

Polar areas
of protein
nonpolar amino acids
 hydrophobic
 anchors protein
into membrane
 On outer surfaces of
membrane in fluid

polar amino acids
 hydrophilic
 extend into
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extracellular fluid &
into cytosol
Nonpolar areas of protein
Many Functions of Membrane Proteins
“Channel”
Outside
Plasma
membrane
Inside
Transporter
Enzyme
activity
Cell surface
receptor
Cell adhesion
Attachment to the
cytoskeleton
“Antigen”
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Cell surface
identity marker
Membrane carbohydrates
 Play a key role in cell-cell recognition

ability of a cell to distinguish one cell
from another
Glycoprotein
 antigens
Glycolipid
important in organ &
tissue development
 blood types
 basis for rejection of
foreign cells by
immune system

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Synthesis and Sidedness of
Membranes
 Membranes have
distinct inside and
outside faces

Membrane proteins
and lipids are made
in the ER
 Proteins have a
directional
orientation in the
membrane
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RULE: What is on the inside of the
vesicle, ends up on the outside of the
plasma membrane
Inside Cell
PM
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Outside Cell
- Membrane fusion proteins will fuse the
membrane of the transport vesicle to the
plasma membrane…
Inside Cell
PM
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Outside Cell
- Notice that the proteins inside the vesicle
are now outside the cell. And the proteins
stuck in the vesicle membrane will now be in
the plasma membrane…
Inside Cell
PM
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Outside Cell
RULE: What is on the inside of the
vesicle, ends up on the outside of the
plasma membrane
Inside Cell
PM
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Outside Cell
Movement Across the
Cell Membrane
cell needs materials in & products or waste out
OUT
IN
food
carbohydrates
sugars, proteins
amino acids
lipids
salts, O2, H2O
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OUT
IN
waste
ammonia
salts
CO2
H2O
products
Passive Transport
 The diffusion of a substance across a
membrane with no energy investment

Diffusion is the tendency of molecules
of any substance to spread out evenly
into the available space
 Down the concentration gradient
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movement from HIGH  LOW concentration
1. Simple Diffusion
 Move from HIGH to LOW concentration
“passive transport”
 No energy needed
 Nonpolar,
hydrophobic
molecules
 Lipids, O2, CO2

movement of O2
 Can easily diffuse
 Water is small enough
and can slowly diffuse
 OSMOSIS
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LOW
HIGH
2. Osmosis
 Osmosis is the diffusion
of water across a
semipermeable
membrane

Tonicity
 Is the ability of a cell to
gain or lose water
 Depends on the amount of
solutes in the solution
Diffusion of water from
HIGH concentration of H2O  LOW concentration of H2O
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Concentration of water
 Direction of osmosis is determined by
comparing total solute concentrations

Hypertonic - more solute, less water

Hypotonic - less solute, more water

Isotonic - equal solute, equal water
water
hypotonic
hypertonic
net movement of water
HIGH
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concentration of H2O  LOW concentration of H2O
1
Managing water balance
 Hypotonic

a cell in fresh water or pure water

high concentration of water around cell
 problem: cell gains water,
Animal cells
swells & can burst
burst
 example: Paramecium
 ex: water continually enters
Paramecium cell
 solution: contractile vacuole
 pumps water out of cell
 Uses ATP

plant cells
Plant cells are
healthiest
 Turgor pressure
 the pressure of water inside a plant cell
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pushing against the cell membrane
freshwater
2
Managing water balance
 Hypertonic
I’m shrinking,
a cell in salt water I’m shrinking!
 low concentration of water
around cell

 problem: cell loses water &
can die
 example: shellfish
 solution: take up water or
pump out salt
Plasma membrane

plant cells
shrinks = wilt
 plasmolysis = wilt
 can recover
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saltwater
3
Managing water balance
 Isotonic
That’s
perfect!

animal cell immersed in
mild salt solution

no difference in concentration of
water between cell & environment
 problem: none
 no net movement of water
flows across membrane equally, in
both directions
I could
 cell in equilibrium
be better…

 volume of cell is stable
 example:
blood cells in blood plasma
 slightly salty IV solution in hospital
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balanced
Do you understand Osmosis…
.05 M
.03 M
Cell (compared to beaker)  hypertonic or hypotonic
Beaker (compared to cell)  hypertonic or hypotonic
Which way does the water flow?  in or out of cell
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How Will Water Move Across
Semi-Permeable Membrane?
Solution A has 100 molecules of
glucose per ml
Solution B has 100 molecules of
fructose per ml
How will the water molecules move?
There will be no net movement of water since the
concentration of solute in each solution is equal
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How Will Water Move Across SemiPermeable Membrane?
Solution A has 100 molecules of
glucose per ml
Solution B has 75 molecules of
fructose per ml
How will the water molecules move?
There will be a net movement of water from
Solution B to Solution A until both solutions have
equal concentrations of solute
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How Will Water Move Across SemiPermeable Membrane?
Solution A has 100 molecules of
glucose per ml
Solution B has 100 molecules of
NaCl per ml
How will the water molecules
move?
Each molecule of NaCl will dissociate to form a Na+ ion and a
Cl- ion, making the final concentration of solutes 200 molecules per
mil. Therefore, there will be a net movement of water from
Solution A to Solution B until both solutions have equal
concentrations of solute
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3. Facilitated Diffusion
 Diffusion through protein channels




channels move specific polar molecules or ions
across cell membrane
facilitated = with help
A type of passive
open channel = fast transport
transport
HIGH
No energy needed
High to low
concentration
 Channel proteins
 Water or specific solute
 Carrier proteins
LOW
HIGH
 Undergoes a subtle shape change
 Ex: Glucose carrier protein
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LOW
1991 | 2003
Aquaporins
 Water moves rapidly into & out of cells

evidence that there were water channels
 protein channels allowing flow of water
across cell membrane
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Peter Agre
Roderick MacKinnon
John Hopkins
Rockefeller
Active Transport
 Cells may need to move molecules against
concentration gradient



conformational shape change transports solute
from one side of membrane to other
protein “pump”
conformational change
“costs” energy = ATP
LOW
 Phosphorylation

Examples
 Sodium Potassium Pump
 Proton Pump
ATP
HIGH
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Ion Pumps
 Ion pumps maintain membrane
potential

In membranes of the chloroplast,
mitochondria, lysosome and nerve cells
 Membrane potential
 Voltage difference across a membrane
 Electrochemical gradient
 Caused by the ion concentration difference
across a membrane
 Electrogenic pump
 Transport protein that generates the voltage
across a membrane
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Sodium Potassium Pump
 Found in nerve cells
1
Na+
Cytoplasmic
binds to
the sodium-potassium pump.
2
[Na+] high
[K+] low
Na+
Na+
Na+ binding stimulates
phosphorylation by ATP.
Na+
Na+
Na+
[Na+] low
[K+] high
Na+
CYTOPLASM
ATP
P
ADP
Na+
Na+
Na+
K+ is released and Na+
3
sites are receptive again;
the cycle repeats.
4
K+
P
K+
Phosphorylation causes the
protein to change its
conformation, expelling Na+ to
the outside.
K+
K+
5
Loss of the phosphate
restores the protein’s
original conformation.
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K+
K+
6
Extracellular K+ binds to the
protein, triggering release of the
Phosphate group.
–
Proton Pumps
 Creates
electrochemical
gradient across a
membrane
 Cotransport

Active transport of one
solute indirectly drives
the active transport
of another solute
 Ex: Sucrose
cotransporter
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ATP
+EXTRACELLULAR
FLUID
+
H+
–
H+
Proton pump
H+
–
–
CYTOPLASM
–
+
H+
H+
+
+
+
H+
How about large molecules?
 Moving large molecules into & out of cell
through vesicles & vacuoles
 Exocytosis

exocytosis
 Transport vesicles migrate to
the plasma membrane, fuse
with it, release their contents

Endocytosis
 The cell takes in
macromolecules by forming a vesicle or
vacuole from the plasma membrane
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Endocytosis
“cell eating”
Vacuole
fuses with
lysosome for
digestion
Pinocytosis
non-specific
process, fluid
“gulped” into
a vesicle
Phagocytosis
“cell drinking”
receptor-mediated
endocytosis
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“ligand-receptor”
triggered by
specific
molecular
signal
Summary
Passive transport. Substances diffuse spontaneously
down their concentration gradients, crossing a
membrane with no expenditure of energy by the cell.
The rate of diffusion can be greatly increased by transport
proteins in the membrane.
Active transport. Some transport proteins
act as pumps, moving substances across a
membrane against their concentration
gradients. Energy for this work is usually
supplied by ATP.
ATP
Diffusion. Hydrophobic
Facilitated diffusion. Many hydrophilic
molecules and (at a slow
substances diffuse through membranes with the
rate) very small uncharged
assistance of transport proteins,
polar molecules can diffuse through the lipid
either channel or carrier proteins.
bilayer.
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