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Membrane Structure
Membranes
Chapter 5
—  Phospholipids
arranged in a bilayer
—  Globular proteins inserted in the lipid bilayer
—  Fluid mosiac model – mosaic of proteins floats
in or on the fluid lipid bilayer like boats on a
pond
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Membrane Components
—  Cellular
— 
Transmission electron microscope (TEM) of the cell membrane
membranes have 4 components
1.  Phospholipid bilayer
– 
Flexible matrix, barrier to permeability
2.  Transmembrane proteins
– 
Integral membrane proteins
3.  Interior protein network
– 
Peripheral membrane proteins
4.  Cell surface markers
– 
Glycoproteins and glycolipids
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Phospholipids
—  Structure
consists of
—  Bilayers
are fluid
bonding of
water holds the 2
layers together
—  Individual
phospholipids and
unanchored proteins
can move through
the membrane
◦  Glycerol – a 3-carbon polyalcohol
◦  2 fatty acids attached to the glycerol
◦  Phosphate group attached to the glycerol
—  Spontaneously
—  Hydrogen
forms a bilayer
◦  Fatty acids are on the inside
◦  Phosphate groups are on both surfaces
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Structure relates to function
Membrane Proteins
—  Various
1. 
2. 
3. 
4. 
5. 
6. 
—  Diverse
functions arise from the diverse
structures of membrane proteins
—  Have common structural features related
to their role as membrane proteins
—  Peripheral proteins
functions:
Transporters
Enzymes
Cell-surface receptors
Cell-surface identity markers
Cell-to-cell adhesion proteins
Attachments to the cytoskeleton
◦  Anchoring molecules attach membrane
protein to surface
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Integral membrane proteins
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Membrane Proteins - Pores
—  Extensive
nonpolar regions within a transmembrane
protein can create a pore through the membrane
—  Span
the lipid bilayer (transmembrane
proteins)
◦  Cylinder of β sheets in the protein secondary structure
called a β-barrel
–  Interior is polar and allows water and small polar
molecules to pass through the membrane
◦  Nonpolar regions of the protein are embedded in
the interior of the bilayer
◦  Polar regions of the protein protrude from both
sides of the bilayer
◦  Transmembrane domain
–  Spans the lipid bilayer
–  Hydrophobic amino acids
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Diffusion Rates
Passive Transport
— 
Passive transport is movement of molecules through
the membrane in which
◦  No energy is required
◦  Molecules move in response to a concentration gradient
— 
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Diffusion is movement of molecules from high
concentration to low concentration down their
concentration gradient
—  Factors
affecting diffusion rate through a
membrane
◦  temperature - ↑ temp., ↑ motion of particles
◦  molecular weight - larger molecules move slower
◦  steepness of concentrated gradient - ↑difference, ↑ rate
◦  membrane surface area - ↑ area, ↑ rate
◦  membrane permeability - ↑ permeability, ↑ rate
◦  Will continue until the concentration is the same in all
regions . Animation
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The Nature of the Membrane and
Molecule Transport
Transport of Polar Molecules
—  Facilitated
—  Major
barrier to crossing a biological
membrane is the hydrophobic interior that
repels polar molecules but not nonpolar
molecules
◦  Nonpolar molecules will move until the
concentration is equal on both sides
◦  Limited permeability to small polar molecules
◦  Very limited permeability to larger polar molecules
and ions
diffusion
◦  Molecules that cannot cross membrane easily may
move through proteins
◦  Move from higher to lower concentration- down
their concentration gradient animation
◦  Channel proteins
–  Hydrophilic channel when open
◦  Carrier proteins
–  Bind specifically to molecules they assist
—  Membrane
is selectively permeable
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Channel proteins
—  Ion
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Carrier proteins
channels
—  Can
help transport both ions and other
solutes, such as some sugars and amino
acids
—  Requires a concentration difference
across the membrane
—  Must bind to the molecule they transport
◦  Allow the passage of ions
◦  Gated channels – open or close in response
to stimulus (chemical or electrical)
◦  3 conditions determine direction
–  Relative concentration on either side of membrane
–  Voltage differences across membrane
–  Gated channels – channel open or closed
◦  Saturation – rate of transport limited by
number of transporters
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Osmosis
—  Cytoplasm
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Osmotic concentration
of the cell is an aqueous
—  When
2 solutions have different osmotic
concentrations
solution
◦  Water is solvent
◦  Dissolved substances are solutes
◦  Hypertonic solution has a higher solute
concentration
◦  Hypotonic solution has a lower solute concentration
—  Osmosis
– net diffusion of water across a
membrane toward a higher solute
concentration
—  When
two solutions have the same osmotic
concentration, the solutions are isotonic
—  Aquaporins facilitate osmosis
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Osmotic pressure
—  Force
needed to stop osmotic flow
in a hypotonic solution gains water causing
cell to swell – creates pressure
—  If membrane strong enough, cell reaches
counterbalance of osmotic pressure driving
water in with hydrostatic pressure driving water
out
—  Cell
◦  Cell wall of prokaryotes, fungi, plants, protists
—  If
membrane is not strong, may burst
◦  Animal cells must be in isotonic environments
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Maintaining osmotic balance
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Active Transport
—  Requires
—  Some
cells use extrusion in which water is
ejected through contractile vacuoles
—  Isosmotic regulation involves keeping cells
isotonic with their environment
◦  Marine organisms adjust internal concentration to
match sea water
◦  Terrestrial animals circulate isotonic fluid
energy – ATP is used directly or
indirectly to fuel active transport
—  Moves substances from low to high
concentration-up their concentration gradient.
—  Requires the use of highly selective carrier
proteins
—  Plant
cells use turgor pressure to push the cell
membrane against the cell wall and keep the
cell rigid
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Sodium–potassium (Na+–K+) pump
Carrier Protein and Active Transport
—  Carrier
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—  Direct
use of ATP for active transport
an antiporter to move 3 Na+ out of the
cell and 2 K+ into the cell
proteins used in active transport
—  Uses
include
◦  Uniporters – move one molecule at a time
◦  Symporters – move two molecules in the same
direction
◦  Antiporters – move two molecules in opposite
directions
◦  Terms can also be used to describe facilitated
diffusion carriers
◦  Against their concentration gradient
—  ATP
energy is used to change the conformation
of the carrier protein
—  Affinity of the carrier protein for either Na+ or
K+ changes so the ions can be carried across the
membrane
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Coupled transport- Secondary Active
Transport
—  Uses ATP
indirectly
the energy released when a molecule moves
by diffusion to supply energy to active transport
of a different molecule
—  Symporter is used
—  Glucose–Na+ symporter captures the energy
from Na+ diffusion to move glucose against a
concentration gradient
—  Uses
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Bulk Transport
Endocytosis
— 
◦ 
◦ 
◦ 
◦ 
Exocytosis
— 
◦ 
— 
Animation
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Movement of substances out of cell
Requires energy
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— 
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Movement of substances into the cell
Phagosytosis – cell takes in particulate matter
Pinocytosis – cell takes in only fluid
Receptor-mediated endocytosis – specific molecules are
taken in after they bind to a receptor
In the human genetic disease familial hypercholesterolemia, the LDL
receptors lack tails, so they are never fastened in the clathrin-coated pits
and as a result, do not trigger vesicle formation. The cholesterol stays in
the bloodstream of affected individuals, accumulating as plaques inside
arteries and leading to heart attacks.
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Exocytosis
— 
◦ 
◦ 
◦ 
Movement of materials out of the cell
Used in plants to export cell wall material
Used in animals to secrete hormones, neurotransmitters,
digestive enzymes
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