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CHAPTER 5
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Membranes
Chapter 5
Membrane Structure
• 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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4
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• Cellular 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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• Both transmission electron microscope (TEM) and
scanning (SEM) used to study membranes
• One method to embed specimen in resin
– 1µm shavings
– TEM shows layers
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• Freeze-fracture visualizes inside of
membrane
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Phospholipids
• Structure consists of
– Glycerol – a 3-carbon polyalcohol
– 2 fatty acids attached to the glycerol
• Nonpolar and hydrophobic (“water-fearing”)
– Phosphate group attached to the glycerol
• Polar and hydrophilic (“water-loving”)
• Spontaneously forms a bilayer
– Fatty acids are on the inside
– Phosphate groups are on both surfaces
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• Bilayers are fluid
• Hydrogen bonding
of water holds the
2 layers together
• Individual
phospholipids and
unanchored
proteins can move
through the
membrane
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• Environmental influences
– Saturated fatty acids make the membrane
less fluid than unsaturated fatty acids
• “Kinks” introduced by the double bonds keep them
from packing tightly
• Most membranes also contain sterols such as
cholesterol, which can either increase or decrease
membrane fluidity, depending on the temperature
– Warm temperatures make the membrane
more fluid than cold temperatures
• Cold tolerance in bacteria due to fatty acid
desaturases
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Membrane Proteins
• Various functions:
1.
2.
3.
4.
5.
6.
Transporters
Enzymes
Cell-surface receptors
Cell-surface identity markers
Cell-to-cell adhesion proteins
Attachments to the cytoskeleton
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Structure relates to function
• Diverse functions arise from the diverse
structures of membrane proteins
• Have common structural features related
to their role as membrane proteins
• Peripheral proteins
– Anchoring molecules attach membrane
protein to surface
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• Anchoring molecules are modified lipids with
1. Nonpolar regions that insert into the internal portion
of the lipid bilayer
2. Chemical bonding domains that link directly to
proteins
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• Integral membrane proteins
– Span the lipid bilayer (transmembrane
proteins)
• 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 arranged in α helices
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• Proteins need only a single transmembrane
domain to be anchored in the membrane, but
they often have more than one such domain
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• Bacteriorhodopsin has 7 transmembrane
domains forming a structure within the
membrane through which protons pass
during the light-driven pumping of protons
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Membrane Proteins
• Pores
– Extensive nonpolar regions within a
transmembrane protein can create a pore
through the membrane
– Cylinder of  sheets in the protein secondary
structure called a -barrel
• Interior is polar and allows water and small polar
molecules to pass through the membrane
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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
• Diffusion is movement of molecules from
high concentration to low concentration
– Will continue until the concentration is the
same in all regions
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• 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
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• Facilitated diffusion
– Molecules that cannot cross membrane easily
may move through proteins
– Move from higher to lower concentration
– 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 channels
– 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
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Carrier proteins
• 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
– Saturation – rate of transport limited by
number of transporters
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Osmosis
• Cytoplasm of the cell is an aqueous
solution
– Water is solvent
– Dissolved substances are solutes
• Osmosis – net diffusion of water across a
membrane toward a higher solute
concentration
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Osmotic concentration
• When 2 solutions have different osmotic
concentrations
– Hypertonic solution has a higher solute concentration
– Hypotonic solution has a lower 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
• Cell 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 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
• 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
• Plant cells use turgor pressure to push the
cell membrane against the cell wall and
keep the cell rigid
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Active Transport
• Requires energy – ATP is used directly or
indirectly to fuel active transport
• Moves substances from low to high
concentration
• Requires the use of highly selective carrier
proteins
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• Carrier proteins used in active transport
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
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Sodium–potassium (Na+–K+) pump
• Direct use of ATP for active transport
• Uses an antiporter to move 3 Na+ out of
the cell and 2 K+ into the cell
– 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
• Uses ATP indirectly
• Uses 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
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Bulk Transport
•
Endocytosis
–
–
–
–
•
Exocytosis
–
•
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
Movement of substances out of cell
Requires energy
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• 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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