Lecture 2 Exam

Membrane Structure and
Function

2
Membrane Structure
• Phospholipids arranged in a bilayer
• Globular proteins inserted in the lipid
bilayer
• Fluid mosaic model – mosaic of proteins
floats in or on the fluid lipid bilayer like
boats on a pond

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The Structure of Membranes

Polar
hydro-
philic
heads
Nonpolar
hydro-
phobic
tails
Polar
hydro-
philic
heads
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Phospholipid Bilayer

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The Structure of Membranes
• Biologists use the fluid mosaic model to
describe a membrane’s structure – a
patchwork of diverse protein molecules
embedded in a phospholipid bilayer
• Plasma membrane exhibits selective
permeability
• Globular proteins embedded in a
membrane’s phospholipid bilayer perform
various functions

Extracellular Fluid
Carbohydrate Glycolipid
Transmembrane
proteins
Glycoprotein
Peripheral
protein
Cholesterol
Filaments of
cytoskeleton
Cytoplasm
Extracellular
matrix protein
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Fluid-Mosaic Model

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The Structure of Membranes
Stabilized to ECM and Cytoplasm
ECM
Cytoplasm

8
Cellular membranes have 4 components
1. Phospholipid bilayer
• Flexible matrix, barrier to permeability
2. Transmembrane proteins
• Integral membrane proteins
3. Interior protein network
• Peripheral or Intracellular membrane proteins
4. Cell surface markers
• Glycoproteins and glycolipids
The Structure of Membranes

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5.1 The Structure of Membranes
Cytoplasm
Cytoplasm
ECM

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The Structure of Membranes
• Both transmission electron microscopy (TEM)
and scanning electron microscopy (SEM) used
to study membrane structure
• One method of preparing a specimen, tissue is
embedded in hard epoxy matrix
• Cut with microtome (1μm shavings)
• Shavings placed on grid
• Beam of electrons applied with TEM
• Resolution good enough to see phospholipid
bilayer
• False color added to enhance details

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The Structure of Membranes
• Computer enhanced TEM image of membrane
structure:

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The Structure of Membranes
• Freeze-fracturing a specimen is another method
to visualize inside of membrane
• Tissue embedded in medium
• Quick frozen with liquid nitrogen
• Frozen tissue is “tapped” with knife, causing
crack between phospholipid bilayers
• Proteins, carbs, pits, pores, channels, or any
other structure affiliated with membrane will
pull apart and stick with one or the other side
of the split membrane

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The Structure of Membranes
• Freeze-fracturing a specimen is another method
to visualize inside of membrane
• Tissue embedded in medium
• Quick frozen with liquid nitrogen
• Frozen tissue is “tapped” with knife, causing
crack between phospholipid bilayers
• Proteins, carbs, pits, pores, channels, or any
other structure affiliated with membrane will
pull apart and stick with one or the other side
of the split membrane

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The Structure of Membranes
• Freeze-fracture technique:

14
RECALL: Phospholipids
• Amphipathic phospholipid 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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Phospholipids: Membrane’s Foundation
• Phospholipid bilayer is fluid
• Stable because of water’s affinity for hydrogen
bonding never stops
• Hydrogen bonding of water holds 2 layers together
• Water drives phospholipids into bilayer configuration
• Water does NOT have any effect on the mobility of
phospholipids an their nonlipid neighbors in the
bilayer
• Because phospholipids interact relatively weakly with
one another, individual phospholipids and
unanchored proteins are comparatively free to move
about within the membrane

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Phospholipids: Membrane’s Foundation
• Fluidity of membrane can be demonstrated by
fusing cells and watching their proteins intermix
over time

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Phospholipids: Membrane’s Foundation
• Fluidity of membrane varies with lipid composition and
changes in environment
• Saturated fatty acids make the membrane less fluid
than unsaturated fatty acids
• Warm temperatures make the membrane more fluid
than cold temperatures
• Cold tolerance in bacteria due to fatty acid
desaturases (can introduce double bonds into
fatty acids)
• Lipid composition of the ER membrane, Golgi
apparatus, and plasma membrane are distinct
• Composition affects fluidity, thickness, shape of
membrane

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Proteins: Multifunctional Components
• Cell membranes contain complex assembly of
proteins embedded within them
• Flexible organization permits broad range of
interactions with environment
• Transporters
• Enzymes
• Cell-surface receptors
• Cell-surface identity markers
• Cell-to-cell adhesion proteins
• Attachments to the cytoskeleton

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Proteins: Multifunctional Components

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Proteins: Multifunctional Components
• Structure conveys 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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Proteins: Multifunctional Components
• Anchoring molecules are modified lipids with:
• Nonpolar regions that insert into the internal
portion of the lipid bilayer
• Chemical bonding domains that link directly to
proteins

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Proteins: Multifunctional Components
Integral Membrane Proteins
• Span lipid bilayer (transmembrane proteins)
• Nonpolar regions of protein embedded the
interior of the bilayer
• Polar regions of the proteins protrude from both
sides of the bilayer
• Transmembrane domain
• Spans the lipid bilayer
• Hydrophobic amino acids in α helices
• Proteins need only a single transmembrane
domain to be anchored in the membrane, but
often have more than one such domain

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Proteins: Multifunctional Components
• Transmembrane domains
Cytoplasm

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Proteins: Multifunctional Components
Pores
• Extensive nonpolar regions within a transmembrane protein
can create a pore through the membrane
• Cylinder of β-pleated sheets in the protein secondary
structure called a β-barrel
• Open on both ends
• Interior is polar, allows
water and small polar
molecules to pass
through
Cytoplasm

25
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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Passive Transport Across Membranes
Diffusion is the tendency of particles to spread out
evenly in an available space
• Particles move from an area of more concentrated
particles to an area where they are less
concentrated
• This means that particles diffuse down their
concentration gradient
• Eventually, the particles reach dynamic equilibrium
where there is no net change in concentration on
either side of the membrane

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Molecules of dye
Pores
Membrane
Net diffusion Net diffusion Equilibrium
Passive Transport Across Membranes

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Net diffusion Net diffusion Equilibrium
Net diffusion Net diffusion Equilibrium
Passive Transport Across Membranes

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RECALL: Membrane Properties
• Major barrier to crossing a biological membrane
is the hydrophobic interior that repels polar
molecules, but not nonpolar molecules
• Nonpolar molecules will readily move across
membrane until 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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Passive Transport – Simple Diffusion
• 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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Passive Transport – Facilitated Diffusion
Molecules that cannot cross membrane easily may
move through proteins
• Channel proteins – hydrophilic channel when
open
• Carrier proteins – bind specifically to molecules
they assist
• Does not require energy
• Relies on concentration gradient, move from region
of higher concentration to lower concentration
• Membrane is selectively permeable... channels and
carriers usually selective for one type of molecule

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Passive Transport – Channel Proteins
Ion channels
• Allow the passage of ions through channel
proteins
• Gated channels – open or close in response to
stimuli (chemical or electrical)
• Direction determined by conditions
• Relative concentration on either side of
membrane
• Voltage difference across membrane
• State of the gate (channel open or closed)

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Passive Transport – Carrier Proteins
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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Channel protein Carrier protein
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reproduction or distribution without the prior written consent of
Passive Transport Mechanisms

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Channel protein Carrier protein
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reproduction or distribution without the prior written consent of
Passive Transport Mechanisms

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Passive Transport - Osmosis
• One of the most important substances that crosses
membranes by passive transport is water
• Cytoplasm of the cell is an aqueous solution
• Water is solvent
• Dissolved substances are solutes
• The net diffusion of water
across a selectively
permeable membrane is
called osmosis

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Passive Transport - Osmosis
• One of the most important substances that crosses
membranes by passive transport is water
• Cytoplasm of the cell is an aqueous solution
• Water is solvent
• Dissolved substances are solutes
• The net diffusion of water
across a selectively
permeable membrane is
called osmosis

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Passive Transport - Osmosis
If a membrane, permeable to water but not to a
solute, separates two solutions with different
concentrations of solute, water will cross the
membrane, moving down its own concentration
gradient, until the solute concentration on both
sides is equal...

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Passive Transport - Osmosis
If a membrane, permeable to water but not to a
solute, separates two solutions with different
concentrations of solute, water will cross the
membrane, moving down its own concentration
gradient, until the solute concentration on both
sides is equal...

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Passive Transport - Osmosis
Solute
molecule
Selectively
permeable
membrane
Water
molecule
Solute molecule
with cluster of
water molecules
Osmosis
H2O
Lower
concentration
of solute
Higher
concentration
of solute
More equal
concentrations
of solute

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Passive Transport - Osmosis
• Osmotic concentration
• Tonicity describes the ability of a surrounding
solution to cause a cell to gain or lose water
• Tonicity of a solution depends on its concentration of
solutes relative to concentration of solutes inside cell
• Hypertonic solution - higher solute concentration
• Isotonic solution - same osmotic concentration
• Hypotonic solution - lower solute concentration
• Aquaporins (water channels) facilitate osmosis

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Passive Transport - Osmosis
• Osmotic concentration
• Tonicity describes the ability of a surrounding
solution to cause a cell to gain or lose water
• Tonicity of a solution depends on its concentration of
solutes relative to concentration of solutes inside cell
• Hypertonic solution - higher solute concentration
• Isotonic solution - same osmotic concentration
• Hypotonic solution - lower solute concentration
• Aquaporins (water channels) facilitate osmosis

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Passive Transport - Osmosis
• 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 and 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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Passive Movement - Osmosis
How will animal cells be affected when placed into
solutions of various tonicities?
• In an isotonic solution, concentration of solute is
the same on both sides of a membrane, and the cell
volume will not change
• In a hypotonic solution, solute concentration is
lower outside the cell (compared to inside), water
molecules move into the cell, cell will expand and
may burst
• In a hypertonic solution, solute concentration is
higher outside the cell (compared to inside), water
molecules move out of the cell, cell will shrink

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5.3 Proteins: Multifunctional Components
Animal
cell
Plant
cell
Hypotonic solution
(lower solute levels)
Isotonic solution
(equal solute levels)
Hypertonic solution
(higher solute levels)
H2O
Lysed Normal Shriveled
Turgid (normal) Flaccid
Plasma
membrane
Shriveled (plasmolyzed)
H2O H2O H2O
H2O
H2OH2O

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Passive Movement - Osmosis
• 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
cell rigid

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Active Transport Across Membranes
• Requires energy – ATP is used directly or indirectly
• Moves substances against its concentration gradient,
from an area of low concentration to an area of high
concentration
• Requires the use of highly selective carrier proteins
• One of most important functions of any cell
• Enables cell to take up additional molecules of a
substance that is already present in its cytoplasm in
concentrations higher than in the extracellular fluid
• Enables cell to move substances out of its cytoplasm
and into extracellular fluid, despite higher external
concentrations

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Active Transport Across Membranes
Carrier Proteins
Carrier proteins used in active transport
• 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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Active Transport Across Membranes
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 Na+ or K+ changes
so the ions can be carried across the membrane

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Active Transport Across Membranes

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Transport
protein
Solute
ATP
Solute binds
to transport
protein.
ATP provides
energy for
change in
protein shape.
Protein returns
to original shape
and more solute
can bind.
1 2 3

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Active Transport Across Membranes
Coupled Transport
Coupled transport
• Uses ATP indirectly
• Uses the energy released
when a molecules moves
by diffusion to supply
energy to active transport
to 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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Active Transport - Bulk Movement
Material to be transported packaged within a vesicle
that fuses with the membrane
• Endocytosis
• Movement of substance into the cell
• Phagocytosis – cell takes in particulate matter
• Pinocytosis – cell takes in only fluid
• Receptor-mediated endocytosis – specific
molecules are taken in after bind to a receptor
• Exocytosis
• Movement of substance out of cell
• Requires energy

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Bulk Transport by Endocytosis

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Bulk Transport by Endocytosis
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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Bulk Transport by Exocytosis
Exocytosis
• Discharge 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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reproduction or distribution without the prior written consent of
Enzyme
Enzyme
O2 CO2
Diffusion of small
nonpolar molecules
Attachment
protein
Receptor
protein
Channel
protein
Active
transport
protein ATP
Junction
protein
Glyco-
protein
Junction
protein

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Membrane Transport Summary
Is the substance
nonpolar?
Yes
No
Is the substance
moving down its
concentration
gradient?
Yes
No
Is the substance
very large?
No
Yes
Is the substance
entering or
leaving the cell?
Exocytosis
Facilitated
diffusion
Simple diffusion