Chapter 11
Membrane Structures
Plasma Membrane
• The ‘container’ for the cell
– Holds the cytoplasm and organelles
together
• Barrier for the cell
– Bacteria have a single membrane
– Eukaryotes have outer plasma
membrane and internal membranes
• Endoplasmic reticulum
• Nuclear membrane
• Membrane-bound organelles
Cell Membranes
Cell Membrane Functions
• Receives signals from outside the cell
for internal cellular activities
• Imports and exports molecules
• Movement of the cell
General Structure
• A lipid bilayer that contains 2 sheets of
lipids interdispersed with proteins
Lipid Structure
• Hydrophilic head –
H2O loving – due to
polar group in the
head
• Hydrophobic tail –
H2O hating – due to
the long hydrocarbon
tails
Review of Lipids
Lipids
• Most abundant lipid
is the phospholipid
• Phospholipids have a
PO4 group in the 3rd
–OH group of the
glycerol instead of
hydrocarbon
• This can attach a
hydrophilic group
– Choline –
phosphyltidylcholine
– Polar amino acids like
serine phosphatidylserine
Amphipathic Molecules
• Contain both a hydrophilic and a
hydrophobic portion to the molecule
• Form a bilayer because of this
• Other molecules are amphipathic
– Steroids
– Glycolipids – lipid with a sugar attached
rather than a phosphate group
Other Lipid Molecules
Reminder
• Hydrophilic molecules can dissolve in H2O
due to the polarity of both of these
molecules
– H bonds and other non-covalent interactions
may aid in this
Reminder
• Hydrophobic molecules will be “caged” by the
polar molecules – requires energy
• Why when fats or oils are placed in water that
they usually sit as a glob on the surface
Membrane
• Amphipathic molecules have both components
so the hydrophilic head molecules interact with
the aqueous solution and the hydrophobic tails
will interact with each other
Lipid Bilayer
• Due to amphipathic property the
membrane can reseal after an ‘injury’
• Bilayer is fluid – the orientation of
the lipids and the outer aqueous
surroundings keeps the lipids in the
bilayer
– The lipid can move around the layer – like
one person moving in a crowded room
• Not the same as flexible – entire
membrane bending
Liposomes
• Can study membranes by using artificial
membrane structures called liposomes
• Can follow the movement of lipids in each
of the layers
What We Know
• Lipids cannot move
from one layer to
another without
the aid of proteins
• Lipids can exchange
places with
neighbors
• Lipids can rotate
around their axis
Importance of Hydrocarbon
• Hydrocarbon tail will determine the
fluidity of the membrane just as it
does in fats and oils
• 2 components are important
– Length of hydrocarbon chain
• 14 to 24 C but usually 18 to 20 C per tail
– Level of unsaturation (# of C=C bonds)
• 1 tail has 1 or more C=C bonds (unsaturated)
• Other tail is saturated (no C=C bonds)
Unsaturated Hydrocarbons
• Each C=C bond
causes a kink or
bend in the tail
– Can’t pack tightly in
the layer
– More lipids that
have unsaturated
tails the more fluid
the membrane
Membrane Fluidity
• Enables the membrane proteins to
diffuse rapidly
• Simple means of distributing lipids
and proteins
• Allows membranes to fuse with one
another
• Evenly distributed during daughter
cell formation
Cholesterol in the Membrane
• Cholesterol is added to
areas that have lots of
unsaturated lipids to
help fill in the gaps
between the tails
• Helps to stiffen and
stabilize the bilayer
– Less fluid
– Less permeable
Membranes are Asymmetrical
• Inner surface is different from the
outer surface
– Types of lipids in each layer
• Proteins in the bilayer have a specific
orientation due to its function
New Membrane
• New lipids are added on one side of the
membrane
• Enzyme called flippase used to put the lipid
in the other half of the bilayer
– Flippase may be selective for the type of lipids
that it puts on either surface
Asymmetry
• New membrane comes
from the SER
• Vesicle buds off the SER
and when fuses with the
plasma membrane, the
orientation is maintained
• Membranes have distinct
inner and outer surface
– Inner – cytosolic face
• Adjacent to the cytosol
– Outer – non-cytosolic face
• Adjacent to the cell exterior
or the interior of an
organelle
Special Lipids
• Glycolipids are found only on the noncytosolic surface
– Sugar added in the Golgi apparatus
– No flippase to move the glycolipid to the
cytosolic surface
• Inositol phospholipids are only on the
cytosolic surface
– Functions to relay signals on cytosolic surface
that pass through the membrane
Membranes as Barriers
• Because of the
hydrophobic interior
of the bilayer
• Membrane is
impermeable to ions
and large charged
molecules and require
special membrane
proteins to transport
across
Membrane Proteins
• Carry out the functions of the membrane (Table 11-1)
– Transporters – Na+ pump to move Na+ across
– Linkers – integrins to link intercellular components to extracellular
ones
– Receptors – to bind a compound that sends a signal to the rest of
the cell
– Enzymes – perform chemical reactions in the membrane
Association with Membrane
• Transmembrane – span the entire membrane
• Linked by lipids – on either surface of the
membrane
• Interaction with transmembrane proteins
Transmembrane Proteins
• Protein has hydrophilic and hydrophobic
portions
– Hydrophilic will interact with the aqueous
solutions on either surface
– Hydrophobic will be in contact with the
hydrophobic interior of the bilayer
• Also called integral membrane proteins
Peripheral Membrane Proteins
• Proteins that are attached to either
surface of the bilayer
• Those attached to lipids are covalently
linked
• Those that interact with other
transmembrane proteins are attached by
noncovalent interactions
– H bonds, hydrophobic and hydrophilic
interactions
Membrane Spanning Proteins
• Must have
hydrophobic side
chains in the area
that spans the
membrane
• Peptide backbone is
polar
– Not real happy in
the hydrophobic
interior
 Helix Span Interior
• Interior forces the
peptide backbone
to form  helix
• Non-polar R groups
are on the outside
of the helix
• Transmembrane
usually span the
membrane once
– Receptors – collect
signal, pass on the
the inside of cell
Membrane Pores
• When protein spans the
membrane several times
usually form pores that
allow molecules to move
back and forth through
the membrane
• Multiple  helix span
membrane
– Hydrophilic on the inside
of the channel
– Hydrophobic on the outer
surface of the channel
 Barrel
•  barrels are made
of  sheets that
are curved into a
cylinder
• Again the
hydrophilic line the
inner side and
hydrophobic the
outer surface
• Larger pore than 
helix pore
Detergents
• Used to remove the
proteins from the
membrane
• Amphipathic molecules
• Have a single hydrocarbon
tail
• Form small clusters in
aqueous solutions called
micelles
• SDS and Triton X-100
common in the laboratory
Removal of Proteins
Bacteriorhodopsin – Pumps Out H+
Photosynthetic Reaction Center
Cell Cortex
• Membrane is very fragile and support comes from a
meshwork on the cytosolic surface
• Spectrin is an important protein in the cell cortex –
links with transmembrane proteins by an attachment
protein
Carbohydrates on Cell Surface
• Many of the plasma membrane proteins have
sugars attached to them
– Short oligosaccharides – glycoproteins
– Long polysaccharides - proteoglycans
• Sugars on the surface make up the
glycocalyx
– Keeps cells moist and slippery
– Used as cell recognition (lectins) and adhesion
molecules
Glycocalyx – Cell Coat
Role of Glycocalyx
Protein Movement
• Proteins can move through the layer of the
membrane similar to the lipids
• Can’t flip from one side to the other
Membrane Domains
• Cells can restrict the
movement of proteins
by
– Cell cortex attachment
– Extracellular
attachment
– Attachment to other
cells
– By diffusion barriers
• Tight junction –
continuous barrier
between adjacent cells
Restriction by Location
• Apical side – facing opening
• Basal side – bottom of the cell
• Lateral sides – side surfaces