BIOL 266 – CELL BIOLOGY
Lecture 5:
Membrane structure
The plasma membrane serves as a barrier that separates
the interior of the cell from the surrounding medium
The plasma membrane prevents the contents of the cell
from escaping and mixing with the surrounding medium
Plasma
membrane
Interior
of the cell
Surrounding
medium
Some functions of the plasma membrane
(1) Import and export of molecules:
Nutrients pass inward across the plasma membrane
Waste products pass outward across the plasma
membrane
Plasma
membrane
Nucleus
Interior
of the cell
Some functions of the plasma membrane
(2) Receiving information:
Some proteins in the plasma membrane act as
sensors to enable the cell to respond to changes in its
environment
Changes in the
surrounding medium
Plasma
membrane
Interior
of the cell
Nucleus
Some functions of the plasma membrane
(3) Capacity for movement and expansion:
When the cell grows or changes shape, the plasma
membrane enlarges its area by addition of new
membrane and it can deform without tearing
Interior
of the cell
Plasma
membrane
Nucleus
Internal membranes enclose intracellular
compartments
The membranes that surround the organelles of
eukaryotic cells separate one aqueous phase – the cell
cytosol – from another – the interior of the organelle
Internal
membrane
Plasma
membrane
Interior
of the cell
Surrounding
medium
Internal membranes serve as selective
barriers between the cell cytosol and
the interior of individual organelles
Internal
membrane
Plasma
membrane
Interior
of the cell
Surrounding
medium
endoplasmic
reticulum
peroxisome
nucleus
lysosome
Golgi
apparatus
mitochondrion
transport
vesicle
plasma
membrane
The membranes of the endoplasmic reticulum, Golgi apparatus,
mitochondria and other membrane-bounded organelles
maintain the characteristic differences in composition between
these organelles
endoplasmic
reticulum
peroxisome
nucleus
lysosome
Golgi
apparatus
mitochondrion
transport
vesicle
plasma
membrane
Internal membranes act as more than just barriers: subtle
differences between them, especially differences in the
membrane proteins, are largely responsible for giving each
organelle its distinct character
lipid bilayer
(5 nm)
lipid molecule
protein molecule
All cell membranes are composed of lipids and proteins and
have a common general structure
The lipid component consists of many millions of lipid
molecules arranged in two closely apposed sheets, forming a
lipid bilayer
hydrophilic head
hydrophobic tails
The lipids in cell membranes combine two very different
properties in a single molecule:
they have a hydrophilic (“water-loving”) head and one
or two hydrophobic (“water-hating”) hydrocarbon tails
Molecules with both hydrophilic and hydrophobic properties
are termed amphipathic
The simplest lipids are fatty acids
A fatty acid consists of a long hydrocarbon chain (16 to 18 carbon
atoms) terminating in a carboxyl group at one end
A negatively
charged
carboxyl
group
In saturated fatty acids all of the carbon atoms
are bonded to the maximum number of hydrogen
atoms  No double bonds between carbon atoms
The double
bond
introduces a
kink in the
hydrocarbon
chain
Unsaturated
fatty acids
contain one or
more double
bonds between
carbon atoms
The most abundant membrane lipids are the phospholipids
Phospholipids consist of two fatty acids linked to a polar head group
In the glycerol phospholipids, the two fatty acids are bound to carbon atoms in glycerol
The third carbon atom of glycerol is bound to a phosphate group, which is in turn
frequently attached to another small polar molecule, such as:
- choline
- serine
- inositol
- ethanolamine
CHOLINE
polar
(hydrophilic)
head
PHOSPHATE
of phospholipid in most
cell membranes is
phosphatidylcholine
nonpolar
(hydrophobic)
tails
ACID
The most common type
FATTY ACID
GLYCEROL
+
-
In the glycerol phospholipids, the two fatty acids are bound to carbon
atoms in glycerol
The third carbon atom of glycerol is bound to a phosphate group, which is
in turn frequently attached to another small polar molecule, such as:
- choline
- serine
- inositol
- ethanolamine
Choline
Phosphate
Glycerol
Fatty acids
The net charge of the polar head =
(+1) + (-1) = 0
In the glycerol phospholipids, the two fatty acids are bound to carbon
atoms in glycerol
The third carbon atom of glycerol is bound to a phosphate group, which is
in turn frequently attached to another small polar molecule, such as:
- choline
ETHANOLAMINE
- serine
The net charge of the polar head =
(+1) + (-1) = 0
-
GLYCEROL
ACID
- ethanolamine
PHOSPHATE
FATTY ACID
- inositol
+
Phosphatidylethanolamine
In the glycerol phospholipids, the two fatty acids are bound to
carbon atoms in glycerol
The third carbon atom of glycerol is bound to a phosphate group,
which is in turn frequently attached to another small polar
molecule, such as:
SERINE
- choline
-
- serine
PHOSPHATE
- inositol
The net charge of the polar head =
(+1) + (-1) + (-1) = -1
-
ACID
GLYCEROL
FATTY ACID
- ethanolamine
+
Phosphatidylserine
In the glycerol phospholipids, the two fatty acids are bound to
carbon atoms in glycerol
The third carbon atom of glycerol is bound to a phosphate group,
which is in turn frequently attached to another small polar
molecule, such as:
- choline
- serine
- inositol
Serine
Phosphate
Glycerol
- ethanolamine
The net charge of the polar head =
(+1) + (-1) + (-1) = -1
Fatty
acids
Phosphatidylserine
In the glycerol phospholipids, the two fatty acids are bound to
carbon atoms in glycerol
The third carbon atom of glycerol is bound to a phosphate group,
which is in turn frequently attached to another small polar
molecule, such as:
INOSITOL
- choline
PHOSPHATE
- serine
- inositol
-
GLYCEROL
The net charge of the polar head = -1
ACID
FATTY ACID
- ethanolamine
Phosphatidylinositol
In the glycerol phospholipids, the two fatty acids are bound to
carbon atoms in glycerol
The third carbon atom of glycerol is bound to a phosphate group,
which is in turn frequently attached to another small polar
molecule, such as:
- choline
- serine
- inositol
Inositol
Phosphate
Glycerol
- ethanolamine
The net charge of the polar head = -1
Fatty
acids
Phosphatidylinositol
The net charge of the polar head =
(+1) + (-1) = 0
+
PHOSPHATE
-
SERINE
ACID
The third carbon atom of serine is bound to a
phosphate group, which is in turn attached to
another small polar molecule, choline
CHOLINE
FATTY ACID
A non-glycerol phospholipid, the sphingomyelin,
contains two hydrocarbon chains of fatty acids
linked to a polar head group formed from serine
rather than from glycerol
Sphingomyelin
Glucose or
Galactose
In addition to glycerol phospholipids and
non-glycerol phospholipids, many cell
membranes contain glycolipids
SUGAR
ACID
Glycolipids consist of two hydrocarbon
chains of fatty acids linked to a polar head
group formed from serine, which is in turn
attached to a polar carbohydrate molecule
(glucose or galactose)
FATTY ACID
SERINE
Glycolipid
OH
In addition to
glycerol
phospholipids, nonglycerol
phospholipids and
glycolipids, all cell
membranes contain
cholesterol
Hydrophilic head
(hydroxyl group)
CH3
CH
Cholesterol
Cholesterol consists
of four hydrocarbon
rings rather than
linear hydrocarbon
chains of fatty acids
CH3
CH3
CH2
Hydrophobic
tail (sterol)
CH2
CH2
CH
CH3
CH3
Is amphipathic, with a hydrophilic
head and one hydrophobic tail
Hydrophilic molecules dissolve in water because they contain
charged atoms or polar groups and therefore can form
electrostatic bounds with water molecules
CHOLINE
-
+
H+
OH+
PHOSPHATE
GLYCEROL
H+
O-
+
Water (polar molecule)
Charged atoms and polar groups
CHOLINE
H+
OH+
-
+
PHOSPHATE
GLYCEROL
electrostatic bond
H+
OH+
H+
OH+
H+
O-
H+
OH+
Hydrophobic molecules are insoluble in water
because all of their atoms are uncharged and
nonpolar and therefore cannot form bonds
with water molecules
H+
OH+
H+
OH+
H+
OH+
FATTY ACID
H+
OH+
H+
OH+
ACID
H+
OH+
H+
OH+
H+
OH+
H+
OH+
H+
OH+
Cage-like structure of
water molecules
around the
hydrophobic molecule
The formation of cage structure of water molecules around
the hydrophobic molecule requires energy
The energy cost is minimized if the hydrophobic molecules
cluster together
H+
OH+
H+
OH+
H+
OH+
H+
OH+
H+
OH+
H+ OH+
ACID
FATTY ACID
H+
OH+
FATTY ACID
H+
OH+
ACID
H+
H+
OOH+
H+
H+
OH+
H+
OH+
H+
OH+
Amphipathic molecules, such as membrane phospholipids, are
subject of two conflicting forces:
(1) the hydrophilic head is attracted to water
(2) the hydrophobic tail shuns water and aggregates with other
hydrophobic tails
This conflict is resolved by the formation of a phospholipid (lipid)
bilayer – the most energetically favorable state of phospholipid
molecules in water
water
lipid bilayer
water
A phospholipid bilayer
water
lipid bilayer
water
The hydrophilic heads face the water at each of two
surfaces of the sheet of molecules
The hydrophobic tails are all shielded from the water and
lie next to one another in the interior of the sandwich
Energetically unfavored
Planar phospholipid bilayer
with edges exposed to water
Sealed compartment formed
by phospholipid bilayer
Energetically favored
Phospholipid bilayers
spontaneously close in
on themselves to form
sealed compartments
The closed structure
is stable because it
avoids the exposure of
the hydrophobic
hydrocarbon tails to
water
Synthetic lipid bilayers: liposomes
water
water
Closed spherical
vesicles, called
liposomes, form if
pure phospholipids
are added to water
Synthetic lipid bilayers: flat bilayers
Phospholipids are applied to a small hole (~ 1 mm in diameter)
in a partition that separates two aqueous compartments
Flat bilayer forms across the hole
water
water
Flat lipid bilayer
The fluidity of lipid bilayers
1st type of phospholipid mobility in a lipid bilayer - lateral
diffusion:
Lipid molecules within a monolayer constantly exchange places
with their neighbors
Lateral diffusion
The fluidity of lipid bilayers
2nd type of phospholipid mobility in a lipid bilayer – rotation:
Lipid molecules within a monolayer rotate very rapidly around
their long axis
Rotation
The fluidity of lipid bilayers
3rd type of phospholipid mobility in a lipid bilayer – “flip-flop”
movement
Lipid molecules very rarely flip from one monolayer to the
other
Flip-flop
The fluidity of a lipid bilayer depends on
the nature of the hydrocarbon tails:
The closer and more regular the
packing of the tails, the less fluid the
bilayer will be
Membrane fluidity depends on:
(1) length of the hydrocarbon tails
(2) level of saturation of the hydrocarbon
tails with respect to hydrogen
(3) presence of the sterol cholesterol
Length of the hydrocarbon tails:
A shorter chain length reduces the tendency of the
hydrocarbon tails to interact with one another and
therefore increases the fluidity of the bilayer
Less fluid
bilayer 2
FATTY ACID
nonpolar
(hydrophobic)
tails
FATTY ACID
bilayer 1
More fluid
Level of saturation of the hydrocarbon tails
with respect to hydrogen:
Lipid bilayers that contain a large proportion of unsaturated
hydrocarbon tails are more fluid than those with lower proportions
More fluid
FATTY ACID
FATTY ACID
FATTY ACID
FATTY ACID
bilayer 2
ACID
FATTY ACID
ACID
FATTY ACID
bilayer 1
Less fluid
Each double bond ( - HC = CH - ) between adjacent carbon atoms
in an unsaturated tail creates a small kink in the hydrocarbon tail
Presence of cholesterol:
phospholipid
head group
Fatty
acid tails
polar
hydroxyl cholesterol
group
Cholesterol inserts into the
membrane with its polar
hydroxyl group close to the polar
head groups of the phospholipids
The rigid hydrocarbon rings of
cholesterol interact with – and
partly immobilize - the regions of
the fatty acid chains that are
adjacent to the phospholipid head
groups
Cholesterol stiffens the bilayer
and makes it less fluid
The asymmetry of the lipid bilayer:
The lipid compositions of the two leaflets (monolayers) of the lipid bilayer
in many membranes, including the plasma membrane, are different
phosphatidylcholine
sphingomyelin
cholesterol
glycolipid
Outside (extracellular) leaflet
Plasma
membrane
phosphatidylserine
Inside (cytosolic) leaflet
cholesterol
phosphatidylinositol
phosphatidylethanolamine
Glycolipids and sphingomyelin are only in the extracellular (outside) leaflet
This asymmetry is generated by: Both glycolipids and sphingomyelin are
produced by enzymes exposed to the Golgi lumen and are not substrates
for lipid translocators (flippases)
The asymmetry of the lipid bilayer:
The lipid compositions of the two leaflets (monolayers) of the lipid bilayer
in many membranes, including the plasma membrane, are different
phosphatidylcholine
sphingomyelin
cholesterol
glycolipid
Outside (extracellular) leaflet
Plasma
membrane
phosphatidylserine
Inside (cytosolic) leaflet
cholesterol
phosphatidylinositol
phosphatidylethanolamine
Phosphatidylcholine, a glycerol phospholipid molecule that have choline
in its head group is mostly in the extracellular (outside) leaflet
The asymmetry of the lipid bilayer:
The lipid compositions of the two leaflets (monolayers) of the lipid bilayer
in many membranes, including the plasma membrane, are different
phosphatidylcholine
sphingomyelin
cholesterol
glycolipid
Outside (extracellular) leaflet
Plasma
membrane
phosphatidylserine
Inside (cytosolic) leaflet
cholesterol
phosphatidylinositol
phosphatidylethanolamine
Glycerol phospholipids that contain a terminal primary amino group, namely
phosphatidylethanolamine and phosphatidylserine, are mostly in the inside (cytosolic)
leaflet
This asymmetry is generated by: flippases, the phospholipid translocases that move
these phospholipids from the extracellular leaflet to the cytosolic leaflet
The asymmetry of the lipid bilayer:
The lipid compositions of the two leaflets (monolayers) of the lipid bilayer
in many membranes, including the plasma membrane, are different
phosphatidylcholine
sphingomyelin
cholesterol
glycolipid
Outside (extracellular) leaflet
Plasma
membrane
phosphatidylserine
Inside (cytosolic) leaflet
cholesterol
phosphatidylinositol
phosphatidylethanolamine
Cholesterol is distributed equally in both leaflets
This lipid spontaneously shuttles (flip-flops) between the leaflets
The relative permeability of a synthetic lipid
bilayer to different classes of molecules
Small hydrophobic molecules
18 – 46
daltons
Small uncharged polar molecules
92 – 180
daltons Large uncharged polar molecules
Charged molecules (IONS)
O2, CO2, N2,
benzene
H2O, glycerol,
ethanol
amino acids,
glucose,
nucleotides
H+, Na+, K+, Ca2+,
Mg2+, Cl-, HCO3-