Schedule
1. The Plasma Membrane, the Lipid Bilayer
❖ Chemical structure of cell membranes
❖ Membrane proteins
2. Principles of Membrane Transport
❖ Transporters, passive and active membrane
transport
❖ Ion channels and the electrical properties of
membranes
Schedule of the present lecture
1. The Plasma Membrane, the Lipid Bilayer
❖ Chemical structure of cell membranes
❖ Membrane proteins
2. Principles of Membrane Transport
❖ Transporters, passive and active membrane
transport
❖ Ion channels and the electrical properties of
membranes
1. Plasma Membrane, Lipid Bilayer
Cell membranes are crucial to the life of the cell
The plasma membrane
❖ encloses the cell
❖ defines its boundaries
❖ maintains the essential differences between
cytosol and extracellular environment
❖ Encloses organelles inside eukaryotic cells:
endoplasmic reticulum, Golgi apparatus, mitochondria …
Chemical Structure of Cell Membranes
Cell structures with the membrane
❖ Cytoplasmic membrane
❖ Nucleus membrane
❖ Membrane of endoplasmatic reticulum
❖ Membrane of Golgiho complex
❖ Mitochondrion
❖ Chloroplast, chromoplasts, and leukoplasts
❖ Lysozomes and spherosomes
❖ Peroxisomes and glyoxisomes
❖ Vacuoles
❖ in ATP (adenosin triphosphate) synthesis
❖ in the transmembrane transport
of small molecules
❖ in cell signalling and cell adhesion
❖ protein traffic in cell compartments
The role of the cell membranes
The plasma membrane is
❖ very thin film of lipid (fatty) and protein molecules
❖ these molecules are hold together by
noncovalent interactions
Despite of their different functions, all
biological membranes have a common
general structure
Cell membranes are dynamic, fluid structures. Most of their
molecules move about in the plane of the membrane.
The lipid molecules are arranged as
a continuous double layer
Cell membranes are dynamic, fluid structures. Most of their
molecules move about in the plane of the membrane.
Protein molecules span the lipid bilayer and serve
structural links through the lipid bilayer to the
extracellular environment or an adjacent cell.
The lipid bilayer constitute the basic
structure of all cell membranes
Lipid molecules constitute
about 50 % of the mass of
the cell membranes
Lipid molecules in cell membranes are amphiphilic =
= they have:
❖ hydrophilic (polar) end (“water-loving“)
❖ hydrophobic (nonpolar) end (“water-fearing“)
Picture : The parts of phosphoglyceride molecule
Phospholipids are the most
abundant membrane lipids
HYDROPHILIC (POLAR)
HEAD GROUP
TWO HYDROPHOBIC
(NONPOLAR) TAILS
The tails are usually fatty acids
They can differ in lengthy
One tail has one or more
cis double bonds
…creates a small kink in the tail
Picture : The parts of phosphoglyceride molecule
Each
Differences
in the length
and saturation of
the fatty acid tails
influence how
phospholipid
molecules pack
against one
another thereby
affecting the
fluidity of the
membrane.
The double bonds make
it more difficult to pack
the chains together,
thereby making the lipid
bilayer more difficult to
freeze.
The influence of cis-double bonds in
hydrocarbon chains
In addition, because the hydrocarbon chains of
unsaturated lipids are more spread apart, lipid bilayer
containing them are thinner than bilayers formed
exclusively from saturated lipids.
The lipid layers
in many cell membranes contain: Phosphatidyl -
ethanolamine
- serine
- choline❖ Phospholipids
phosphoglycerides (they have glycerol)
sphingomyeline (it have sphingosine)
❖ Cholesterol
❖ Glycolipids
galactocerebrosides
gangliosides
PHOSPHATE
GLYCEROL
Ethanolamine Serine Choline
Chemical Structure of Three Groups of
Phosphoglycerides: 1. Phosphatidyl-ethanolamine
2. Phosphatidyl-serine
3. Phosphatidyl-choline
❖ Phospholipids
SPHINGOSINE
PHOSPHATE
Choline
Chemical
Structure of
Sphingomyelins
❖ Phospholipids
Eukaryotic plasma membranes contain large amounts of
cholesterol. Cholesterol is sterol.
..to which is attached a single polar hydroxyl group
and a short non polar hydrocarbon chain
It contains a rigid ring structure…
❖ Cholesterol
Cholesterol molecule interacts with two
phospholipid molecules in one monolayer of a lipid
bilayer
❖ Cholesterol
❖ Lipid molecules which contain sugar are called
glycolipids.
❖ Glycolipids are found on the surface of all plasma
membranes. They have the most extreme asymmetry in
their membrane distribution.
❖ They generally constitute about 5% of the lipid molecules
in the outer monolayer. They are also found in some
intracellular membranes.
❖ In the plasma membrane of nerve cells have been
identified more than 40 different gangliosides.
❖ Glycolipids
Chemical Structure of Glycolipid molecules
1. Galactocerebrosides
contain sugar galactose. They are
neutral glycolipid (uncharged)
2. Gangliosides
contain oligosaccharides
as sugar and
sialic acid residues (one
or more) which give
gangliosides negative
charge (-).
❖ Glycolipids
Different cell membranes contains
various types of lipids
Red circle mark the largely represented type of lipid in given
type of cell membrane.
The lipid composition of the two monolayers of the lipid
bilayer are strikingly different.
phosphatidyl-
serine
phosphatidyl-
choline
phosphatidyl-
ethanolamine
sphingomyelin
glycolipids
Cholesterol (not shown) is distributed roughly equally in
both monolayers
phosphatidyl-
choline
sphingomyelin
phosphatidyl-
ethanolamine
phosphatidyl-
serine
Asymmetry of the Lipid Bilayer
OUTER MONOLAYER (touch extracellular space)
Almost all of the phospholipid molecules contain CHOLINE chain
in their molecules (= phospatidyl-choline + sphingomyeline)
INNER MONOLAYER (touch cytosol)
Almost all of the phospholipid molecules contain terminal
primary amino group in their molecules
(= phosphatidyl-ethanolamin +
The
inner monolayer is negative charged, but outer monolayer is neutral
has negative
charge
Asymmetry of the Lipid Bilayer
phosphatidyl-serin)
Asymmetry of the Lipid Bilayer
Lipid asymmetry is functionally important especially in
signaling pathways (converting extracellular signals into
intracellular signals).
On the cytosolic face of the plasma membrane
(inner monolayer) are
❖ specific proteins proteinkinase C (PKC), phospholipase C
❖ specific lipid head groups phosphatidylinositol
❖ specific lipid kinases phosphoinositide 3-kinase (PI3-
kinase)
Membrane Proteins
Each type of cell membrane have characteristic functional
properties
The membrane's specific tasks perform proteins in cell
membrane, therefore there are various types of proteins
The amount of proteins in cell membrane differ
❖ in myelin membrane proteins consist less than 27% of the
membrane mass
❖ in membranes of mitochondria a total of 75 % of the mass
are proteins
❖ Transmembrane proteins
They extend through the lipid bilayer
On either side is the part of their mass
❖Proteins located entirely in the one side of
membrane
Proteins are anchored in the internal or external
side of membrane
Membrane proteins are
They are amphiphilic, having hydrophobic and
hydrophilic regions
pass through the membrane
and interact with the
hydrophobic tails of the lipid
molecules in the bilayer
are exposed to water on
either side of the membrane
❖ Transmembrane proteins
This picture shows the different ways in which the
membrane proteins can associate with the membrane
 Most transmembrane proteins are throught
to extend across the bilayer as single α helix
= single-pass transmembrane proteins
some of these proteins are covalently attached to
fatty acid chain inserted in cytosolic monolayer
This picture shows the different ways in which the
membrane proteins can associate with the membrane
 …or as multiple α helices
= multipass transmembrane proteins
This picture shows the different ways in which the
membrane proteins can associate with the membrane
 Other transmembrane proteins extend
across the bilayer as a rolled–up β sheet
(β barrel)
❖Proteins located entirely in the one
side of membrane (internal or external side).
They are attached to the lipid chain, phosphaditylinositole
group of phospholipids or to the other
transmembrane protein.
Sometimes these proteins (enzymes, signaling proteins) are
anchored by GPI (glyco phosphatidyl-inositol) anchor into
correct place of cytosolic side of the membrane.
This picture shows the different ways in which the
membrane proteins can associate with the membrane
 Other membrane proteins exposed at only one
side of the membrane - some of these are anchored
to the cytosolic surface by an amphiphilic
α helix
This picture shows the different ways in which the
membrane proteins can associate with the membrane
 Others are attached to the bilayer only
(specifically) by a covalent bond with lipid
chain or fatty acid chain or a prenyl group or
via a oligosaccharide linker to PI (phosphatidylinositol)
– called GPI (glyco-PI) anchor 
This picture shows the different ways in which the
membrane proteins can associate with the membrane
  Many proteins are attached to the
membrane only by noncovalent interactions
with other membrane proteins
❖ Functions of membrane proteins
❖ Cell surface receptors (e.g. GPCR = G protein
coupled receptors)
Transmembrane proteins can functions as
The signal molecule in the
extracellular side binds on
the extracellular part of
receptor protein.
This binding generate
intracellular signal on the
opposite side of the plasma
membrane.
❖ Functions of membrane proteins
❖ Transporters (carriers or permeases)
Transmembrane proteins
Transporters bind the specific solute and undergo a series of
conformational changes to transfer the bound solute across
the membrane.
❖ Functions of membrane proteins
❖ Channels (inorganic ions)
Transmembrane proteins
Channel proteins form a water-filled pore across the
bilayer through which specific inorganic ions can diffuse.
❖ Functions of membrane proteins
❖ Intracellular signalling molecules
(proteins of Src family, Ras, Raf proteins,…)
❖ Enzymes
(proteinkinases, adenylylcyclases, phospholipases)
Proteins, which are only on one side of the
lipid bilayer
These proteins are often associated exclusively with either
the lipid monolayer or a protein domain on that side.
Transmembrane protein always has a
unique orientation in the membrane
In order to membrane protein is attached to fatty
acid chain or a prenyl group. The covalent
attachment of certain type of lipid can help to
localise a protein to a membrane after its synthesis
in the cytosol.
The lipid anchors are:
Myristoyl anchor, Palmitoyl anchor, Farnesyl anchor
A fatty acid chain (myristic acid) is attached via
an amide linkage to an N-terminal glycine
1) Myristoyl anchor
2) Palmitoyl anchor
A fatty acid chain (palmitic acid) is attached via a
thioester linkage to a cysteine
3) Farnesyl anchor
A prenyl group (either farnesyl or
longer geranylgeranyl group) is
attached via a thioether linkage
to a cysteine residue
Many of the membrane proteins are
glycosylated
Because most trans membrane proteins in animal
cells are glycosylated*, carbohydrates extensively
coat the surface of all eukaryotic cells.
*The sugar residues are added in the lumen of the ER and
the Golgi apparatus.
1. The Plasma Membrane, the Lipid Bilayer
Summary
Biological membranes consist of a continuous
double layer of lipid molecules in which
membrane proteins are embedded
The lipid bilayer is fluid – The membrane lipid
molecules are amphiphilic and they are able to
diffuse rapidly within their own monolayer.
Cells contain 500-1000 different lipid species –
there are three major classes of membrane
lipids – phospholipids, cholesterol and
glycolipids.
The lipid composition of the inner and outer
monolayers a different, reflecting the different
functions.
Plasma membrane lipid bilayer play an important
part in cell signalling (see lecture Principles of
cell communication – signaling pathways)
1. The Plasma Membrane, the Lipid Bilayer
Summary