Chapter 7
Membranes
Functions of membranes
1.
Boundaries and serve as permeability barriers.
2.
Sites of specific proteins and therefore of specific functions.
3.
Membrane proteins regulated the transport of solutes.
4.
Membrane proteins detect and transmit electrical and chemical signals.
5.
Membrane proteins medicate cell adhesion and cell-to-cell
communication.
The Prominence of membrane around and
within Eukaryotic cells
Figure 7-1
Models of Membranes structure
1.
2.
3.
Lipid nature of membrane: It is the pioneering work of
Chlarles Overton in the 1890s.
Lipid monolayer: It is the working of Irving Langmair in
1900s.
Lipid bilayer: 1935s Hugh Davson & James Danielli
suggested that proteins are present in membranes and its
consist of lipids bilayer that are coated on both sides with
thin sheet of proteins .
Models of Membranes structure
1.
2.
3.
Unite membranes: In 1950s J.David Robertson
suggested that all cellular membranes share a
common underlying structure.
Fluid mosaic model :Its consummation in 1970s
was proposed by S.Jonathon singer & Groth Nicolson.
Membrane protein structure: Unwin & Richard
Henderson used electron microscopy to determine
the three dimensional structure of unfixed crystallized
bacteriorhodopsion and its position in the membrane.
Timeline for development of
the fluid mosaic model
The fluid mosaic
model of membrane
structure
Figure 7-5
Membrane Lipids

1.
2.
3.
The main classes of membrane lipids:
Glycolipids
Sterols
Phospholipids
The main classes
of membrane lipids
Figure 7-6
Glycolipids
Glycolipids are formed by the addition of carbohydrate groups to lipids. The
most common glycolipids are:
1.
Cerebrosides: Also known as neutral glycolipids because of the
uncharged sugars as head groups.
2.
Gangliosides
*Both of the above types of glycolipids are found mostly in brain cells and
nerve cell membranes.
Sterols
The major sterols in animal membranes are cholesterol.
It maintains and stabilizes membranes of our bodies.
Plants have different types of sterols similar in function
but different in structure, even fungi have a unique
sterol known as ergosterol.
*Sterols aren’t fount in the inner membrane of
mitochondria or chloroplasts.
Phospholipids
Phospholipids are the most abundant lipids in membranes. There are 2 types of
phospholipids:
A-Phosphoglycerides: glycerol-based
B-Sphingolipids: sphingosine-based
 The most common phophoglyerides are:
1-Phosphatydylcholine
2-Phosphatidlyethanolamine
3-Phophatidylamine

*The portions of any of these lipids vary according to the type of membrane and its
function.
Fatty Acids



Fatty acids are components of all membrane lipids except sterols
The asymmetry of membranes depends partly on the degree of
unsaturation in the fatty acids and the different types of lipids there are.
Unsaturated fatty acids fail to tightly pack because of the kinks made from
double bonds.
kink
Membrane Fluidity
Membrane fluidity is caused by movement of lipids from one
monolayer to the other.
The type of movement's maybe any of the following:
1.
Transverse Diffusion (flip-flop)
2.
Lateral Diffusion (movement in the plane of the
membrane)
3.
Or Rotation of phospholipids about their long axis
One of the techniques used to overview these movements includes
Florescence recovery after photobleaching also known as
FRAP.
Membrane Proteins


1.
2.
3.
4.
5.
6.
A technique called freeze fracturing was used to
support the fluid mosaic model.
Membranes contain:
Integral
Peripheral
Lipid-Anchored proteins
Fatty acid-anchored membrane proteins
Isoprenylated membrane proteins
GPI-anchored membrane proteins
Figure 7-16a
Membrane Proteins
Peripheral Membrane Proteins


Peripheral membrane proteins are bound to membrane surfaces through
weak electrostatic forces and hydrogen bonding with the hydrophilic
portions of the lipid bilayer, perhaps with the polar head groups of
membrane lipids.
Spectrin, Ankyrin, and Band 4.1 are main peripheral proteins found in
erythrocyte membranes. These proteins are found on the inner surface of
the plasma membrane and forms a skeletal network that maintains the
erythrocyte shape.
Integral Membrane Proteins


1.
2.
AB-
Integral membrane proteins have hydrophobic regions embedded
within the membrane which makes it difficult to remove them.
Because of their affinity for the lipid bilayer they are difficult to
extract and study.
Types of integral membrane proteins:
Integral monotopic proteins: Those that protrude only from one
side
Transmembrane proteins: Those that have hydrophilic regions
protruding from both sides of the membrane, they can be:
Single pass: Cross the membrane once (Example: Glycophorin)
Multipass: Cross the membrane several times (Example: Band 3)

Lipid-Anchored Membrane Proteins: Lipid-anchored proteins
are located on either side of the surfaces of the lipid bilayers but are
covalently bonded to lipid molecules embedded within the bilayer.

Fatty Acid-Anchored Membrane Proteins: Its synthesized in
the cytosol and then is attached to a saturated fatty acid usually myristic
acid.

Isoprenylated Membrane Proteins: Its synthesized in the cytosol
as soluble cytosolic proteins then is modified.

GPI-Anchored Membrane Proteins: GPI
(glycosylphophatidylinositol) is a glycolipid found in the outer layer of the
plasma membrane. These types of membrane proteins are made in the ER.
Isolation of Proteins


1.
2.
3.
4.
SDS- Sodium Dodecyle Sulphate: Allows integral
membrane proteins to be isolated in addition to being
fractionated and analyzed by electrophoresis.
Membrane protiens have many functions which include:
Enzymatic
Electron transporters
Transport proteins that transport nutrients
Transport ATPase
Glycosylated Proteins


The addition of carbohydrate groups to the protein is called
glycosylation. If the linkage of the carbohydrate is to a nitrogen
atom of an amino group then its called N-Linked Glycosylation. If
the carbohydrate group is linked to an oxygen atom of a hydroxyl
group its known as O-Linked Glycosylation.
Glycoproteins and glycolipids that protrude from the outer surface
of plasma membranes form a surface coat called glycocalyx.