2. • Introduction
• Membranes are highly viscous, pliable structures that
define the borders of the cell and keep the cell functional.
• They are described as a continuous sheet of lipid molecules
arranged as a molecular bilayer (lipid bilayer), 4-5mm thick.
• They form closed compartments around cellular
protoplasm to separate one cell from another and
therefore permit cellular individuality.
• Membranes also form specialized compartments within
the cell (intracellular membranes) which help to form
morphologically distinguishable structures (organelles)
such as the mitochondria, ER, sarcoplasmic reticulum,
Golgi complexes, secretory granules, lysosomes, and the
nuclear membrane.
• They localize enzymes, function as integral elements in
excitation-response coupling, and provide sites of energy
transduction, such as in oxidative phosphorylation.
3. • They are selectively permeable, allowing certain
substances to move freely in and out the cell, while
some others cannot move freely but require the use
of some specialized structure/ transport mechanisms
or even energy.
• The selective permeabilities are provided mainly by
channels and pumps for ions and substrates.
• The plasma membrane exchanges materials with the
extracellular environment by exocytosis and
endocytosis.
• Special areas of membrane structure called the gap
junctions exist through which adjacent cells exchange
material.
• The plasma membranes therefore play key roles in
cell-cell interactions and in transmembrane signaling.
4.
5.
6. • SOME IMPORTANT FACTS TO NOTE ABOUT THE
CELL MEMBRANE:
• The individual lipids and proteins of the plasma
membrane are not covalently linked (see later),
• The entire structure is therefore flexible,
allowing changes in the shape and size of the
cell.
• As the cell grows, newly made lipid and protein
molecules are inserted into its plasma
membrane.
• Cell division produces two cells, each with its
own membrane.
7. • Growth and cell division occurs without loss
of membrane integrity.
• In addition to plasma membrane (external
cell membrane); eukaryotic cells also contain
internal membranes that form the boundaries
of organelles such as mitochondria,
peroxisomes, and lysosomes.
• The external and internal membranes have
essential features in common.
8. CHEMICAL COMPOSITION OF MEMBRANES
• Lipids and proteins are the two major
components of cell membrane.
• Membranes also contain a small amount of
various polysaccharides as glycoprotein and
glycolipid, but not free carbohydrate.
• The three major kinds of membrane lipid are
phospholipids, glycolipids and cholesterol.
9. lipid composition varies between membranes.
• Tissue and various cell membranes have a
distinctive lipid composition.
• The membranes of a specific tissue (e.g liver)
in different species contain very similar classes
of lipids.
• The plasma membrane exhibits the greatest
variation in percentage composition because
the cholesterol content is affected by the
nutritional state of the animal.
10. • Myelin membrane of neuronal axons is rich in
sphingolipids such as sphingomyelin.
• Intracellular membranes – for example,
endoplasmic reticulum contain primarily
glycerophospholipids and little sphingolipids.
• The membrane lipid composition of
mitochondria, nuclei, and rough endoplasmic
reticulum is similar, with that of Golgi complex
being somewhere between that of other
intracellular membranes and the plasma
membrane.
11. • The amount of cardiolipin is high in the inner
mitochondrial membrane and low in the outer
membrane, with essentially non in other
membranes.
• The choline-containing lipids,
phosphatidylcholine and sphingomyelin are
most common, followed by
phosphatidylethanolamine.
• The constancy of composition of various
membranes suggests a relationship between
their lipids and the specific functions of those
membranes.
12. Membrane proteins
• Membrane proteins are classified based on the
ease of removal of the protein from the
membrane
• Integral (or intrinsic) membrane proteins.
• They span the lipid bilayer and are in contact
with the aqueous environment on both sides.
• Integral proteins contain sequences rich in
hydrophobic amino acid which interact with the
hydrophobic hydrocarbons of the lipids, thereby
stabilizing the protein lipid complex.
13. • The removal of integral membrane proteins
thus requires disruption of the membrane by
detergent or organic solvents and when
isolated, usually contain tightly bound lipid.
• Many integral proteins are glycoproteins.
• Integral proteins are embedded and are
asymmetric in the membrane.
14. • They have a defined, rather than a random
orientation.
• The orientation of proteins is determined by their
primary structure.
• Just like non-membrane proteins, they contain
specific domains for ligand binding; for catalytic or
transport activity, and for attachment of
carbohydrate or lipid.
• For some proteins, the amino and carboxyl termini
are both on one side of the membrane, whereas for
others they are on opposite sides, frequently with
the carboxyl terminus on the cytoplasmic side.
• Some integral proteins form multiple subunits
structures in order to carry out their function (see
diagram).
16. PERIPHERAL (EXTRINSIC) MEMBRANE PROTEINS
• They are located on the surface of membranes
and can be easily removed without disrupting
the lipid bilayer.
• Some bind to integral membrane proteins.
• Negatively charged phospholipids of
membranes interact with positively charged
regions of proteins and produce electrostatic
binding.
• In some cases Ca2+ mediates the binding.
17. • Several peripheral proteins have short
sequences of hydrophobic aminoacids at one
end that serve as a membrane anchor, eg.
Cytochrome b5 is attached to the endoplasmic
reticulum by such an anchor at the carboxyl
terminus.
• Other peripheral membrane proteins have
specific conserved domain that binds non-
covalently to the inositol 3 phosphate head
group of phosphatidylinositol fixed in the
membrane.
19. • Diagram illustrates the multiple types of binding
of proteins in or to the lipid bilayer
• (a) a single transmembrane segment
• (b) multiple transmembrane segments
• (c) bound to an integral protein
• (d) bound electrostatically to the lipid bilayer
• (e) attached by a short terminal hydrophobic
sequence of amino acids; and
• (f) noncovalent binding to a phosphatidy
linositol (PI) in the membrane.
20. • The peripheral proteins are released by
treatment with salt solutions of different
ionic strength, extremes of pH or cleavage of
covalently bound lipid that serves to attach
the protein to the membrane.
21. • LIPID ANCHORED PROTEINS
Some peripheral membrane proteins as mentioned
earlier are attached to the membrane by covalently
linked lipid.
• The lipid is inserted into the lipid membrane, anchoring
the protein to the membrane e.g GPI ANCHOR
• phosphatidylinositol is attached to a glycan (consisting
of ethanolamine, phosphate, mannose, mannose,
mannose, and glucosamine).
• The glycan is covalently bound to the carboxyl terminus
of a protein by ethanolamine and the glucosamine is
linked covalently to phosphatidylinositol.
• The fatty acyl groups of phosphatidylinositol are then
inserted into the lipid membrane, thus anchoring the
protein.
• This form of attachment is referred to as a glycosyl
phosphatidylinositol (GPI) anchor.
• See below
23. • Note :This type of anchoring is important
because release and reattachment of the
protein to the anchor can be controlled,
thereby allowing protein regulation of the
activity of the protein.
24. membrane carbohydrate
• Carbohydrates are present in membranes as
oligosaccharide, covalently attached to proteins
(glycoproteins) and to lipids (glycolipids).
• The sugars in the oligosaccharides include
glucose, galactose, mannose, fucose, N-
acetylgalactosamine, N-acetylglucosamine and
N-acetylneuraminic acid (sialic acid).
• The carbohydrate is on the extracellular surface
of plasma membrane and the luminal surface of
endoplasmic reticulum.
• There is little or no free carbohydrate in
membrane.(see diagram)
25. Roles of membrane proteins
• Membrane proteins have a variety of functions. They
serve as
• Mediators of transmembrane movement of charged and
uncharged molecules
• Receptors for the binding of hormones and growth
factors
• Enzymes involved in transduction of signals
• Molecular pumps
• Some integral membrane proteins have a structural role
to maintain the shape of the cell
• Roles of protein bound carbohydrates of membrane
include
(1)Cell-cell recognition
(2) adhesion and
(3) receptor-action
26. FLUID MOSAIC MODEL OF MEMBRANE STRUCTURE
• The fluid mosaic model, now widely accepted,
was proposed by S. J. Singer and G.L Nicholson
in the early 1970s.
• This model of membrane structure was likened
to icebergs (membrane proteins) floating in a
sea of predominantly phospholipid molecules.
• The fluid mosaic model therefore describes the
structure of the plasma membrane as a mosaic
of components —including phospholipids,
cholesterol, proteins, and carbohydrates—that
gives the membrane a fluid character.
27. • The membrane consists of a bimolecular layer
of lipid with proteins inserted in it or bound to
either surface,
• Those embedded in either the outer or inner
leaflet of the lipid bilayer, that is, Loosely
bound to the outer or inner surface of the
membrane are the peripheral proteins (see
earlier).
• Integral membrane proteins are firmly
embedded in the lipid layers and are also
called transmembrane proteins
28. • Many of the proteins and lipids have externally
exposed oligosaccharide chains.
• Both lipids and proteins are considered to diffuse
laterally in the membrane.
• Their (lipids and proteins) distribution in cellular
membranes are thus not homogenous.
• Many of the properties of cellular membrane in
this regard include, fluidity, and flexibility that
permits change of shape and form, ability to self
seal and impermiability.
• Membranes are asymmetric structures.
29.
30.
31. • Although membrane models suggest that
some proteins are randomly distributed
throughout or on the membrane, there is a
high degree of functional organisation with
definite restrictions on the localization of
some proteins.
