3. Introduction
• Cell membrane are crucial to the life of the cell.
• The plasma membrane encloses the cell, defines its
boundaries, and maintains essential difference betn
cytosol and extra cellular environment.
4. • separates the living cell from its surroundings.
• 5 nm thick, controls traffic into and out of the
cell.
• selectively permeable, allowing some
substances to cross more easily than others
• Major macromolecules in membranes are
lipids, proteins, and some carbohydrates
• Made of a bilayer of phospholipids. With polar
heads, hydrophilic, and non-polar tails,
hydrophobic.
5. STRUCTURE
All biological membrane has common general
structure.
Thin film of lipid and protein held together by
non covalent interactions.
6.
7. Fluid mosaic model
• The membrane is represented as a fluid mosaic model, a fluid
environment with a mosaic of proteins and carbs. embedded
or attached that serve several functions.
• On the basis of the dynamic properties of proteins in
membranes, S. Jonathan Singer and Garth Nicolson proposed
the concept of a fluid mosaic model for the overall
organization of biological membranes in 1972.
• Membrane proteins are free to diffuse laterally in the lipid
matrix unless restricted by special interactions.
8.
9. Membrane Movement and
Cholesterol
• Most of the lipids and some
proteins can drift laterally in
the plane of the membrane,
but rarely flip-flop from one
layer to the other.
• Cholesterol is wedged between
phospholipids molecules in the
plasma membrane of animals
cells. It restrains the movement
of the phospholipids in warm
temps. and maintains fluidity
by preventing tight packing at
cold temps.
10. Cells can Change their Membrane
Composition
• Cells can modify the lipid make-up of
membranes to compensate for changes in
fluidity caused by changing temperatures.
– Ex, winter wheat, increases the percentage of
unsaturated phospholipids in the autumn.
– This lets them prevent their membranes from
solidifying during winter.
11. MEMBRANE LIPIDS ARE AMPHIPATHIC
MOLECULS
Approximately 50,00,000 lipid molecules
present in 1x1 um area of lipid bilayer.
Amphipathic –hydrophilic and hydrophobic
Most of lipid is phospholipids.
Have polar head group and 2 non polar tail.
Tail is fatty acids differ in length (14 and 24
carbon atoms).
1or2 cis double bonds creates small kink in tail
12.
13. Individual units are
wedge-shaped
(cross section of head
greater than that
of side chain)
Individual units are
cylindrical (cross section
of head equals that of side
chain)
(a) Micelle (b) Bilayer (c) Liposome
Aqueous
cavity
Amphipathic lipid aggregates that form in water. (a) In micelles, the hydrophobic
chains of the fatty acids are sequestered at the core of the sphere. There is virtually no
water in the hydrophobic interior.
(b) In an open bilayer, all acyl side chains except those at the edges of the sheet are
protected from interaction with water.
(c) When a two-dimensional bilayer folds on itself, it forms a closed bilayer, a three-
dimensional hollow vesicle (liposome) enclosing an aqueous cavity.
14. • TYPES OF MEMBRANE LIPIDS
1.Phospholipid
2.Glycolipid
3.Cholesterol
PHOSPHOLIPIDS
• Phospholipids are abundant in all biological
membranes
• Four components
fatty acids, glycerol,
phosphate, alcohol.
16. • Phospholipids are built from glycerol,
3-carbon alcohol, or sphingosine, a more
complex alcohol.
A)PHOSPHOGLYCERIDES
• Glycerol is back bone to which two fatty acid
chain and a phosphorylated alcohol are
attached.
• Simplest phosphoglycerides
18. Sphingomyelin
• Sphingomyelin is a phospholipid found in
membranes that is not derived from glycerol.
Instead, the backbone in sphingomyelin is
sphingosine, an amino alcohol that contains a
long, unsaturated hydrocarbon chain
• In sphingomyelin, the amino group of the
sphingosine backbone is linked to a fatty acid
by an amide bond. In addition, the primary
hydroxyl group of sphingosine is esterified to
phosphoryl choline
20. • Glycolipids,
• Glycolipids, as their name implies, are sugar-
containing lipids. Glycolipids in animal cells
are derived from sphingosine.
• The amino group of the sphingosine
backbone is acylated by a fatty acid
• In Glycolipids, one or more sugars are
attached to this group.
• The simplest glycolipid, called a cerebroside,
contains a single sugar residue, either glucose
or galactose.
21.
22. CHOLESTROL
• Cholesterol is a lipid with a structure quite
different from that of phospholipids. It is a
steroid, built from four linked hydrocarbon
rings.
• It constitutes almost 25% of the membrane
lipids in certain nerve cells but is essentially
absent from some intracellular membranes.
23. Lipid Bilayers Are Highly Impermeable to Ions and Most
Polar Molecules
• lipid bilayer membranes have a very low
permeability for ions and most polar
molecules.
• Water is a conspicuous exception to this
generalization; it readily traverses such
membranes because of its small size, high
concentration, and lack of a complete charge.
25. Proteins
• Membranes are very complex and dynamic containing
many different parts.
• Proteins decide most of the membrane’s functions.
• Contain lipids and carbohydrates also
• The collection of molecules in the membrane vary from
membrane to membrane
• All of the structures in the membrane serve various
functions like cell recognition proteins.
• Typically contains 50% of proteins.
26. 2 Types of Proteins
• Peripheral proteins are not embedded in the lipid bilayer,
they are loosely bounded to the surface.
• Integral proteins penetrate, often completely spanning the
membrane (a transmembrane proteins)
Integral and Peripheral Membrane Proteins.
27. • Peripheral membrane proteins are bound to
membranes primarily by electrostatic and
hydrogen-bond interactions with the head
groups of lipids.
Many peripheral membrane proteins are bound
to the surfaces of integral proteins, on either
the cytosolic or the extracellular side of the
membrane.
Others are anchored to the lipid bilayer by a
covalently attached hydrophobic chain, such
as a fatty acid.
28. Integral proteins
• The firm attachment of integral proteins to
membranes is the result of hydrophobic
interactions between membrane lipids and
hydrophobic domains of the protein.
30. • Types I and II have only one transmembrane
helix; the amino-terminal domain is outside
the cell in type I proteins and inside in type II.
Type III proteins have multiple
transmembrane helices in a single
polypeptide.
In type IV proteins, transmembrane domains of
several different polypeptides assemble to
form a channel through the membrane
Type V proteins are held to the bilayer primarily
by covalently linked lipids
type VI proteins have both transmembrane
helices and lipid (GPI) anchors
31. Type Description Examples
Integral proteins
or
transmembra
ne proteins
Span the membrane and have a
hydrophilic cytosolic domain, which
interacts with internal molecules, a
hydrophobic membrane-spanning
domain that anchors it within the cell
membrane, and a hydrophilic
extracellular domain that interacts
with external molecules. The
hydrophobic domain consists of one,
multiple, or a combination of α-helices
and β sheet protein motifs.
Ion channels,
proton pumps, G
protein-coupled
receptor
32. Lipid anchored proteins
Covalently-bound to single
or multiple lipid molecules;
hydrophobically insert into
the cell membrane and
anchor the protein. The
protein itself is not in
contact with the
membrane.
G proteins
Peripheral proteins
Attached to integral
membrane proteins, or
associated with peripheral
regions of the lipid bilayer.
These proteins tend to have
only temporary interactions
with biological membranes,
and, once reacted the
molecule, dissociates to
carry on its work in the
cytoplasm.
Some enzymes, some
hormones
33. FUNCTIONS
• The cell membrane physically separates the
intracellular components from the
extracellular environment, thereby serving a
function similar to that of skin
• The cell membrane also plays a role in
anchoring the cytoskeleton to provide shape
to the cell,
• The barrier is selectively permeable and able
to regulate what enters and exits the cell
• The membrane also maintains the cell
potential
35. Carbohydrates in the Membrane
• Used for cell to cell recognition, the ability of a cell to
distinguish one type of neighboring cell from another.
– important in cell sorting and organization as tissues and
organs in development.
• Basis of immune response. Ex. WBC and T-cell response
• Membrane carbohydrates are usually branched
oligosaccharides with fewer than 15 sugar units
• vary from species to species, individual to individual, and
even from cell type to cell type within the same individual.
36. Crossing the Membrane
• steady traffic of small molecules and ions moves across
the plasma membrane in both directions
– Ex, sugars, amino acids, and other nutrients enter a
muscle cell and waste products leave
• membranes are selectively permeable so all this traffic is
under some control. Esp. the large molecules.
• Passage is controlled in part due to the hydrophobic core
of the membrane. So other hydrophobic molecules cross
easily while polar molecules and ions have difficulty.
• Proteins assist and control the transport of ions and polar
molecules.
37. Transport Proteins
• ions and polar molecules can cross the lipid bilayer by
passing through transport proteins that span the
membrane.
– Some transport proteins have a hydrophilic channel
– Others bind molecules and carry passengers across the
membrane physically
• Each transport protein is specific
– Ex. Gluclose transport in liver. Not fructose.
39. Exocytosis and Endocytosis
• Ways of getting large molecules in and out of
the cell.
• Phagocytosis is cell eating and involves solids.
• Pinocytosis is cell drinking and involves liquids.
40.
41.
42. GAP JUNCTION
• Gap junctions, also known as cell-to-cell
channels, serve as passageways between the
interiors of contiguous cells.
• Gap junctions are clustered in discrete regions
of the plasma membranes of apposed cells.
Electron micrographs of sheets of gap
junctions show them tightly packed in a
regular hexagonal array
44. • A cell-to-cell channel is made of 12 molecules
of connexin, one of a family of
transmembrane proteins with molecular
masses ranging from 30 to 42 kd.
• Each connexin molecule appears to have four
membrane-spanning helices.
• Six connexin molecules are hexagonally
arrayed to form a half channel, called a
connexon or hemhannel
• Two connexons join end to end in the
intercellular space to form a functional
channel between the communicating cells
45. • Cell-to-cell channels differ from other
membrane channels in three respects:
(1) they traverse two membranes rather than
one
(2) they connect cytosol to cytosol, rather than
to the extracellular space or the lumen of an
organelle
(3) The connexons forming a channel are
synthesized by different cells.
46. • Gap junctions form readily when cells are
brought together. A cell-to-cell channel, once
formed, tends to stay open for seconds to
minutes
• They are closed by high concentrations of
calcium ion and by low pH.
• The closing of gap junctions by Ca 2 + and H +
serves to seal normal cells from traumatized
or dying neighbors.
• They also controlled by membrane potential
and Harmon induced phosphorylation.
47.
48. • All polar molecules with a mass of less than
about 1 kd can readily pass through these cell-
to-cell channels.
• Thus, inorganic ions and most metabolites
(e.g., sugars, amino acids, and nucleotides)
can flow between the interiors of cells joined
by gap junctions.
• In contrast, proteins, nucleic acids, and
polysaccharides are too large to traverse these
channels.
• Gap junctions are important for intercellular
communication.
