Transcript plasma membrane

PLASMA MEMBRANE
Submitted by
Dr Madhurima Sharma
LIPID COMPONENTS OF THE PLASMA MEMBRANE
The outer leaflet consists predominantly of phosphatidylcholine,
sphingomyelin, and glycolipids, whereas the inner leaflet contains
phosphatidylethanolamine, phosphatidylserine, and
phosphatidylinositol. Cholesterol is distributed in both leaflets. The
net negative charge of the head groups of phosphatidylserine and
phosphatidylinositol is indicated. The structures of phospholipids,
glycolipids, and cholesterol
PLASMA MEMBRANE
• Like all other cellular membranes, the plasma
membrane consists of both lipids and proteins.
The fundamental structure of the membrane is
the phospholipid bilayer, which forms a stable
barrier between two aqueous compartments. In
the case of the plasma membrane, these
compartments are the inside and the outside of
the cell. Proteins embedded within the
phospholipid bilayer carry out the specific
functions of the plasma membrane, including
selective transport of molecules and cell-cell
recognition.
Phospholipids
• One of the principal types of lipid in the membrane
include the phospholipids . These have a polar head
group and two hydrocarbon tails. An example of a
phospholipid is shown in this figure (right). The top
region beginning with the NH3 is the polar group. It is
connected by glycerol to two fatty acid tails. One of the
tails is a straight chain fatty acid (saturated). The other
has a kink in the tail because of a cis double bond
(unsaturate d). This kink influences packing and
movement in the lateral plane of the membrane
• Plasma Membrane Structure: The
Fluid Mosaic Model
The fluid mosaic model was developed by S.J.
Singer and Garth Nicolson in 1972. As per this
theory, the plasma membrane consists of
carbohydrates and different types of lipids and
proteins. And the model is named in accordance
to the structure of the plasma membrane; the
membrane is not rigid, but more of a fluid type,
containing various molecules like a mosaic
pattern. These molecules are arranged in a
proper manner, which helps in selective
movement of the substances. :
• Lipid Bilayer: The fundamental part of the plasma
membrane structure is the lipid bilayer. Types of lipids
present in the plasma membrane are phospholipids,
cholesterol and glycolipids. However, as majority of the
molecules are of phospholipid type (containing a
phosphate group), the two lipid layers are better known
as phospholipid layers.
The lipid tails are water repelling (hydrophobic), while
phosphate heads are water-attracted (hydrophilic). The
phospholipid bilayer is arranged in a specific fashion,
with the hydrophobic tails orienting towards the inside
(facing each other) and the hydrophilic head aligning to
the outside. Thus, both sides of the plasma membrane,
one that faces the cytosol and the other facing the
outside environment, are hydrophilic in nature.
Cell Membrane
• The cell membrane is a biological membrane that
separates the interior of all cells from the outside
environment.
• The cell membrane is selectively-permeable to ions and
organic molecules and controls the movement of
substances in and out of cells.
• It consists of the phospholipid bilayer with embedded
proteins.
• Cell membranes are involved in a variety of cellular
processes such as cell adhesion, ion conductivity and
cell signaling and serve as the attachment surface for
the extracellular glycocalyx and cell wall and intracellular
cytoskeleton.
• Membrane Proteins: Another key component of
the plasma membrane is proteins, which help in
selective transport of the macromolecules like
sucrose, amino acids and ions. Based on the
location of proteins with reference to the
phospholipid bilayers, there are two types of
proteins:
• Integral membrane proteins attach to the lipids
of the bilayered structure. And integral proteins
that traverse the phospholipid bilayer are called
transmembrane proteins.
• Peripheral membrane proteins are indirectly or
loosely attached to the membrane. They are
non-covalently connected with the lipids or ends
of the integral proteins.
• Carbohydrates: In addition to
phospholipids and proteins, the cell
membrane also consists of carbohydrates,
basically glycoproteins and glycolipids.
These molecules are exclusively arranged
in the outer side of the cell membrane,
wherein the carbohydrate portions are
exposed to the external surface of the cell.
• plasma membrane function, the primary roles
are cell recognition and regulation of cellular
transport. Small molecules like oxygen, nitrogen,
etc. are allowed to enter or leave the cell freely
(passive transport), while larger molecules like
amino acids are passed selectively through the
membrane against the concentration gradient
(active transport). For example, through active
transport waste materials are expelled, while
allowing other useful substances to enter inside
the cell
PLASMA MEMBRANE
• Encloses PROKARYOTIC and EUKARYOTIC
CELLS.
• Visible only under electron microscope.
• Semipermeable
• Elastic
• Living
• Dynamic
• Cells of bacteria and plants have it between cell
wall and cytoplasm.
Plasma membrane is a FLUID MOSAIC
assembly of molecules of
• LIPIDS(phospholipids and cholesterol)
• PROTEINS
• CARBOHYDRATES
• All biological membranes including the
plasma membrane and internal
membranes of eukaryotic
cells(i.e.membranes bounding ER,
nucleus,mitochondria,chloroplast,golgi
apparatus,lysosomes,peroxisomes etc.)
are similar in structure(i.e.fluid mosaic)
and selectively permeable but differ in
other functions.
• Plasma membrane is also called
CYTOPLASMIC MEMBRANE
• CELL MEMBRANE or
• PLASMALEMMA
FLUID MOSAIC MODEL
• Proposed by S.J.SINGER and
G.NICHOLSON in 1972.
• Cell membrane is composed of a highly
viscous matrix of 2 layers of phospholipid
molecules,having globular proteins and
carbohydrates associated with it.
PHOSPHOLIPIDS
• Phospholipid molecules have polar heads
directed outward and nonpolar tails
pointing inwards.This forms a WATER
RESISTANCE BARRIER which allows
only LIPIDS to pass through it.
• The bilayer is 35A thick.
• Each layer is called a leaflet.
• The two leaflets show ASYMMETRY.
MEMBRANE FLUIDITY
All lipids in biomembranes are laterally mobile in
their own leaflet exchanging places with their
neighbours.
• MEMBRANE CHOLESTEROL is the major
determinant of membrane fluidity.
• Double bonds in unsaturated hydrocarbon
chains also tend to increase fluidity of the
phospholipid bilayer by making it more difficult
to pack chains together.
• Lipid molecules rarely move from one leaflet to
the other. This is called FLIP FLOP
MOVEMENT.
PROTEINS
• There are two types of proteins
• PEREPHERAL PROTEINS-located superficially,are
loosely bound to the membrane,held by protein-protein
interactions or protein-lipid head interaction.
• INTEGRAL PROTEINS-embedded in phospholipid
matrix,held in place with polar heads and non-polar tails
of lipid molecules by strong hydrophilic or hydrophobic
interactions or both.Some integral proteins are small
molecules who project only on one surface of the bilayer
while some are large enough to extend on both sides of
the bilayer and are called TRANS MEMBANE OR
TUNNEL PROTEINS.These work as channels for the
passage of water soluble materials.
PROTEIN MOLECULES ACT AS
• CARRIER MOLECULES
• RECEPTOR MOLECULES
• ENZYME MOLECULES
CARRIER MOLECULES
• Bind and transport specific molecules into and
out of the cell and cell organelles.
• Allows selective exchange of materials between
cell and extracellular fluid and between cell
organelles and intracellular fluids.
• Carrier proteins are also called PERMEASES.
• Active membranes have more carrier proteins.
RECEPTOR MOLECULES
• Mediate in the flow of information also
called SIGNAL TRANSDUCTION e.g.
hormones like epinephrine and insulin bind
to membrane receptors and produce their
effect in the cells.
ENZYME MOLECULES
• Membranes require enzyme molecules for
their functions e.g.inner mitochondrial
membrane carries enzymes for the
electron transport chain of respiration.
CARBOHYDRATES
• Present as short branched or unbranched chains of
sugars(oligosaccharides) attached either to
ECTOPROTEINS forming GLYCOPROTEINS or to the
polar ends of phospholipids at the external surface of the
plasma membrane forming GLYCOLIPIDS.
• Different combinations of sugar residues give an endless
variety to glycolipids.
• Oligosaccharides form a coat called GLYCOCALYX
which enables the cells to interact with one another.
• They can recognize self from non self-membrane
specificity.
• Helps during embryological development,formation of
tissues ,in defence against foreign bodies
MEMBRANE ASYMMETRY
• Cytoplasmic and exoplasmic leaflets have
different lipids.
• The two faces of the membrane have
different proteins.
• Glycoproteins and glycolipids are formed
on exoplasmic face only.
CELL MEMBRANES ARE FLUID
AND DYNAMIC
• Constituent molecules can move freely.
• Membranes are constantly renewed during a
cells life.
• Can repair minor injuries.
• Grow with the growth of the cell and cell
organelles.
• Expand and contract during cell movement and
during changes in the cell shape.
• Allow recogonition of self and fusion.
• Control flow of materials through them.
MODIFICATIONS OF CELL
MEMBRANES
FOLDING
• Invaginations-pores, endocytotic vesicles.
• Evaginations - microvilli, interdigitations,ciliary and flagellar
sheaths,neurilemma.
INTERCELLULAR JUNCTIONS
• Desmosomes
• Intercellular bridges
• Tight junctions
• Gap junctions
• Septate junctions
• Plasmodesmata
EXTRACELLULAR COATS
• Chitin
• Mucoprotein or glycoprotein
• Mucopolysaccharide
• Basement membrane
FUNCTIONS
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Keeps the cell contents in place
Protects cell from injury
Regulates flow of materials into and out of the cell
Forms organelles within the cytoplasm
Junctions keep cells together
Infolds help in intake of materials by endocytosis
Outfolds increase surface for absorption of nutrients
Outfolds form protective sheaths around cilia and flagella
FUNCTIONS
• Receptor molecules permit flow of information
into the cell
• Oligosaccharides help in recogonising self from
non self
• Makes metabolism possible by controlling flow of
information and materials
• Permits exit of secretions and wastes by
exocytosis
• Controls cellular interactions necessary for
tissue formation and defence against microbes
• Helps in cell movement by forming pseudopodia