 The Plasma membrane or the cell
membrane is a thin, biological
membrane present in all eukaryotic and
prokaryotic cells that forms a boundary
between the cell and its environment
and regulating the flow of materials in
to and out of cell.
 The cells maintain an approprite
amount of all molecules within them to
function effectively.
 So, this plasma membrane acts as a
semi permeable membrane allowing
the entry and exits of certain materials.
 It is like a gaurd at a gated community
who inspects those who enter and
leave, to make sure that only people
and things needed in the community
are there.
 The cell membrane was discovered by
a Swiss botanist Carl Naegeli and C
Carmer in 1855.
 Despite the existance microscopes
from 1600, no-one thought that the cell
membrane existed because all they
could see was the cell wall.
 Carl Naegeli and Carmer noted that the
surface of the cell was not continuous
and that it was impermeable to
pigments added to the solution around
the cell.
 They also found that the photoplasmic
surface was more dense and viscous
when compared to the cytoplasm.
 They called this surface as the plasma
membrane.
• The first insight into chemical nature of
the membrane was obtained by Ernst
Overton in 1890s.
• He knew that the nonpolar solutes
dissolved very easily in the non polar
solvents than polar solvents and the
polar solutes had opposite solubility.
• So, he realised that the substances
entering the cell had to be dissolved in
the outer boundary of cell is due to
lipids.
• Irvin Langmuir, in 1917, during his
research in nature of oil film, found that
the membrane was made of monolayer
of lipids and they were arranged
vertically with hydrocarbon chains
away from water and carboxyl groups in
contact with the surface of water.
• This finding was a key in understanding
the lipid bilayer and cell membrane
structure.
• The two Dutch scientists E. Gorter and
F. Grendel in 1925 were the first to find
that the membrane was made of two
layers of lipids ( lipid bilayer) with
hydrophilic heads and hydrophobic
tails, but they could not explain about
the solute permeability or the surface
tension.
• In 1935, Hugh Davson and James
Danielle proposed that the membrane
is made of lipid bilayer and on both
outer and inner surface there was a
lining of globular proteins.
• In 1950 they found that selective
permeability was because of the
presence of protein lined pores within
the lipid bilayer, which allowed the
passage of polar solutes and ions into
and out of cell.
• It was in 1972 that S. Jonathan singer
and Garth Nicholson proposed the
Fluid Mosaic Model which is
considered as the central dogma of
membrane biology.
• It describes the structure of cell
membrane as a lipid bilayer with
proteins embeded in it and which is
free to move laterally within the
membrane.
• It was first proposed by S. J. Singer
and G. Nicholson in 1972 to describe
the structure of the plasma membrane.
• The fluid mosaic model describes the
plasma membrane as that which
surrounds the cell, which is made up of
two layers of phospholipids and at
body temperature is fluid.
• Embedded within this membrane is
variety of protein molecules that acts
as channels and pumps.
• It contains carbohydrates, cholesterol
and other lipids.
• The protein and other substances such
as cholesterol become embedded in
the lipid bilayer, giving the membrane
the look of the mosaic.
• Since the plasma membrane has the
consistency of vegetable oil at body
temperature, the proteins and other
substances are able to move freely
laterally.
• That is why the plasma membrane is
described as a fluid mosaic model.
• The fulid mosaic model thoery thereby
states that plasma membrane structure
is a lipid bilayer with mosaic of
proteins embedded in it and moves
freely parallel to the surface of the
membrane.
• The fluidity of lipid bilayer was shown
by the technique of fluorescence
recovery.
• The fluorescent dye is used to tag the
lipids and a high density laser beam is
used to bleach the dye in a tiny spot on
the cell surface.
• When observed under fluorescent
microscope, it is seen that within
seconds the bleached spot became
fluorescent again.
• This explained the lateral diffusion of
phospholipids.
1. MEMBRANE LIPIDS
2. MEMBRANE PROTEINS
1. MEMBRANE LIPIDS
PHOSPHOLIPIDS
CHOLESTEROL
GLYCOLIPID
2. MEMBRANE PROTEINS
INTEGRAL MEMBRANE
PROTEINS
PERIPHERAL PROTEINS
GLYCOPROTEINS
• To maintain cell functions, many
biological molecules enter and leave
the cell.
• All materials that the cell gets from its
environment or sends to the
environment, Passes through this
semipermeable plasma membrane.
• Membrane transport is esseltial for
cellular life.
 Chemicals that can pass through the
membrane are:Water
Carbon dioxide
Oxygen
Small polar molecules such as ammonia
Lipids such as cholesterol
 Chemicals that cannot pass through the
membrane are:All ions including hydrogen ions
Large polar molecules like glucose
Aminoacids
Macromolecules such as proteins,
polysachharides
Small uncharged polar molecules like
water, urea, ethanol, have an exceptions as
they can diffuse through the lipid bilayer.
There are certain factors that affect the
diffusion across the cell membrane:
Size of solute
Solute polarity
Temperature
Lipid solubility
The substances to be moved binds to these
proteins and this complex will bind to a
receptor site and then be transported across
the membrane.
This process does not require energy as
molecules are moving down the concentration
gradient.
Polar and charged solutes such as glucose,
fructose, galactose and some vitamins are
transported by facilitated diffusion.
A solution with lower solute concentration
than inside of cell is called hypotonic
solution.
It causes the cell to swell and burst as it
causes movement of water to inside of cell.
A solution with higher solute concentration
than inside of cell is called hypertonic
solution.
This causes osmosis of water from inside of
cell to outside leading to shrinkage of cell.
Eg, transportation of sodium out of cell and
potassium into the cell.
 There are two forms of active transports:-
 When the process uses chemical energy in
the form of ATP, redox energy or photon
energy to transport substances across the
membrane, it is called primary active
transport.
 The energy is derived directly from the
breaskdown of ATP or some other high
energy phosphate compounds.
 The proteins act as pumps to transport
ions.
 Most of the enzymes that perform this
transport are transmembrane ATP-ase.
 A primary ATP-ase which is universal to all
animal cells is sodium- potassium pump
which maintains the cell potential.
 When the process uses electrochemical
gradient to transport substances, it is called
secondary active transport.
 Here the energy is derived secondarily from
energy that has been stored in the form of
ionic concentration differences between the
two sides of a membrane, created in the first
place by primary active transport.
 The pore forming proteins act as channels
across the cell membrane for transporting
substances.
 The energy stored in Na+, H+ concentration
gradient is used to transport other solutes
or ions.
 This pump is called a P-type ion pump
because the ATP interactions phosphorylate
the transport protein and causes a change in
its confirmation.
 It is an antiporter enzyme located in the
plasma membrane of the cells, which
transport potassium ions from the extra
cellular fluid to the cytoplasm and sodium
ions from the cytoplasm to outside of the
cell.
 The pump is present in all the cells of the
body, and it is responsible for maintaining
the sodium and potassium concentration
difference across the cell membrane as well
as establishing a negative electrolyte
potential inside the cells.
 It was discovered by Danish scientist Jens
Christian Skou in 1950.
 It was investigated by the passage of
radioactively labelled ions across the plasma
membrane.
 It showed that the sodium and potassium
ions on both sides were interdependent
which suggested that the same carrier
protein transported both the ions.
 This carrier protein is a complex of two
globular proteins namely αsubunit
andβsubunit which has receptor sites for
transport of three sodium ions out of cell for
every two potassium ions pumped in.
4. Now, two potassium ions binds at the
receptor sites present on the portion of
protein that is near to outside of the
carrier protein.
5. The ATP is then activated and the energy
released causes confirmational change in
the protein causing potassium ions to be
released into the cell.
6. The returns to its first stage-steady to
receive new sodium ions, so that the
cycle can begin all over again.
• It is the movement of substances out of the
cell in the form of the secondary vesicles,
which fuses with the plasma membrane and
then releases its contents into the
extracellular fluid.
• It is important in the expulsion of waste
materials out of the cell, and the secretion of
enzymes and hormones.
• Neurotransmitters, digestive enzymes,
hormones are released from cell by
exocytosis.
• It is the movement of substances from extra
cellular fluid into cell in the form of vesicles.
• The large polar molecules that cannot pass
through the plasma membrane enters the cell
by endocytosis.
• This process requires energy in the form of
ATP.
Pinocytosis
 It attracts the substance tobe absorbed
by forming a membrane depression or a
coated pit on the membrane.
 When sufficient molecules have been
attracted, the pocket will pinch off
forming a coated vesicle in the
cytoplasm.
 Inside the cytoplams the vesicle shed off
their coats and then fuse with other
membrane bound structures releasing
their contents.
 E.g, Uptake of iron, cholesterol by the
cell occurs by receptor mediated
endocytosis.
• Cell junction is a type of structure that
exists in the tissues and organs.
• It is a multi-protein complex that
occurs between the neighbouring cells
which helps in communication between
them.
• There occurs a specialized
modification of the plasma membrane
at the point of contact, forming a
function or a bridge.
• Also known as occluding junction, is
the closest contact between adjacent
cells providing a tight seal, preventing
the leakage of mlecules cross the cells.
• It is found just beneath the apical
region (portion of cell exposed to
lumen is apical surface) of cell around
the cell circumference.
• Since they are tight seals limiting the
passage of molecules and ions, most
materials actually enter the cells by diffusion
or active transport.
• The tight junction is formed by proteins
called claudins and occludins which are
arranged in strands along the line of junction
creating a tight seal.
• It is usually seen in epithelial cells, ducts of
liver, pancreas and urinary bladder.
• These are protein complexes that occur
at cell to cell junction in epithelial and
endothelial tissues which provides
strong mechanical attachments
between adjacent cells.
• It is built from proteins cadherins and
catenins.
• The cytoplasmic face of the cell has actin
filaments and these actin bundules of one
cell joins with the actin bundles of the
neighbouring cells providing a strong
mechanical attachment.
• The space between the neighbouring cell
membranes are about 20-25 nm.
• This kind of junction is seen in heart muscles
and they hold the cardiac muscles together
when it expands and contracts.
• They are specialized intracellular
channels which are brought into
intimate contact with a gap of about 2-3
nm between the adjacent cells.
• They directly form a connection
between the cytoplasm of adjacent cell
so that molecules, ions, electrical
impulse pass directly from cell to cell.
• The intracellular channels are like hollow
cylinders and they are called as connexons.
• These connexons are madeup of proteins
called connexin.
• The two adjcent connexons form a
hydrophilic channel of 3 nm diameter and it
is through this channel that the ions and
molecule pass.
• Gap junction is seen in muscles and nerves.
In heart tissue helps in regular heart beat, in
brain it is seen in cerebellum and it helps in
muscular activity.
• These are intracellular junctions which form
a strong adhesion between adjacent cells.
• It enables the cell to resist any stress.
• The intermediate filaments (presents
intracellularly) of adjacents cells join with
eachother to form the strong adhesions so
that they can function as a single unit.
• They are usually seen in orgns subjected to
mechanical stress like skin, heart and neck
of uterus
• It is a type of cell junction seen in plants.
• These are microscopic channels that connect
the cytoplasm of adjacent cells.
• It penetrates through the cell wall and it
provides an easy route for movement of ions,
small molecules like RNA and proteins.