Fluid Mosaic System-
Plasma Membrane
The Fluid Mosaic Model states that
membranes are composed of a Phospholipids Bilayer with various
protein molecules floating around within it. The 'Fluid' part
represents how some parts of the membrane can move around freely,
if they are not attached to other parts of the cell. The 'mosaic'
part illustrates the 'patchwork' of proteins that is found in
the Phospholipids Bilayer.
All cells
are surrounded by a plasma membrane. Eukaryotic cells also contain internal
membranes and membrane bound organelles. This was originally proposed by Davison
and Daniele and later In 1972, S. Singer and G. Nicolson described the Fluid
Mosaic Model of membrane structure. „According to this model, cell membranes are composed of a lipid bilayer
(two layers) with globular proteins embedded in the bilayer.
The
membrane’s lipid bilayer is mainly 2 layers of phospholipids; the non-polar
tails point (hydrophobic) inward and the polar heads(Hydrophilic) are on the
outer surface.
As the cell membrane consists of a
mosaic of protein and lipids it is called as a fluid mosaic model.
„No cell exists as a closed system. In
order to survive, materials must be transported into and out of the cell,
across the plasma membrane. „In addition,
because different processes take place in different parts of the cell,
materials must be transported from one part of the cell to another.
The phosphor lipids are arranged in bilayers with the polar
head groups oriented towards the extra cellular side and the cytoplasmic side with
a hydrophobic core. Each leaflet is 25A thick and the total thickness is 50 to
80 A.
Some proteins in the membrane are called 'Intrinsic'. This
means that they completely span the Bilayer. Others are called 'Extrinsic'
- they are partly embedded in the Bilayer.
Here choline containing phospholipids are in the external
layers and ethanolamine and serine containing phospholipids are in the inner layer.
The lipid bilayer is said to be fluid in nature because it shows free lateral movement
of its component and their fluidity helps the membrane to perform endocytosis(import
into the cell and exocytosis(export out of the cell). („In endocytosis, part of the plasma membrane envelops small
particles or fluid, then seals on itself to form a vesicle which enters the
cell)
But flip-flop movement is
restricted that means movement of particle from inner to outer layer , or outer
to inner layer is restricted. Flip-flop movement occurs only during apoptosis. The
fluidity of the membrane is maintained by the length of hydrocarbon chain, and nature
of polar head group. Membrane also contains cholesterol. As cholesterol concentration
increases ,membrane breaks and loss of fluid in the outer surface , but more
fluid in the core.effect of cholesterol on fluidity is vary with different
temperature.
Membrane protein
The peripheral or extrinsic protein exists on the surface of
the bilayer. They are attached by ionic and polar bonds to polar heads of
lipids and they can be easily separated.
The integral (intrinsic) are deeply embedded in the bilayer and
they cannot be easily separated. Eg. Hormones
receptors.
