The fluid mosaic model proposes that the cell membrane is composed of a fluid bilayer of phospholipids embedded with integral and peripheral proteins. Cholesterol is also present within the bilayer, helping to maintain fluidity. This model improved upon earlier theories by describing the membrane as a fluid structure with movable components, rather than a rigid sandwich. It remains the most widely accepted structure of the cell membrane.

1. MEMBRANE BIOLOGY
TOPIC: CELL MEMBRANE
SUBMITTED BY:
EVELIN GEORGE,
19BCU006,
2ND
Bsc,. BIOCHEMISTRY

2. CELL MEMBRANE
 Introduction
Plasma membrane surrounds the organelles of the cells. It forms a barrier between cell
organelles and the outside environment. A plasma membrane plays different roles
depending upon its location. For e.g., a plasma membrane encloses the cytoplasm as well
the organelles of the cell. It is a tough, rigid and flexible layer made up of proteins, lipids,
and carbohydrates.
 Plasma Membrane Function
Following are the important plasma membrane function:
 The plasma membrane forms a barrier between the cell organelles from the outside
environment.
 It allows only a certain molecules to pass through it.
 It facilitates communication and signalling between the cells.
Key Points on Cell Membrane and Plasma Membrane
The cell membrane is a type of plasma membrane that encloses the cell and all its
components.
 Both the membranes are selectively permeable and regulate the entry and exit of
components.
 The cell membrane is the only membrane involved in cytokinesis.
Plasma membrane and cell membrane are often confused to be similar terms. The plasma
membrane encloses the organelles of the cell, whereas, the cell membrane encloses the
entire cell components.

3.  DEVELOPMENT IN THE THEORIES
Charles Overton was the first one to suggest that cell membrane is made up of
lipids. In the last years of the 19th century Overton did experimental work,
allowing the distinction to be drawn between the cell wall of plants and
their cytoplasmic membrane.He studied the permeability of a range of biological
materials to around 500 chemical compounds. In 1900, Overton proposed a
biomembrane model "Overton Biomembrane Model" which stated that
biomembranes are made up of lipids. He gave this statement on the basis of
observation of transport of lipid soluble substances across the biomembranes.
The next development came in the year 1935 when James Danielli and Hugh
Davson stated that some protein molecules are also sprinkled on the surface
of the membrane. This theory is famously called the Sandwich Theory or
more technically called the Lamellar Theory.
Later on, David Robertson came up with the Robertson model. Named after
the proposer David Robertson, his model was based on the observation of a
red blood cell. He found that it had 3 layers: 2dark staining layers and 1 light
staining layer in between. So the outer layers were assumed to be the
protein layers and the inner one, the lipid bilayer. Therefore this supported
the Lamellar or Sandwich Theory. But David Robertson went one step further
in the protein front and suggested that proteins are not necessarily globular.
According to the hypothesis, there is a membrane consisting of a
phospholipid bilayer sandwiched between two protein monolayers. David
Robertson also said that all the various membranes that are in a cell are of
just two types: unit membranes or multiples of these unit membranes.
Examples of unit membranes are Endoplasmic Reticulum, the Golgi, the
Lysosomes, etc. and examples of double unit membranes are Mitochondria,
Plastid, etc.

4. THE THREE FAMOUS MEMBRANE THEORY
BEFORE FLUID MOSAIC MODEL
1.Lamellar Theory:
“Sandwitch model” proposed by Danielli and Davson (1935), the plasma membrane
exhibit trilaminar structure i.e., a lipid layer is bounded by protein layer on both
the sides. In this model, proteins are usually represented in globular form.
Subsequently electron microscopic studies favour the idea of trilaminar
structure of plasma membrane.
2. Unit Membrane Theory:
The universal occurrence of trilaminar pattern of membranes of cells led
Robertson to propose in 1959, the concept of a ‘unit membrane’ with a trilaminar
appearance. The unit membrane forms not only plasma membrane but also
contains other membranes in most of the cell organelles.All the unit membranes
consist of a bimolecular layer of phospholipids covered by a layer of proteins.
Phospholipid is the main part of lipid. For example, lecithin, cephalin etc.

5. Besides it, other lipids like cholesterol, cerebroside and ganglioside as well as
some polysaccharides; it is likely that these molecules provide stability to plasma
lemma. Phospholipids are oriented with their fatty acid groups towards each
other and their water soluble ends (polar groups) facing outwards.
3. Micellar Theory:
Hilleir and Hoffman (1963) proposed this theory. According to them the plasma
membrane includes a mosaic of globular subunits (micelles) which are closely
packed together having a central core of lipid molecules with hydrophilic (water
loving) polar end. Each lipid micelle measures about 40-70 A in diameter.

6. FLUID MOSAIC MODEL: THE BEST ACCEPTED
MODEL TILL DATE
The fluid mosaic model was proposed by S.J. Singer and Garth L. Nicolson. This
model explains the structure of the plasma membrane of animal cells as a mosaic
of components such as phospholipids, proteins, cholesterol, and carbohydrates.
These components give a fluid character to the membranes. Components of
plasma membranes are
 Cholesterol – It is located between the phospholipids and phospholipid
bilayers. It helps the plasma membrane to retain the fluidity and prevents
the compaction of hydrophilic tails at low temperatures and their expansion
at high temperatures.

7.  Phospholipids – Phospholipids are the main fabric of the plasma membrane.
amphipathic molecules with a hydrophilic head and a hydrophobic tail and
attached to a glycerol molecule by a covalent bond.
 Proteins – The plasma membrane has three types of proteins they are
(1) Integral Proteins: These proteins form channels to allow the movement of
large molecules and ions across the hydrophobic layer of the membrane.
(2) Peripheral Proteins: These are found embedded in a single leaflet of the
membrane. They carry signals from one segment of the membrane and relay it to
the another.
(3) Glycoproteins: They stabilize the membrane and are responsible for
intercellular communication.
 Carbohydrates – These are attached to proteins on the outside membrane
layer.
REFERENCE:
 BOOKS OF LEHNINGER AND SATHYANARAYANA
 WEBSITE OF NCBI, BYJU’S, SCIENCE DIRECT