The plasma membrane surrounds cells and organelles, and is composed of a phospholipid bilayer with embedded proteins. It has several important functions, including acting as a selective barrier, receptor site, and anchor for cytoskeletal fibers. The membrane controls interactions between the cell and its environment, and its composition can vary between cell types. Early models proposed membranes were a lipid monolayer or bilayer, while later models incorporated proteins embedded within or attached to the lipid bilayer. The trilaminar appearance seen with electron microscopy provided evidence for a common basic structure of biological membranes.

1. Plasma Membrane
Introduction to Plasma Membrane:
All cells and many subcellular organelles are bounded by thin membranes of phospholipid bilayer. With
the help of light microscopy, it is not possible to identify the cell membrane or plasma membrane.
Scientists were able to identify the membrane with the advent of the electron microscope.
It shows that every cell is surrounded by a membrane and also the cell has a complex internal
membranous structure. Again, membranes make some compartments inside the cytoplasm to perform
some specific functions as in mitochondria, chloroplasts, lysosomes etc.
Membrane-bound enzymes also perform certain specific reactions which are needed for certain cellular
activities. Proteins present in the membrane help in the transport of certain molecules from inside and
outside of the cell.
Proteins also help in anchoring some cytoskeletal fibres to give the cell its shape. So, the membrane is a
highly differentiated dynamic structure that controls the behaviour of the cell. It is the most
multifunctional cellular structure.
Functions
a. Interactions of series of enzymatic processes for performing several cellular events and in the
production of chemical energy (ATP) by confining macromolecules in a small space.
b. It acts as a receptor site for some agents like hormones, neurotransmitters, immune proteins.
c. It helps in the conversion of signal conveyed by some extracellular agents as stated above.
d. It prevents the loss of different macromolecules.
e. It protects the cell from the uptake, of some harmful materials.
f. The cell membrane interacts with other adjacent cells in forming tissues and organs during
organogenesis and embryonic development.
Composition of Plasma Membrane:
Plasma membranes or biological membranes are composed of lipids, proteins and small amounts of
carbohydrate. The ratio of proteins to lipid varies considerably among different membranes
Phospholipids are present in almost all the membranes such as Phosphatidylcholine, Phosphatidylserine,
Phosphatidyl-ethanol-amine, Sphingomyelin.
Cholesterol is common in the membrane of mammalian cells. Cardiolipin is found only in the inner
mitochondrial membrane. The plant plasma membrane has a high sterol to phospholipid molar ratio.

2. Cholesterol and various sterol esters are found in the plant plasma membrane.
Chemical Constituents
Carbohydrates are bound to the membrane in the form of glycoproteins when attached to proteins or
glycolipids when attached to lipids. Carbohydrates are found in the membrane of eukaryotic cells. They
are not present in the chloroplast lamellae, mitochondrial membrane and other membranes of cell
organelles.
The major component of the plant plasma membrane is carbohydrate in the form of glycolipids,
glycoproteins and various cell wall polysaccharides. Although the structure and function of the plant
plasma membranes is fundamentally similar, but little work has been done on plant plasma membrane
as compared to the animal system.
Hence the nature of lipids and proteins is not clearly known in plant system. The plant cell membrane
has to perform some other functions than in animal cell, particularly in mediating the transport of
solutes into and out of the cell.Further, it has to perform in synthesizing the cell wall micro fibrils and to
transmit hormonal and environmental signals during growth and differentiation. The knowledge of the
membrane is based mainly on cells of prokaryotic and animal systems.
Morphology of the Plasma Membrane:
An eukaryotic cell of 20 μm diameter has a membrane of less than 10 nm thickness, i.e., 1/2000 of the
cell diameter. Before the invention of electron microscopy, cytologists observe, a very thin refractile
outline of the cell through a light microscope.Actually, the existence of the membrane was taken into
account on the basis of experiment on cell osmosis in plant cells as early as 1877 by Pfeffer. At that time
only the semipermeable properties of plasma membrane was known.
The structure and function of the plasma membrane is known with the refinement of the techniques
used in the Transmission Electron Microscope (TEM). The knowledge of many aspects of the plasma
membrane in plants is scanty as compared to the plasma membrane of animal and bacteria.However,
the basic structure and function of the plasma membrane is similar to that in animals, fungi and
bacteria. The plasma membrane is composed primarily of proteins and lipids in all cases. The plasma
membrane is seen as a thin wavy line around the surface of the protoplast under the electron mi-
croscope.
The tripartite structure of the plasma membrane with dark-light-dark structures can be seen at higher
magnification. The lighter structure of the membrane is about 35A° thick while the two dark layers show
thickness of 30- 35A° in each case.
Numerous small vesicles and cell organelles are also bounded by membranes. There are other cellular
bodies which are invaginations of the plasma membrane. In plant cells, the plasma membrane has a
continuity throughout the tissues by plasmodesmata.

3. Models of Plasma Membrane:
Previously, membranes were thought to be a static structure functioning only to separate the cell from
the external environment. It has now been established that membranes are involved almost in most of
the cellular activities.
Thus, the knowledge of how the different components of the membranes are organised in the plasma
membrane of different cell and cellular organelles is essential in understanding the mechanism of cell
function.
We already know that membranes are composed of lipids, proteins and small amounts of carbohydrate.
The chemical composition of the membrane is not constant for all cell types. There is considerable
variation in the amount of proteins and lipids present in the membrane structure of different organisms.
The ratio of protein to lipid varies from 80 : 20 in bacteria to 20 : 80 in some nerve cells. But in most of
the membranes the ratio is about 50 : 50. The lipid components of the membrane consists of
phospholipids, glycolipids or steroids. Due to this diversity in membrane-composition, different ideas or
models have been proposed to show the structure and organisation of membrane.
Lipid Monolayer Model of Langmuir:
The first scientific attempt to know the structure of membrane was made by Langmuir (1881-1957) who
suggested that the membrane was composed of phospholipids one molecule thick. It was shown by an
experiment in which the phospholipid was spread on water.

4. This formed a layer one molecule thick on water surface. Phospholipids are known to be amphipathic
molecule which contains both hydrophilic and hydrophobic regions.
Langmuir interpreted from his model that the hydrophilic or ‘head’ groups of the lipid molecules remain
attached to the water surface and the hydrophobic ‘tails’ remain free towards the air
Lipid Bilayer Model of Gorter and Grendel (1925):
E Gorter and F Grendel proposed a lipid bilayer model (Fig. 2.2) of membrane structure from their
experiments on Red Blood Cells. When lipids extracted from Red Blood Cells were spread on the water
surface, it was found that lipids were also spread as one layer on water. But it covers twice the area on
the water surface than that of the surface area of the cell from which the lipid is extracted.
From these observations they came to the following conclusions:
Lipid bilayer model of Gorter and Grendel
i. Lipids are present in the membrane as a bilayer.
ii. Hydrophilic head groups are towards the aqueous environments of the two membrane surfaces.
iii. Hydrophobic tails are away from the water and present in the interior of the membrane.
iv. These types of structure of lipid bilayer would be most stable.
The model of Gorter and Grendel gives a new impetus to membrane research as they first tried to
describe the structure of membrane at the molecular level.
The Danielli-Davson Model (1934):
Harvey and Danielli’s observations on surface tension experiment led doubt on the model of Gorter and
Grendel. Their results showed that the surface tension of cell membranes was higher than that of pure

5. lipids. Hence they concluded that biological membranes could not be of lipids alone.
Later, Danielli and Davson proposed a molecular model of the membrane in which hydrophilic head
groups of the lipid molecule is covered on both sides by protein layer. The proteins are attached to the
hydrophilic head groups of lipid bilayer by ionic bonds.
But, in this model, the distance between ends of the fatty acid chains (hydrophobic tails) is not specified.
Later, the observations made through polarised light and X-ray diffraction on myelin membrane by
Schmidt and others (1936, 1941) confirmed the existence of lipids as bilayer.
With the advent of electron microscopy, first visible structure of plasma membrane was noted. But the
detailed analysis of membrane structure was not possible at that time as Osmium tetroxide was used as
the only fixative in electron microscopy.
As Osmium tetroxide did not preserve membrane structure, only a single line was found on the cell
surface. Later, Robertson (1964, 1966) used Permanganate as a fixative instead of Osmium tetroxide and
showed the trilamellar structure of the biological membrane.
Robertson’s Model:
With the appearance of permanganate fixed membranes in all cell systems, a general idea has
propounded that there is a basic identical general membrane structure in all cell forms. Again, it has
been noted through electron microscope that there are two electron-dense lines separated by a lightly
stained zone.

6. As the three layers of the membrane were observed, membranes were said to have a trilaminar ar-
rangement. These trilaminar appearance of the membrane are found in prokaryotic and eukaryotic
plasma membrane, endoplasmic reticulum, mitochondrial, chloroplast and nuclear membrane. The
presence of common structure in almost all biological membranes led Robertson to postulate Unit
membrane hypothesis.
Fluid-Mosaic Model
According to this model, proposed by S.J. Singer and G Nicholson, the principle of
membrane-organisation is as follows:
i. Lipids are present in two layers.
ii. Proteins are arranged in two ways:
(a) Some jure embedded in the lipid layer, called Integral proteins, and
(b) Some are present on surface of the lipid bilayer, called the Peripheral proteins.
iii. The lipid layer is usually in liquid-crystal line, i.e., fluid state.
With the use of different sophisticated techniques, it has been established that lipid exists in the
membrane as a bilayer. This has been further confirmed by comparing the properties of natural
membrane with artificial membranes.
Physiological Function # 1. Permeability:
The plasma membrane is a thin, elastic membrane around the cell which usually allows the movement

7. of small ions and molecules of various substances through it. This nature of plasma membrane is termed
as permeability.
Osmosis
The plasma membrane is permeable to water molecules. To and fro movement of water molecules
through the plasma membrane occurs due to the differences in the concentration of the solutes on its
either side. The process by which the water molecules pass through a membrane from a region of higher
water concentration to the region of lower water concentration is known as osmosis (Gr., osmos =
pushing).
The process in which the water molecules enter into the cell is known as endosmosis, while the reverse
process which involves the exit of the water molecules from the cell is known as exosmosis.
Physiological Function # 3. Diffusion or Passive Transport:
When molecules of two kinds are placed together they tend to mix with each other by a process known
as diffusion. The diffusion of certain solutes or substances takes place through the plasma membrane.
Such diffusing solute particles require no energy for the diffusion process through the plasma
membrane. The diffusion of ions through the plasma membrane depends on the concentration and
electrical gradients.
Physiological Function # 4. Active Transport:
When the molecules or ions move through the plasma membrane from low concentration to higher
concentration, they require energy for such movement. The energy is provided by adenosine
triphosphate (ATP) which is produced by the oxidative phosphorylation in the mitochondria. The active
transport of the molecules occurs in nerve cells and kidney cells.
Through the pores of plasma membrane, some chemical compounds such as urea, formamide and
glycerol could pass actually. Brachet (1957) has shown that sometimes large molecules of certain
proteins penetrate the cell.
Physiological Function # 5. Endocytosis and Exocytosis:
The plasma membrane participates actively in the ingestion of certain large-sized foreign or food
substances. The process by which the foreign substances are taken in and digested is known as
endocytosis. In the process of exocytosis, the cells which have secretory function such as pancreatic cells
pass out their enzymatic secretions outside the cell.
(i) Pinocytosis:
When the ingestion of fluid material in bulk takes place by the cell through the plasma membrane, the
process is known as pinocytosis.

8. (ii) Phagocytosis:
Sometimes the large-sized solid food or foreign particles are taken in by the cell through the plasma
membrane. The process of ingestion of large-sized solid substances by the cell is known as phagocytosis.
Prof. Ali Goraya
M.phil Cell and Molecular Biology
agoraya34@gmail.com
cell# 03478898416