Cell Membrane- Definition, Composition, Structure, Functions

1. Cell membrane- Definition, Composition, Structure,
Functions
Mr. Prabhat Kumar
Singh
Ph.D. research scholar
Department of Molecular Biology And Biotechnology, ANDUA&T,
Kumarganj, Ayodhya-224229

2.  Membranes are lipid structures that separate the contents of the compartment
they surround from its environment.
 Plasma membranes separate the cell from its environment while other
membranes define the boundaries of organelles and provide a matrix upon
which complex chemical reactions can occur.
 The plasma membrane, also known as the cell surface membrane or
plasmalemma, defines the boundary of the cell.
 It is a phospholipid bilayer with embedded proteins that encloses every living
cell.
 It regulates the movement of materials into and out of the cell and facilitates
electrical signaling between them.
Cell Membarne (Plasma membrane)

3.  It is said to be semi-permeable because it allows certain molecules but not
others to enter into the cell.
 It serves some specific functions such as controlling the flow of nutrients
and ions into and out of the cells, mediating the response of a cell to external
stimuli (a process called signal transduction), and interacting with bordering
cells.
 The term cell membrane was given by for the
membrane covering of the protoplast.

4. There are two types of cell membrane. They are:
1. Cytoplasmic membrane: which surrounds the whole protoplasm.
2. Internal membrane: which surrounds various cellular organelles and vacuoles.
 Inside constituents are proteins, carbohydrates and, nucleic acids. They are usually
large in size. They are soluble in water and consists of charge.
 The outside environment of the cell is a bit different. The cell needs the nutrients
too for its growth and development. Such nutrients along with the toxic substances
which are harmful to the cell may be present in the outside environment.
 There can be the presence of ions, acids, and alkalis.
 Here the cell membrane acts as the barrier or like the checkpoints.
 The small molecules, solute, and lipid-soluble can go inside the cell membrane but
the large molecules, water-soluble substances can’t go inside.

5.  It is impermeable to them. So it has got various mechanism by which such
substances can be imported and exported.
 Different types of transport systems are available like active transport and
passive transport.
 Similarly, there is the presence of facilitators and pumps.
 It allows only those things essential for cells to go inside whereas it limits the
entry of such toxic substances.
 All the waste products of the cell are made exit to the outside.
 Cell membrane helps to maintain homeostasis.
 It provides protection to all the internal organelles of the cell.

6. Composition of Cell membrane
 Most of the cell membrane is composed of 40-50 % protein and 50-60 % lipids.
 Membrane lipids are of three types: a) Phospholipids b) Glycolipids c) Steroids
 In the different membrane, the proportion of the lipid varies:
The composition of the plasma membrane is:
 Phospholipids- 55%
 Glycolipids- 5%
 Steroids- 20%
 Other lipids- 20%
The composition of the bacterial membrane:
 Cholesterol- 70%
 Phospholipids- 30%

7. Structure of the cell membrane
The structure of the cell membrane is explained by the different models. They are as
follows :
1. Danielli-Davson Trilaminar sandwich model
2. Fluid mosaic model of the Singer and Nicolson
3. Unit membrane model of Robertson
Fig: Structure of the cell (Plasma) membrane

8. 1. Danielli-Davson Trilaminar sandwich model
 It was proposed by in the year 1935.
 This model suggests the cell membrane as the solid and the stable structure.
 Four molecular layers are present in it i.e. two phospholipids and two protein layers.
 It consists of the phospholipid. It is based on the basis of physical and chemical
properties of the plasma membrane.
 The plasma membrane is considered to be a protein-lipid-protein sandwich. The
plasma membrane is of three-layer.
 The middle one is the phospholipid bilayer.
 Outer and inner denser protein is a monolayer.
 Protein and lipid are present in the P-L-L-P pattern.
 Protein molecules contain both polar and non-polar side chains.

9.  They are globular and aid in giving stability to the plasma membrane.
 Each phospholipid molecule is a polar molecule and consists of the
hydrophilic and the hydrophobic end.
 Head (glycerols) of the phospholipid molecule: They are directed in the
opposite directions.
 They are associated with the protein molecules by the Hydrogen bond, ionic
bond, and electrostatic force of attraction.
 Tail end ( Fatty acids): They are held together by the weak Vander wal force.
They lie facing each other.
 The plasma membrane is a porous membrane having microscopic pores of 7-
10 A.

10. Fig: Danielli-Davson Trilaminar sandwich model

11. Drawbacks of Danielli-Davson Trilaminar sandwich model
 The cell membrane is the dynamic structure but not the stable
structure.
 The cell membrane is not a solid structure. But rather it is the
semisolid (quasifluid) structure.
 It doesn’t explain the functional specificity and variability in the
biomembrane.
 It doesn’t explain the movements like active transport and movement
of the water-soluble substances.
 Active and bulk transport of material through the membrane cannot be
explained.

12. 2. Fluid mosaic model of the Singer and Nicolson
 It was proposed by Singer and Nicholson in the year 1972 AD.
 This model suggests that the cell membrane is a quasifluid (semi-solid) and
dynamic structure.
 This model is also known as the protein icebergs in a sea of phospholipids.
 In this model, the arrangement of the lipids and the integral proteins is in a
mosaic style.
 Between the lipids and the proteins, there is interaction which results in the
fluidity of the membrane.
 The interaction is hydrophobic. Two types of globular proteins are present
which are embedded in the phospholipid bilayers.

14.  Membrane proteins are of two types:
 Intrinsic protein
 Extrinsic protein
Intrinsic protein:
 Within the phospholipid bilayer, those intrinsic proteins are found to be embedded
inside.
 Intrinsic or integral proteins or tunnel proteins are soluble in nature. Integral proteins
form a channel for the passage of water, ions, and other water-soluble small-sized
solutes.
 It has got two parts:
Hydrophilic head: It is polar in nature. It protrudes out from the cell membrane.
Hydrophobic tail: It is non-polar in nature. They are present inside of the membrane
facing towards the center.

15. Extrinsic protein
 They are present on two surfaces of the membrane in the floating
form.
 Extrinsic proteins are attached to the phosphoryl surface.
 They are electrically charged too. Ionic bonds or calcium bridges help
in the attachment.
 They are loosely attached.
 They are also called peripheral proteins.
 They are located outside of the lipid bilayer.
 They are soluble in nature.

16. Fig: Structure of Intrinsic protein and Extrinsic protein

17. Types of Membrane Proteins
Membrane protein are of five types:
 Structural protein: It helps in maintaining stability.
 Channel protein: It is involved in the transport of water and some
dissolved substances.
 Carrier proteins: For active transport
 Enzymes: For different metabolic activities
 Receptor protein: For transport of hormones and conduction of
nerve impulses.

18. Advantages of the fluid mosaic model
 This model explains the dynamic and quasifluid structure.
 Variability of the cell membrane is explained.
 It explains the transport of solute and solvent through the cell membrane.
 It explains the active and bulk transport of material through the cell
membrane.
 Lipid: protein ratio supports this model.
This fluid mosaic model is the scientific and widely accepted model of the cell
membrane.

19.  J.D. Robertson proposed the unit membrane model of the plasma
membrane in 1959 A.D. The Biomembrane model proposed by
Robertson stated that these are made up of three layers and out of these
three layers, two are protein layers and one is a phospholipid layer.
The unit membrane model has the following features:
1. A biomembrane is a lipoproteins (having fat and protein) structure and
trilaminar (three layers).
2. Protein molecules are extended and not globular
3. Unit Membrane Model of Robertson

20. 3. Each phospholipid (two fatty acids, a phosphate group, and a glycerol
molecule) is a polar molecule (attracting something) with a hydrophilic
(water-loving) head of glycerol and hydrophobic (water-hating) tail of two
fatty acid chain.
4. Heads of phospholipid molecules are directed in opposite directions while the
tails of the two layers face each other.
 Additional Information: -Both the Danielli and Davson model and
Robertson’s model of the unit membrane are collectively called lamellar
models.
 Although each of these membranes may serve different functions, they all
share two common properties: they exhibit both a selective permeability to
lipid solvents as well as a high electrical resistance.

21. Fig: Unit Membrane Model

22. Cell recognition and communication:
 In the cell membrane, glycolipids and glycoproteins are present. It helps in
recognizing the cell.
 It is essential during the defense against microbes and tissue formation.
 The binding of the ligands to the specific receptors induces signal transduction.
Blood grouping:
 Based on the antigens present in the cell membrane, blood grouping can be
done and separated as A, B, AB, O.
Functions of the cell membrane

23. Digestion:
 In the gastrointestinal tract, microvilli are present.
 They are the modified cell membrane that helps in the digestion of food
materials.
Locomotion:
 In the amoeba, pseudopodia are present for locomotion which is the modified
structure of the cell membrane.
Active transport:
 The carrier protein present in the cell membrane helps in the active transport
of materials.

24. Bulk transport:
 By the process of endocytosis and pinocytosis, bulk materials are transported.
Exocytosis:
 It helps in the removal of the waste materials and the secretory materials.
Osmosis:
 It shows semi-permeable activity for the transport of water by osmosis.
Diffusion:
 The exchange of gases with the external environment occurs by diffusion.
 Diffusion is of two types: simple diffusion and facilitated diffusion.

25. Metabolism:
 Cell membrane performs metabolic functions.
 Several enzymes are present on the cell surface which is involved in the
breakdown of extracellular nutrients.
 Some are involved in the biosynthesis of the cell wall.
 Incase of prokaryotes, respiratory enzymes are found in the plasma
membrane.