# INTRODUCTION:

-> The outer thin membrane or the layer of living cell is known as cell membrane.

-> It is also known as plasma membrane in animal cells , which is thin, flexible, selectively permeable barrier that surrounds cytoplasm of cell.

-> In the plant cells , it is also known as plasmalemma.

-> It seperates the cell interior from the external environment and regulates movement of substances in and out of the cell.

-> It is present in all types of cell eukaryotic as well as prokaryotic.

-> It's major function includes - Protection, Transportation, Communication, and signalling.

-> The term cell membrane was given by CARL NAGELI and C. CRAMER (1885) for the membrane covering of protoplast.

# COMPOSITION OF CELL MEMBRANE:

-> Most of the cell membrane is composed of 40-50% proteins and 50-60% lipids.

* LIPIDS - Approximately 40-60% of cell membrane is made up of lipids .

-> Membrane lipids are of three types:- a) phospholipids b) glycoplipids c) steroids .

A) PHOSPHOLIPIDS - They are the main components of the cell membrane. Each molecule of phospholipid has hydrophilic head which faces outside of membrane and hydrophobic tail which faces inwards . Both of them combinely form bilayer of cell membrane.

-> They are Amphipathic in nature.

B) GLYCOLIPIDS - They are lipids with carbohydrates ( Sugar attached).

-> They are found mostly on extra cellular surface of membrane.

-> The lipid tail are embedded in membrane and the sugar moiety exposed outside of membrane.

-> These glycoplipids are often derived from sphingosine. 

-> They have hydroxyl group at one end and small hydrocarbon tail at the another end.

-> They provide fluidity and stability to the membrane.

D) PROTEINS - Approx 30-50% of cell membrane iss made up of it.

There are two types of proteins present in the cell membrane:

1) INTRINSIC/ INTEGRAL PROTEINS:

- They are embedded in lipid bilayer.

- They span across membrane ( Transmembrane proteins ).

- Main function includes transportation ( through channels and carriers) and even function as receptors.

2) EXTRINSIC/ PERIPHERAL PROTEIN:

- They are loosely attached to the membrane surface.

- Their main function is signalling, maintaining cytoskeleton an enzymatic activity.

E) CARBOHYDRATES -

- They are present on extra cellular surface.

- They are often attached to lipids and protein.( Glycoplipids and glycoprotein).

- Major function includes cell recognition, adhesion and protection.

# FLUID MOSAIC MODEL:

-> This model was given by SINGER and NICOLSON in 1972 A.D.

-> Most accepted model of cell membrane.

. MOSAIC : Different molecules like lipids, proteins, carbohydrates are arranged in mosaic like pattern.

-> Between lipids and proteins, there is interaction which results in fluidity of membrane.

-> This model suggests that cell membrane is a semi solid and dynamic structure.

-> It is also known as proteins icebergs in a sea of phospholipids.

-> Variability of cell membrane is explained.

-> It explains transport of solute and solvent through cell membrane.

-> LIPID : PROTEIN ratio supports this model.

-> The interaction is hydrophobic. 

-> Two types of globular proteins are present which are embedded in phospholipid bilayer.

-> Membrane Proteins are of various types:

A) INTRINSIC PROTEINS - 

-> Within phospholipid bilayer, these proteins are found to be embedded inside.

-> Also known as integral or tunnel proteins which are soluble in nature.

-> These proteins form a channel for the passage of water, ions, and other water soluble small sized solutes.

-> It has got two parts:

1) HYDROPHILIC HEAD: It is polar in nature.It protudes out from the cell membrane.

2) HYDROPHOBIC TAIL: It is non polar in nature . They are present inside of cell membrane facing towards center.

B) EXTRINSIC PROTEINS - 

-> They are present on the surfaces of membrane in floating form.

-> These proteins are attached to phosphoryl surface.

-> They are electrically charged too . Ionic bonds or calcium bridges help in attachment.

-> and They are loosely attached to membrane.

-> Also known as peripheral proteins.

-> They are soluble in nature and are located outside of lipid bilayer.

C) STRUCTURAL PROTEIN -

-> It helps in maintaining stability.

D) CHANNEL PROTEINS -

-> It is involved in transport of water and some dissolved substances.

E) CARRIER PROTEINS -

-> For active transport.

F) ENZYMES -

-> For different metabolic activities.

G) RECEPTOR PROTEINS -

-> For transport of hormones and conduction of nerve impulses.

# FUNCTIONS OF CELL MEMBRANE:

1) CELL RECOGNITION AND COMMUNICATION -

-> In the cell membrane, glycoplipids and glycoproteins are present . It helps in recognising the cell.

-> It is essential during the defense against microbes and tissue formation.

-> The binding of ligands to specific receptors induces signal transduction.

2) BLOOD GROUPING -

-> Based on antigens present in cell membrane, blood grouping can be done and seperated as A, B, AB, O.

3) DIGESTION -

-> In gastrointestinal tract, microvilli are present.

-> They are modified cell membrane that helps in digestion of food materials.

4) LOCOMOTION -

-> In the amoeba, pseudopodia are present for the locomotion which is modified structure of cell membrane.

5) ACTIVE TRANSPORT -

-> The carrier protein present in cell membrane helps in active transport of materials.

6) BULK TRANSPORT -

-> By process of endocytosis and pinocytosis, bulk materials are transported.

7) EXOCYTOSIS -

-> It helps in removal of waste and secretory materials.

8) OSMOSIS -

-> It shows semi permeable activity for transport of water by osmosis.

9) DIFFUSION -

-> The exchange of gases with external environment occurs by diffusion.

10) METABOLISM -

-> Cell membrane performs metabolic functions.

-> Several enzymes are present on cell surface which is involved in breakdown of extracellular nutrients.

-> Some are involved in biosynthesis of cell wall.

-> Incase of prokaryotes, respiratory enzymes are found in plasma membrane.

# PROPERTIES OF CELL MEMBRANE:

A) ASYMMETRY-  Outer and inner membrane have different composition.

B) SEMI - PERMIABILITY - Allows selective entry or exit in the cell.

C) DYNAMIC NATURE - Molecules move within membrane.

D) CELL SEALING - Automatically reseal if damaged slightly.

# SPECIALIZATION OF CELL MEMBRANE:

A) MICROVILLI - Increase surface area.

Ex: Intestine.

B) LIPID RAFTS - They are cholesterol rich domains important in signalling.

-> Receptors for hormones, neurotransmitters and growth factors.

“Made of a delicate phospholipid bilayer with embedded proteins, the cell membrane is a dynamic guardian—protecting, regulating transport, enabling communication, and maintaining the very balance of life.”