# INTRODUCTION:

-> The outer thin membrane or the layer of living cells is known as the cell membrane. It is also known as the plasma membrane in animal cells, which is a thin, flexible, selectively permeable barrier that surrounds the cytoplasm of a cell. In the plant cells, it is also known as the plasmalemma.

-> It separates the cell interior from the external environment and regulates the movement of substances in and out of the cell. It is present in all types of eukaryotic and prokaryotic cells.

-> The term cell membrane was given by CARL NAGELI and C. CRAMER (1885) for the membrane covering of the protoplast. Its major function includes - Protection, Transportation, Communication, and signalling.

# WHAT IS THE CELL MEMBRANE MADE OF?

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

* LIPIDS - Approximately 40-60% of the cell membrane is made up of lipids. Membrane lipids are of three types:- a) phospholipids b) glycoplipids, and c) steroids.

A) PHOSPHOLIPIDS - They are the main components of the cell membrane. Each molecule of phospholipid has a hydrophilic head which faces outside the membrane and a hydrophobic tail which faces inwards. Both of them combinely form a bilayer of the 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 the membrane. The lipid tails are embedded in the membrane and the sugar moiety is exposed outside of the membrane. These glycoplipids are often derived from sphingosine. 

D) PROTEINS - Approx 30-50% of the cell membrane is made up of it. There are two types of proteins present in the cell membrane:

1) INTRINSIC/ INTEGRAL PROTEINS:

- They are embedded in the lipid bilayer. They span across the membrane ( Transmembrane proteins). The main function includes transportation ( through channels and carriers) and even functions as a receptor.

2) EXTRINSIC/ PERIPHERAL PROTEIN:

- They are loosely attached to the membrane surface. Their main function is signalling, maintaining the cytoskeleton, and enzymatic activity.

#  SOME SPECIALISED PROTEINS ARE:

A) Channel Proteins — transport water and dissolved substances.

B) Carrier Proteins — power active transport.

C) Receptor Proteins — receive hormonal and neural signals.

D) Enzymes — catalyse metabolic reactions at the cell surface.

E) Structural Proteins — maintain membrane shape and stability.

E) CARBOHYDRATES -

- They are present on extra cellular surface. They are often attached to lipids and proteins. ( Glycoplipids and glycoproteins). Major function includes cell recognition, adhesion and protection.

# FLUID MOSAIC MODEL: How scientists visualise it 

-> This model was given by SINGER and NICOLSON in 1972 A.D.The most accepted model of the cell membrane. It is also known as protein icebergs in a sea of phospholipids.

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

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

This model suggests :

-> That the cell membrane is a semi-solid and dynamic structure.

-> Variability of the cell membrane is explained.

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

-> LIPID: PROTEIN ratio supports this model.

-> The interaction is hydrophobic. 

# WHAT DOES THE CELL MEMBRANE ACTUALLY DO?

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 defence against microbes and tissue formation. The binding of ligands to specific receptors induces signal transduction.

2) BLOOD GROUPING -

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

3) DIGESTION -

-> In the gastrointestinal tract, microvilli are present. They are modified cell membrane that helps in the digestion of food materials.

4) LOCOMOTION -

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

5) ACTIVE TRANSPORT -

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

6) BULK TRANSPORT -

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

7) EXOCYTOSIS -

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

8) OSMOSIS -

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

9) DIFFUSION -

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

10) METABOLISM -

-> The cell membrane performs metabolic functions. Several enzymes are present on the cell surface that are involved in the breakdown of extracellular nutrients. Some are involved in the biosynthesis of the cell wall. In the case of prokaryotes, respiratory enzymes are found in the plasma membrane.

# KEY PROPERTIES OF CELL MEMBRANE:

A) ASYMMETRY- The outer and inner membranes have different compositions.

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

C) DYNAMIC NATURE - Molecules move within the membrane.

D) CELL SEALING - Automatically reseals if damaged slightly.

# SPECIALISED STRUCTURE 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.”