Cell Membrane (Plasma Membrane)
FSc Part-1 Comprehensive Biology Study Guide
1. Definition & Introduction
The Cell Membrane (also known as the plasma membrane or cytoplasmic membrane) is a thin,
delicate, elastic, and selectively permeable living outer boundary that encloses the cytoplasm of a
cell. It separates the internal metabolic machinery of the cell from its external environment,
regulating the passage of materials into and out of the cell to maintain homeostasis.
2. Historical Theories & Structural Models
Over the years, several models were proposed to explain the complex structure of the plasma
membrane:
• Unit Membrane Model (Davson-Danielli): Proposed that the cell membrane consists of a lipid
bilayer sandwiched between two continuous layers of proteins (like a sandwich). This model
was later rejected because it could not explain the dynamic transport properties and flexibility of
the membrane.
• Fluid Mosaic Model (Singer and Nicolson, 1972): The most widely accepted structural model.
According to this model, the membrane is a fluid structure consisting of a "sea" of liquid lipids in
which various proteins are embedded or floating like "mosaics". The lipid bilayer provides
fluidity, while proteins perform specific functional tasks.
3. Chemical Composition
The cell membrane is chemically composed of three major components:
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FSc Part-1 Comprehensive Biology Study Guide
1. Definition & Introduction
The Cell Membrane (also known as the plasma membrane or cytoplasmic membrane) is a thin,
delicate, elastic, and selectively permeable living outer boundary that encloses the cytoplasm of a
cell. It separates the internal metabolic machinery of the cell from its external environment,
regulating the passage of materials into and out of the cell to maintain homeostasis.
2. Historical Theories & Structural Models
Over the years, several models were proposed to explain the complex structure of the plasma
membrane:
• Unit Membrane Model (Davson-Danielli): Proposed that the cell membrane consists of a lipid
bilayer sandwiched between two continuous layers of proteins (like a sandwich). This model
was later rejected because it could not explain the dynamic transport properties and flexibility of
the membrane.
• Fluid Mosaic Model (Singer and Nicolson, 1972): The most widely accepted structural model.
According to this model, the membrane is a fluid structure consisting of a "sea" of liquid lipids in
which various proteins are embedded or floating like "mosaics". The lipid bilayer provides
fluidity, while proteins perform specific functional tasks.
3. Chemical Composition
The cell membrane is chemically composed of three major components:
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