Plasma Membrane – Structure, Models, and Functions

🔹 Definition

The plasma membrane is the outer living boundary of the cell that separates the internal contents from the external environment. It is selectively permeable, allowing only certain substances to pass through, thus maintaining homeostasis.

🔹 Origin and Discovery

The concept of a membrane was first suggested in the late 19th century.
Charles Ernest Overton (1895): Proposed that membranes are made of lipids because lipid-soluble substances enter cells more easily.
Gorter & Grendel (1925): Experimentally proved that the plasma membrane is a lipid bilayer by studying erythrocytes.
J. David Robertson (1959): Coined the concept of the Unit Membrane (trilamellar structure).
Singer & Nicolson (1972): Proposed the Fluid Mosaic Model, which remains the most accepted model.

🔹 Literal Meaning and Term

“Plasma” → Greek word plásma = “something molded or formed.”
“Membrane” → Latin word membrana = “a thin skin.”
The term “Plasma Membrane” was first used by Cortical biologists in the early 20th century, later standardized in modern cell biology.

🔹 Key Structural Features

Thickness: ~7–8 nm.
Basic structure: Lipid bilayer.

Molecules present:
1. Phospholipids (amphipathic: hydrophilic head + hydrophobic tail).
2. Proteins:
Fibrillar proteins (rigid, extended → early models).
Globular proteins (dynamic, floating → fluid mosaic model).

Cholesterol (maintains fluidity in eukaryotes).
Carbohydrates (as glycolipids and glycoproteins → glycocalyx).

🔹 Models of Plasma Membrane

1. Bilamellar (Lipid Bilayer) Model – Gorter & Grendel (1925)

Lipids arranged in bilayer.
Heads outward, tails inward.
✅ First proof of bilayer.
❌ Ignored proteins.

2. Trilamellar (Sandwich) Model – Danielli & Davson (1935)

Protein–Lipid–Protein sandwich.
Fibrillar proteins covering bilayer.
✅ Explained selective permeability.
❌ Proteins are not static → outdated.

3. Unit Membrane Model – Robertson (1959)

All membranes have universal trilamellar structure.
✅ Supported by electron microscopy.
❌ Could not explain membrane-specific properties.

4. Fluid Mosaic Model – Singer & Nicolson (1972)

Most accepted model today.
Membrane = fluid lipid bilayer + mosaic of globular proteins.
Explains: transport, receptors, signaling, enzymatic activity.
✅ Supported by freeze-fracture & labeling studies.

🔹 Special Terms Related to Plasma Membrane

Amphipathic molecules: Molecules with dual nature (hydrophilic head + hydrophobic tail).
Fibrillar proteins: Extended, rigid proteins (early models).
Globular proteins: Round, mobile proteins (modern model).
Glycocalyx: Outer sugary coat (glycolipids + glycoproteins), functions in recognition and immunity.

🔹 Functions of Plasma Membrane

Selective permeability – regulates entry/exit of molecules.
Transport: Passive (diffusion, osmosis), active transport.
Cell recognition: Glycoproteins & glycolipids as ID markers.
Communication: Receptor proteins for hormones & signals.
Attachment: Anchors cytoskeleton and extracellular matrix.
Protection: Glycocalyx acts as a protective coat.
Enzymatic functions: Membrane enzymes catalyze reactions.
Flexibility: Provides shape, allows endocytosis & exocytosis.

🔹 Techniques to Study Plasma Membrane

Transmission Electron Microscopy (TEM).
Scanning Electron Microscopy (SEM).
Confocal Microscopy.
Total Internal Reflection Fluorescence Microscopy (TIRFM).
Atomic Force Microscopy (AFM).
X-ray Crystallography.
Lipidomics (Mass spectrometry of lipids).
FRAP (Fluorescence Recovery After Photobleaching).

🔹 Exam-Oriented Summary

Discovered by: Overton (1895, lipid nature).
Lipid bilayer proved by: Gorter & Grendel (1925).
Term “Unit Membrane” by: Robertson (1959).
Most accepted model: Fluid Mosaic Model (Singer & Nicolson, 1972).
Thickness: 7–8 nm.
Special features: Amphipathic phospholipids, cholesterol, proteins (fibrillar + globular), glycocalyx.
Functions: Selective barrier, transport, recognition, signaling, enzymatic activity, protection.