Select the correct statements regarding cell membrane in eukaryotic cell. I. Membrane of human RBCs has approximately 52% protein. II. Major phospholipids are arranged in a bilayer. III. Extensions of the plasma membrane into the cell form mesosomes. IV. Tails towards the inner part of lipids are hydrophobic and thus protected from aqueous medium. V. Glycocalyx is present on the outer surface of the plasma membrane. Choose the correct answer from the options given below:

Fluid Mosaic Model

திரவ மொசைக் மாதிரி

NCERT Class 11 · Cell: The Unit of Life · 1 NEET question

Stop visualizing the plasma membrane as a static wall. It is a dynamic, quasi-fluid structure where proteins drift like icebergs in a lipid sea. If the membrane were rigid, endocytosis and cell division would be physically impossible.

The idea

Proposed by Singer and Nicolson in 1972, the Fluid Mosaic Model defines the membrane as a phospholipid bilayer with integral and peripheral proteins. The quasi-fluid nature of lipids enables the lateral movement of proteins within the bilayer. This fluidity is crucial for cell growth, secretion, endocytosis, and the formation of intercellular junctions.

Why it matters

The fluid nature is essential for cellular functions like growth, secretion, endocytosis, and cell division.

NEET lens

NEET focuses on the 'lateral movement' aspect and the specific roles of integral versus peripheral proteins. Do not confuse this with the earlier Davson-Danielli model. Remember: Fluidity is the key to functionality; questions often test the relationship between lipid mobility and membrane transport.

Clinical significance

Membrane fluidity is strictly regulated by cholesterol content. In conditions like Acanthocytosis, altered lipid composition leads to 'spur cells' with distorted membranes. If the fluid mosaic integrity fails, the cell loses its selective permeability, leading to immediate osmotic lysis.

Introduction

The fluid mosaic model (Singer and Nicolson, 1972) describes the plasma membrane as a fluid phospholipid bilayer with proteins embedded in it like a mosaic.

Core concept

Phospholipid bilayer: hydrophilic heads (face water outside and inside), hydrophobic tails (face inward). Proteins: integral (span the membrane, channels/transporters), peripheral (on surface, structural/signaling). Cholesterol: between phospholipids, regulates fluidity. Glycoproteins/glycolipids: on outer surface, cell recognition.

Go deeper

Membrane fluidity depends on temperature and lipid composition. Unsaturated fatty acids increase fluidity (bent tails don't pack tightly). Cholesterol acts as 'fluidity buffer' — increases fluidity at low temp, decreases at high temp.

From Class 10

The cell membrane separates the cell from its environment. It's not just a simple barrier — it's a dynamic structure explained by the fluid mosaic model.