Plasma Membrane

Singer and Nicolson described the plasma membrane in 1972 as a quasi-fluid structure in which proteins are embedded within a bilayer of phospholipids, a model summarised in NCERT Class 11, Chapter 8 (Cell: The Unit of Life). The fluid nature allows lateral movement of proteins and lipids within the membrane, which is essential for processes such as cell growth, endocytosis, cell division and the formation of intercellular junctions. The option describing permanent rigidity contradicts the very name 'fluid mosaic', as rigidity would prevent the dynamic functions of the membrane. The claim of complete impermeability is incorrect because the membrane is selectively permeable, allowing controlled passage of certain molecules. The statement about random covalent bonding between lipids and water is chemically wrong, since membrane lipids are amphipathic and arrange themselves so that hydrophilic heads face water while hydrophobic tails point inward, held by non-covalent interactions. A consistency check confirms that only lateral mobility of components matches both the model's name and the observed flexible behaviour of living membranes.