Question

Singer and Nicholson’s model of plasma membrane differs from Robertson’s model in the
A. Number of lipid layers
B. Arrangement of lipid layers
C. Arrangement of proteins
D. Absence of protein in Singer and Nicolson’s model

Answer 1

Hint: Biomembranes are trilaminar structures. Different models were given to explain its structure, out of which ‘Singer and Nicholson model’ is the most accepted one. The Robertson model has two continuous layers of proteins, in between which lipid layers are present, whereas, in the Singer and Nicholson model, proteins are of two types and they differ in the arrangement.

Complete answer:
’Unit Membrane model’ or ‘The Robertson Model’ was given in 1959.
It is a trilaminar model that comprises the electron transparent lipid bilayer sandwiched between dense protein layers. All the membranes have the same basic structures but they have their distinctive composition. Example: internal organelles have more protein than lipids in eukaryotic cells whereas plants and bacterias have more lipids than proteins. These membranes may exist as a single membrane or multiple layers of unit membrane (double membrane in mitochondria or nucleus). This model does not explain the membrane’s stability and permeability.

Fluid mosaic Model/ Singer and Nicolson Model (1972):
It is a trilaminar, lipoprotein model. Phospholipids form a bilayer in the centre and show two types of movements: transition and flip flop. The globular proteins are present in a mosaic pattern (proteins form iceberg in a sea of lipids). There are two types of proteins: extrinsic proteins and intrinsic proteins. Due to quasi fluid state, biomembranes can undergo dynamic changes. As proteins in the Robertson model have two continuous layers, in between which lipid layers are present, whereas, in the Singer and Nicolson Model, proteins are of two types and their arrangement differs.

So, the correct answer is option C.

Note: A Biomembrane is a semi-permeable structure that covers the cell and organelles, which separate them from the external environment. Biomembranes that cover the cell are called cell membrane/plasmalemma and biomembranes which cover the organelles are called a subcellular membrane.