Question

In Singer and Nicolson's model, the extrinsic proteins are
A. Tightly associated with intrinsic proteins but can be easily separated
B. Loosely associated with intrinsic proteins and can be easily separated
C. Loosely associated with intrinsic proteins but cannot be easily separated
D. Tightly associated with intrinsic proteins and cannot be easily separated

Answer 1

The cell membrane can also be called a plasma membrane; it is the membrane that is found in all cells which separate the cell's interior from the outside environment. In-plant cells and bacteria, the cell wall is attached to the cell membrane (plasma membrane) on its outside surface. The cell membrane is made up of lipid bilayer which is semipermeable.

Complete answer:
The cell membrane structure is considered to be quasi fluid in structure in which all lipids and integral proteins are arranged in a mosaic fashion. The membrane has fluidity properties because of the interaction of hydrophobic lipids and proteins; in the continuous structure of phospholipid bilayer, the globular proteins are embedded. This globular protein of the membrane is of two types: extrinsic protein (peripheral) and the intrinsic protein (integral).

The extrinsic proteins are soluble and can be readily dissociated from the membrane and they are situated entirely outside the phospholipid bilayer. It is loosely associated with intrinsic proteins and can be easily separated. Whereas, intrinsic proteins are insoluble and can penetrate either part of the lipid bilayer. The integral proteins are amphipathic which means that they have hydrophilic polar heads that protrude from the membrane surface in which the non-polar regions are situated in the interior of the membrane.

Hence, the correct answer is option B.

Note: The fluid mosaic model of the biomembrane or cell membrane was proposed by SJ Singer and GL Nicolson in the year 1972. According to this model, the protein molecules are embedded in the lipid bilayer. The proteins of the membranes are intrinsic proteins of two kinds that are embedded in the cell membrane and are often called integral proteins. Extrinsic proteins are found outside the membrane of the cell and are bound to it by weak molecular forces (ionic or hydrogen bonds), and are soluble in aqueous solutions.