The thickness of unit membrane is
A. 20 Angstrom
B. 35 Angstrom
C. 55 Angstrom
D. 75 Angstrom

Hint: The unit membrane is the limiting membrane which limits the internal environment of cells and various organelles from the external environment. It is a three-layered structure consisting of an inner lipid layer and two outer protein layers. It also is a fluid phospholipid bilayer with intercalating proteins.

Complete answer:
‘Robertson's unit membrane model’ was proposed by the American bio-scientist J. David Robertson. According to his model, Robertson stated that there are three layers in bio membranes. Out of these three layers, two layers are protein layers and one layer is the phospholipid layer.
In the laboratory, electron bombardment was used on membranes by Robertson and he studied two distinct zones. These two zones which were observed, are the electron-light zone or electron-transparent zone and the other is the electron-dense zone.
In this model, another scientist Brown proposed that there are two electron-dense zones and at the centre, there exists an electron transparent zone. The structure which consists of two electron-dense and one electron-transparent zone is known as the Trilaminar Model. This model is also known as the Tripartite Model. The electron-dense zone is the layer of extended proteins which is known as the Beta Proteins.
The unit membrane mainly executes two functions. At first, this membrane works as a barrier of the cell which keeps enclosed the components of the cell within the cell and also stuffs the unwanted substances to enter into the cell. And the second function of the unit membrane is, to act as a path of transport that is performed as a gate that allows essential nutrients to step into the cell and waste products of the cells to step out from the cell.
According to J David Robertson's unit membrane model, the unit membrane consists of three layers among which the bimolecular phospholipid layer has a thickness of 35 Angstrom and the other two protein layers, each has a thickness of 20 Angstrom. Hence, the total thickness of the unit membrane is 75 Angstrom.

Therefore, the correct option is option D.

Note: The unit membrane is a type of cell membrane which is a tri-layer structure. Among the three layers, one layer is composed of phospholipids which is about in thickness, and the other two layers are composed of proteins which are each in thickness and the total thickness is This unit membrane mainly works as a barrier of the cell and the path of transport of various nutrients and waste products of the cell.