Question

According to the unit membrane structure, the thickness of the plasma membrane is about
a. $35\overset{o}{A}$
b. $20\overset{o}{A}$
c. $75\overset{o}{A}$
d. $100\overset{o}{A}$

Answer 1

Hint: The plasma membrane that surrounds the cell surface and physically separates it’s cytoplasm and other organelles from the surrounding cellular environment. It is composed of lipids, proteins, and carbohydrate traces. 1959, J.David Robertson proposed the unit membrane model.

Complete answer:
According to the unit membrane structure:
• The plasma membrane is a trilaminar structure.
• It is composed of two parallel outer protein layers and an inner phospholipid layer.
• The outer protein layer is dark and osmophilic and the central lipid layer is a light osmophobic layer.
• Outer protein layers are $20\overset{o}{A}$ thick and inner phospholipid layers are 35 thick.
• The total thickness of the unit membrane is $75\overset{o}{A}$
• 50% of membrane mass is composed of the membrane proteins.
• Two types of membrane proteins are found, named integral proteins and peripheral proteins.
• Integral proteins penetrate the lipid bilayer.
• Peripheral proteins are found outside the lipid bilayer.
• The middle layer of the unit membrane comprises three types of lipid molecules. These are phospholipids, cholesterol, and glycolipids.
• Each molecule of the lipid layer involves a hydrophilic head and two hydrophobic tails.

Hence, the correct answer is option (C).

Additional information:
The molecules which have an affinity to water are called hydrophilic or amphipathic molecules.
The molecules which do not possess an affinity to water are called hydrophobic molecules.

Note: The major functions of the plasma membrane are
• It provides a barrier to the cell and its organelles.
• The plasma membrane takes part in exocytosis and endocytosis.
• The plasma membrane allows entering selective materials to enter into as well as an exit out of the cell. Thus it provides selective permeability to the cell.
• Plasma members are involved in cell signaling.