The Plasma Membrane - Practice Questions & MCQ

The Plasma Membrane/Cell Membrane/Plasmalemma

• It is the outermost covering in the animal cells while it is present below the cell wall in fungal and plant cells. 
• It is living, thin, delicate, semi-fluid protective covering that surrounds the cytoplasm. 

The plasma membrane is composed of the following:

• Proteins (20 - 70%) 
• Lipids (20 - 79%) 
• Carbohydrates (1 - 5%) 
• Enzymes (30 types) 
• Water (20%

1. Structure of the plasma membrane
In order to explain the structure of the cell membrane, the following models were proposed:

a)  Sandwich/ Trilaminar Model:

• It was given by Hugh Davson and James Danielli in 1935. 
• They proposed a sandwich model for the plasma membrane. 
• According to them, the plasma membrane was composed of a lipid bilayer that was lined on both its inner and outer surface by a layer of globular proteins. 
• They revised their model in the early 1950s to provide reasons for the selective permeability. 
• They further modified the sandwich structure to have proteins within the lipid bilayer that acted as a conduit for transport.
   

b) Unit Membrane Model: 

• It was proposed by Robertson in 1959. 
• The model proposed that lipids and proteins are present as distinct layers. 
• The membrane is asymmetrical because proteins on both sides of the membranes are different. Outer protein is called mucoid protein as it is associated with carbohydrates. 
• Inner protein is called non-mucoid protein. 
• This model is also known as P-L-L-P model. 
• This model holds true for the membranes of organelles.

Drawback of these models:

• Failed to explain the movement of polar compounds across the plasma membrane.
• Considered membrane as a static entity.
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c) Fluid Mosaic Model:

• It was proposed by Singer and Nicolson in 1972. 
• With recent modifications, it is the most accepted model for the structure of plasma membrane. This model provides for a core of a lipid bilayer which maintains the fluidity of the plasma membrane. 
• The greater the concentration of unsaturated fatty acid residues, the more fluid is the bilayer. The mosaic nature of the plasma membrane is due to its protein content.
• The plasma membrane is quasi fluid structure. The lipids perform the following movements:

               rotational movement 

               lateral diffusion movement

               flip-flop movement / transversion movement (rarely shown) 

• The proteins perform the following movements:

                rotational movement 

                lateral diffusion movement

• The following table depicts the components of the plasma membrane: