Question

Which of the following best explains the fluidity of the plasma membrane ?

• A. The presence of phospholipids in a rigid bilayer.
• B. The movement of proteins and lipids within the bilayer.
• C. The fixed positions of membrane proteins.
• D. The attachment of cholesterol to membrane proteins.

Hint:

Remember the name of the model: \textbf{Fluid Mosaic Model}. "Fluid" refers to the movement of lipids and proteins. "Mosaic" refers to the patchwork pattern of proteins embedded within the lipid bilayer.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
The question asks for the best explanation of the 'fluidity' of the plasma membrane, a key concept in the Fluid Mosaic Model of cell membranes.
Step 2: Detailed Explanation:
The Fluid Mosaic Model describes the plasma membrane as a dynamic, fluid structure, not a static one.

The "fluid" part refers to the fact that the components of the membrane, primarily phospholipids and proteins, are not locked in place.
Phospholipids can move laterally (side-to-side), rotate on their axes, and occasionally "flip-flop" from one layer of the bilayer to the other (though this is rare). This lipid movement is a major contributor to membrane fluidity.
Membrane proteins are also generally free to move laterally within the lipid bilayer, floating like icebergs in a sea of lipids.
This constant movement of both proteins and lipids is the essence of membrane fluidity. It allows the membrane to be flexible, self-sealing, and involved in processes like cell signaling and transport.
Let's evaluate the options:

(A) describes the opposite of fluidity by calling the bilayer "rigid."
(C) is incorrect because most membrane proteins are not fixed; their ability to move is part of the membrane's fluidity.
(D) is incorrect. Cholesterol modulates fluidity by interacting with phospholipids, not by attaching to proteins.
(B) accurately describes the dynamic nature of the membrane, where both lipids and proteins are in constant motion.
Step 3: Final Answer:
The fluidity of the plasma membrane is best explained by the continuous movement of its constituent proteins and lipids within the bilayer.