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Keeping in view the 'fluid mosaic model' for the structure of cell membrane, which one of the following statements is correct with respect to the movement of lipids and proteins from one lipid monolayer to the other (described as flip-flop movement)?
A
While proteins can flip-flop, lipids cannot
B
Neither lipids, nor proteins can flip-flop
C
Both lipids and proteins can flip-flop
D
While lipids can rarely flip-flop, proteins cannot
Detailed Solution
According to the fluid mosaic model of Singer and Nicolson (1972), the quasi-fluid nature of lipids enables the lateral movement of proteins within the overall bilayer.
Lipid molecules move freely sideways within their own monolayer (lateral diffusion) and also rotate.
Flip-flop is the transverse movement of a molecule from one monolayer to the other. For a lipid, the polar head would have to pass through the hydrophobic core of the bilayer, which is energetically unfavourable, so it occurs only rarely (it is helped by enzymes called flippases).
Proteins are much larger and have extensive hydrophilic regions; they cannot pass through the hydrophobic core, so they cannot flip-flop at all. This keeps the two faces of the membrane different (membrane asymmetry).
So the correct statement is: while lipids can rarely flip-flop, proteins cannot.
Lipid molecules move freely sideways within their own monolayer (lateral diffusion) and also rotate.
Flip-flop is the transverse movement of a molecule from one monolayer to the other. For a lipid, the polar head would have to pass through the hydrophobic core of the bilayer, which is energetically unfavourable, so it occurs only rarely (it is helped by enzymes called flippases).
Proteins are much larger and have extensive hydrophilic regions; they cannot pass through the hydrophobic core, so they cannot flip-flop at all. This keeps the two faces of the membrane different (membrane asymmetry).
So the correct statement is: while lipids can rarely flip-flop, proteins cannot.