Question

According to widely accepted, "Fluid mosaic model" cell membranes are semi-fluid, where lipids and integral proteins can diffuse randomly. In recent years, this model has been modified in several respects. In this regard, which of the following statements is incorrect?

• A. Proteins in cell membranes can travel within the lipid bilayer.
• B. Proteins can remain confined within certain domains of the memberane.
• C. Proteins can also undergo flip-flop movements in the lipid bilayer.
• D. Many proteins remain completely embedded within the lipid bilayer.

Answer 1

Answer: (D)

Many proteins remain completely embedded within the lipid bilayer.

• As per the fluid mosaic model for membrane structure, the fatty acyl chains in the interior of the membrane form a fluid hydrophobic region.
• Integral membrane proteins float in the sea of lipid held by hydrophobic interactions with their non polar amino acid side chains.
• Both the proteins and lipids are free to move laterally in the plane of lipid bilayer but movement of either from one face of the bilayer to the other is restricted.
• This view has been changed now and it is generally accepted that proteins may also show flip flop movement. Although it occurs rarely.
• When individual protein molecules and multiprotein complexes in freeze fractured biological membranes are visualized with the electron microscope, some proteins appear on only one face of the membrane; others span the full thickness of the bilayer and protrude from both inner and outer membrane surfaces.
• Among the latter, are some proteins that conduct solutes or signals across the membrane. There are no proteins completely embedded within the interior of membranes.

So, the correct answer is 'Many proteins remain completely embedded within the lipid bilayer'.