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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?

1. Proteins in cell membranes can travel within the lipid bilayer.
2. Proteins can also undergo flip-flop movements in the lipid bilayer.
3. Proteins can remain confined within certain domains of the membrane.
4. Many proteins remain completely embedded within the lipid bilayer.

Correct answer: Proteins can also undergo flip-flop movements in the lipid bilayer.

Solution

Integral membrane proteins can diffuse laterally within the same leaflet and may be restricted to specific domains, but they do not normally flip-flop across the bilayer. Flip-flop is common for some lipids, not for proteins, because the protein’s hydrophilic regions would have to pass through the hydrophobic core.

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