The lipid bilayer of cell membrane exist as
The lipid bilayer shows free lateral movement of its components, hence the membrane is said to be fluid in nature. Fluidity enables the membrane to perform endocytosis and exocytosis. However, the components do not freely move from inner to outer layer or outer to inner layer (flip-flop movement is restricted). During apoptosis (programmed cell death), flip-flop movement occurs. Somewhat similar to a gel.Ref: DM Vasudevan, page no: 11
Solid
Semisolid
Gel
Fibres
membrane structure and function