Singer and Nicolson in 1972 proposed an improved model of the cell membrane, the fluid mosaic model, which is the most widely accepted. According to this model, the quasi-fluid nature of lipid allows lateral movement of proteins within the lipid bilayer. The fluid nature of the cell membrane is important in that it allows the cell to grow, and form intracellular junctions. In addition, it also plays vital roles in secretion, cell division, and cell structure formation.
One of the vital functions of the cell membrane is the transport of molecules across it. Many molecules can move across the membrane without any energy requirement, which is called passive transport. Neutral molecules can readily cross the cell membrane by simple diffusion along the concentration gradient. However, there are other molecules that might require energy such as ATP in order to cross the semipermeable membrane. The energy dependent transport of molecules is called active transport.
In addition, the transport of molecules is facilitated by exocytosis and endocytosis respectively. In the example of exocytosis, molecules are excreted outside the cell membrane through special vesicles. Endocytosis is where the membrane engulfs the molecule using a special mechanism.
