Plasma Membrane Models
The plasma membrane is the living outer boundary of the cellular factory. There are different Cell Membrane Models.
Sandwich Model of Plasma Membrane
Denielli and Davson have proposed that in the plasma membrane a double layer lipid molecule is sandwiched between the two different layers of protein molecules. The lipid layers are found internally and the protein molecules are found externally. The inner ends of lipid molecules are non-polar and hydrophobic where as outer ends are polar and hydrophilic.
Unite Membrane Model of Plasma Membrane
Robertson in the year 1953 proposed the unit membrane model consisting of three layers of the plasma membrane and it is complimentary to the sandwich model. According to this there are two outer layers of protein molecules each with 20 angstroms thick, embracing a central layer of lipid molecules of about 35 angstroms thick for a total thickness of 75 angstroms.
Fluid Mosaic Model of Plasma Membrane
Fluid Mosaic Model of Plasma Membrane is the most widely accepted model today. According to this model there is a double layer of lipid molecules with globular protein molecules and sterols which are arranged differently in different parts of the Plasma Membrane. Some protein molecules penetrate through the lipid layers, others partially penetrate and still others lie outside the lipid layers. In the plasma membrane of the animal cells glycolipids and cholesterol are also present.
Invaginations of the plasma membrane in to the cytoplasm containing fluid are called pinocytic vesicles. The plasma membrane may also possess minute folds called microvillus which increase the surface area of the cells in absorption.