In details

Plasma membrane

It is also called a cell membrane, cytoplasmic membrane or plasmalema.

The whole cell, whether prokaryote or eukaryote, has a membrane that isolates from the outside: the plasma membrane. This membrane is so thin (between 6 and 9 nm) that the finest optical microscopes could not make it visible.

It was only after the development of electron microscopy that the plasma membrane could be observed. At large magnifications obtained by the electron microscope, transverse sections of the membrane appear as a lighter line between two darker, delimiting the outline of each cell.

Chemical constitution of the plasma membrane

Studies with isolated plasma membranes reveal that their most abundant components are phospholipids, cholesterol and proteins. This is why it is often said that plasma membranes have lipoprotein constitution.

The molecular organization of the plasma membrane

Once phospholipids and proteins were identified as the main molecular components of the membrane, scientists began to investigate how these substances were organized.

The fluid mosaic model

The arrangement of molecules on the plasma membrane has been recently elucidated, and lipids form a continuous double layer into which protein molecules fit together. The double layer of phospholipids is fluid, oily in consistency, and proteins continually change their position as if they were pieces of a mosaic. This model was suggested by two researchers, Singer and Nicholson, and was named Fluid Mosaic Model.

Phospholipids have the function of maintaining membrane structure and proteins have several functions. Plasma membranes of a eukaryocyte contain particularly large amounts of cholesterol. Cholesterol molecules increase the barrier properties of the lipid bilayer and due to their rigid flat steroid rings decrease mobility and make the lipid bilayer less fluid.