All cells have a structure called plasma membraneThe or plasmalemma, which acts as a barrier preventing both the outflow of cell contents into the medium and the flow of any particle into the cell. This characteristic shows, therefore, an important capacity for selecting the structure, which can vary from 7 to 10 nom thick.
→ plasma membrane structure
The currently accepted model of the plasma membrane structure is known as fluid mosaic and was proposed by Jonathan Singer and Garth Nicolson in 1972. In this model, the plasma membrane is described as a phospholipid bilayer with the presence of proteins. Due to the movement of lipids through this structure, the membrane has a certain fluidity and is therefore not considered static.
Phospholipid molecules have clusters non-polar (hydrophobic, that is, that do not absorb water) and clusters polar (hydrophilic, ie water-retaining). Non-polar clusters face the center of the plasma membrane, and polar groups face the outer and inner surfaces of the membrane.
The proteins found in the membrane can be called peripheral or integral. At proteins periffics are those that do not cross the plasma membrane and are indirectly linked to it, while the integrals they are embedded in the lipid layer. With the use of some reagents, it is possible to extract fathe peripheral membrane proteins, unlike the integral ones, which are only released by the disruption of the lipid bilayer. Whole proteins can be of the transmembranes, which are characterized by the presence of exposed portions on both sides of the bilayer.
On the outside of the membrane, the carbohydrates, which can be linked to proteins or lipids. By binding to proteins, carbohydrates form the glycoproteins. In the case of binding with lipids, they form the glycolipids. The result of these links forms a poorly delimited layer, called the glycocalyx.
→ Plasma membrane function
One of the main functions of the plasma membrane is to control what enters and leaves the interior of the cell, a property known as selective permeability. As it is formed by a lipid bilayer, it is impermeable to most water-soluble molecules. As a result, the vast majority of ions and molecules need the protein mediated to cross it. In addition to selective permeability, the plasma membrane has proteins that they guarantee the interaction between the cells and the reception of signals from the environment.
By Ma. Vanessa dos Santos
Source: Brazil School - https://brasilescola.uol.com.br/o-que-e/biologia/o-que-e-membrana-plasmatica.htm