Origin of plasma membrane:
Initially, Robert Hooke discovered cells in 1665 and thereby the rise of the cell theory occurred. He paved the way for the discovery of cell walls on which for over 150 years, researches have been conducted.
Then, with the onset of research by using permeability and osmosis, plasma membranes became inevitable. At the end of the 19th century, Ernest Overton discovered the presence of lipids in the plasma membrane.
After that, two scientists namely Grendel and Gorter defined the existence of bilayer structure of lipids on the basis of observations using soap bubbles and crystallographic studies in the plasma membrane.
They extracted mature mammalian red blood cells and poured it onto the surface of water. Since, the mature RBCs lack both cytoplasmic organelles and nuclei, the lipid extracted was totally from the plasma membrane. And the calculated ratio of the surface area of the extracted lipid over the water to the lipid surface area in the mature RBCs was 2:1. Hence, they concluded that the structure of lipids is a bilayer in the plasma membrane.
The invention of the leptoscope happened to measure the thickness of the plasma membrane by comparing it with the standard thickness of the known membrane protein. The measurement shield ranges in between 8.6 to 23.2 nm. Then comes the paucimolecular model of Danielle and Davison based on the surface tension between echinoderm eggs and oils.
At last, the fluid mosaic model proposed by Sanger and Nicolson is the widely accepted model for the plasma membrane. It stated that the hydrophilic heads are present in the exterior part of the plasma membrane whereas the hydrophobic tails are present in the interior.
Various names were used to describe the term plasma membrane such as Hautschicht, cytoplasmic membrane and Plasmahaut.
Different models of plasma membrane:
There are four major models namely, the lipid bilayer model, the unit membrane model, the fluid mosaic model and the sandwich model.Lipid bilayer model:
Grendel and Gorter proposed the lipid bilayer model. It was proposed in 1925. It laid the basic foundation for the lipid bilayer structure of the plasma membrane by stating that their hydrophilic heads are towards the outside and their hydrophobic tails are towards the inside. But, it did not provide information on its function.
Sandwich model:
Hugh Davson and James Davson proposed the sandwich model. It was proposed in 1935. It is a three layered model with non permeable proteins in it. It did not explain about the additional functions of the proteins like transmembrane proteins.
Unit Membrane model:
Robertson proposed this model. It was proposed in 1959. It defined that all the plasma membranes possess a central bilayer of phospholipids and a protein layer towards inside and outside.
Fluid mosaic model:
Sanger and Nicolson proposed the fluid mosaic model in 1972. It states that the plasma membrane is a fluid structure with proteins, carbohydrates, cholesterol and phospholipids on it. It is called the fluid mosaic model, because patterns of mosaic are seen with the arrangement of the proteins. It is an important model that is accepted worldwide.
Functions of plasma membrane:
The most important functions of plasma membrane include, communication between cells, adhesion and recognition of cells, environmental interaction, structural support, flexibility, transport, selective permeability, homeostasis, exocytosis and endocytosis.
Communication between cells:
The communication occurs when a signal is received by the receptor proteins on the plasma membrane and this process is known as signal transduction.
Adhesion and recognition of cells:
The class of glycoproteins are allowed to function as cell-to-cell recognition that are important for the responses by the immune system and tissue formation.
Environmental interaction:
It monitors the way the cell membrane is important in the interaction with the environment. It also controls the relationships with molecules and other cells.
Structural support:
It gives shape and structural support to the internal cytoskeleton.
Flexibility:
The flexible nature of plasma membrane allows it to vary the shape which is important in the movement of cells in the blood capillaries.
Transport and selective permeability:
The plasma membrane is also known as the "gate-keeper" as it controls the movement of certain necessary molecules like water, oxygen and many others, in and out of the cell.
Homeostasis:
It is involved in the regulation of molecule movements to possess stability in the balance of the chemical environment within the cell.
Exocytosis and endocytosis:
Exocytosis is a process where the release of certain chemical substances happens and endocytosis is a process which involves the cells engulfing the foreign substances through the formation of vehicles
