The following points highlight the top four historical models of Plasma Membrane. The models are: 1. Lipid and Lipid Bilayer Models 2. Unit Membrane Model (Protein-Lipid Bilayer-Protein) 3. Fluid Mosaic Model 4. Dannelli Model.
1. Lipid and Lipid Bilayer Model:
This model to explain the structure of plasma membrane was given by Overton, Gorion and Grendel. Previously only indirect information was available to explain the structure of plasma membrane. In 1902, Overton observed that substances soluble in lipid could selectively pass through the membranes. On this basis he stated that plasma membrane is composed of a thin layer of lipid.
Subsequently, Gorter and Grendel in 1926 observed that the extracted from erythrocyte membranes was twice the amount expected if a single layer was present throughout the surface area of these cells. On this basis they stated that plasma membrane is made up of double layer of lipid molecules. These models of Gorter and Grendel could not explain the proper structure of plasma membrane but they put the foundation of future models of membrane structure.
2. Unit Membrane Model (Protein-Lipid Bilayer-Protein):
This is also known as unit membrane model. This model was proposed by Davson Daniell and Robertson. When surface tension measurements made on the membranes, it suggests the presence of proteins. After the existence of proteins the initial lipid bilayer model proposed by Gorter and Grendel was modified. It was suggested that surface tension of cells is much lower than what one would expect if only lipids were involved.
It may also be observed that if protein is added to model lipid water system, surface tension is lowered. This suggested indirectly the presence of proteins. On this basis Davson and Danielli proposed that plasma membrane contained a lipid bilayer with protein on both surfaces.
Initially they supposed that proteins existed as covalently bonded globular structures bound to the polar ends of lipids. Subsequently they developed the model in which the protein appears to be smeared over the hydrophilic ends of the lipid bilayer. This model makes its popularity for a long time.
With the availability of electron microscope later, fine structure of plasma membrane could be studied. Definite plasma membrane of 6 nm to 10 nm (10nm = 100 Å; 1 nm = 10_6mm) thickness was observed on surface of all cells, and plasma membranes of two adjacent cells were found to be separated by a space, 1-15nm wide.
It was also observed that the plasma membrane of most of the cells appeared to be three layered. Two outer dense layers were about 2.0nm thick and the middle layer about 3.5nm. The early ideas of Gorter and Grendel and those of Davson and Danielli were first formalized by Robertson in 1959 in the form of his unit membrane concept.
This concept of unit membrane with three layers (two protein layers and one lipid bilayer) only supported the concept proposed earlier by Davson and Danielli. In this unit membrane the less dense middle layer corresponded to hydrocarbon chains of lipids. Thickness of unit membrane (10nm) was found to be greater in plasma membrane than in intracellular membranes of endoplasmic reticulum or golgi complex.
3. Fluid Mosaic Model:
To explain the structure plasma membrane various models have been put forward from time to time. But none was universally accepted. In this relation Gorter and Grendel, Davson and Danielli, etc. proposed model for plasma membrane after that Fluid Mosaic Model for plasma membrane was proposed which was universally accepted.
It was proposed by Singer and Nicholson (1912). This model postulates that lipid and integrated proteins are disposed in a sort of mosaic pattern and all the biological membranes have a quasi-fluid structure where both lipid and protein components are able to perform transitional movement within lipid bilayer.
In this model, lipid molecules may exhibit intra-molecular movement or may rotate about their axis or may display flip-flop movement including transfer from one side of bilayer to the other. Thus this concept implies that main components of the membrane, i.e., lipids, proteins and oligosaccharides are held together by means of non-covalent interactions as suggested by Gitler (1972). A term amphipathy was coined by Hartley (1936) to the molecules having both hydrophilic and hydrophobic groups. Thus lipids and integrated proteins are amphipatic in nature.
Our present knowledge of plasma membrane is based on integration of data from chemical analysis and those from the study of biophysical properties with the help of various types of techniques. These have provided the main components which are integrated in plasma membrane. In this relation, following four major techniques as discoveries have given support.
These are as follows:
(i) Freeze fracture technique was used to study membrane. Freeze fractured electron microscope, revealed the presence of bums and depressions which are 7- 8 nm in diameter. These remains randomly distributed. These were later shown to be intra-membrane protein particles which transverse the bilayer.
(ii) Frye and Edidin (1970) labeled selectively the species specific proteins of human and mouse cells and then fused these cells of the two species to make a heterokaryon. After incubating the heterokaryons for 30-35 minutes at 37°C, human and mouse proteins in these heterokaryons were seen intermixing (as demonstrated by using specific antibodies), so that human and mouse proteins became randomly distributed suggesting that membrane proteins are mobile in the plane of the membrane.
(iii) The process named patching and capping also provide evidence about the mobility of proteins within the lipid bilayer. This process suggested that when ligands like antibodies have more than one sites for binding the specific proteins on the cell surface, the proteins tend to aggregate into clusters through cross-linking. This indicates that proteins diffuse laterally in the bilayer.
(iv) Fluorescence recovery after photo-bleaching (FRAP) has also been used for measuring rates of lateral diffusion of proteins. A cell surface protein of interest is marked with a fluorescent ligand (e.g., antibody). The ligand is bleached in a small area by a laser beam and the time taken for bleached and unbleached fluorescent ligands to diffuse and mix is measured. The rate of diffusion of protein is not constant.
The evidences as above suggested that lipid bilayer has fluid properties enabling membrane proteins to diffuse rapidly. Rotational diffusion of proteins is possible. However no evidence of flip-flop mechanism as suggested for lipids has been available for proteins. Later on it was suggested that not only proteins, but individual lipid molecules are also able to diffuse freely within the lipid bilayers.
And it was found true in synthetic as well as isolated biological membranes which were obtained from mycoplasmas, bacteria and red blood cells. Initially, this was demonstrated in the following two types of synthetic lipid bilayers i.e., liposomes and black membranes, liposomes are spherical vesicles.
These measures from 25nm to 1/nm (1000nm) in diameter Black membranes extend across a hole in a partition between two aqueous compartments. The motion of individual lipid molecules could be measured by ‘spin labeling’.
(The spin of unpaired electron creates a paramagnetic signal that can be detected by electron spin resonance (E.S.R) spectroscopy). The motion and orientation of a spin labelled lipid can be deduced from the ESR spectrum. The lipid molecules can also rotate or readily exchange places within the same monolayer (107 times in a second) with a diffusion coefficient (D) of about 10-8 cm2/sec, so that a lipid molecule could diffuse the length of a large bacterial cell (–2µm) in about one second.
Even when the lipid molecule is static, the hydrocarbon chains are flexible. Similar results were obtained from isolated biological membranes, except that in the natural. On the basis of these facts, Singer and Nicolson proposed a hypothesis to explain the structure of plasma membrane. This is known as fluid mosaic model. Basically this model was modification of Robertson and Davson.
Modifications of Fluid Mosaic Model:
On the basis of fluid mosaic model, it can be stated that cell membrane is a two dimensional oriented solution of integral proteins in the viscous phospholipids bilayer. Now recent work has cleared that although lipids and a fraction of the labeled protein population appear to diffuse freely, movement of other proteins is much more complicated than originally envisioned in the fluid mosaic model.
A substantial fraction of the proteins is confined, at least transiently, to small domains in the membrane of a cell. Most probably by the propelation of cytoskeleton motors, a few membrane proteins undergo rapid, forward-directed transport towards the cell edge. The transient confinement of integral proteins has been seen most clearly for certain cell adhesion molecules including cadherin and neural cell adhesion molecules and for receptors for nutrient and growth factors.
Where proteins get confined, the domains are 300-600 nm in diameter and the confinement lasts for 3-30 seconds. Recently such confinement of proteins has been explained by a ‘membrane skeleton fence’ model. On the basis of this membrane skeleton fence model it may be stated that a spectrin-like mesh-work on the cytoplasmic side of the membrane sterically confines transiently some membrane spanning proteins.
The above features of cell membrane, demanding revision of our original concept of fluid mosaic model, were revealed through the use of at least three new techniques:
(i) Fluorescence recovery after photo-bleaching (FRAP).
(ii) Single particle tracking (SPT).
(iii) Optical laser trap (OLT).
However, it is certain that the plasma membrane is an intriguing mix of dynamic activities, in which its components may randomly diffuse (as proposed in fluid mosaic model), or be confined transiently to small domains or experience highly directed movement. These features cause considerable lateral heterogeneity in the membrane.
1. Microvilli:
The plasma membrane of the epithelial cells of intestine gives out many minute, finger-like cytoplasmic processes which are called microvilli. There may be as many as 3,000 microvilli in a single cell. The villi vary in length from few hundred angstroms to 10µ and in diameter from 800-1400 A°. The gap between microvilli is of 100 A°. They remain covered with a unit membrane which is thicker (100-125 Å) than common plasma membrane. In case of nematodes, microvilli remain covered by fine filaments lying parallel to villi.
They are beaded and adielectronic regions being 130 Å apart. According to Threadgold (1969), cestodes cuticle is emarginated into microtrichs which appear to be a type of microvillus. Their distal ends are adielectronic, proximal part is dielectronic, while centre of microtrichs is filled with an amorphous granular material of variable amount.
At certain places, the microvilli make direct contact with the endoplasmic reticulum thus providing a direct channel into and out of the cytoplasm. The microvilli increase the surface area for absorption thus functioning like villi. Such microvilli are also found in hepatic cells, mesothelial cells of the convoluted tubules of the kidney and in the epithelial cells of gall bladder, uterus and yolk sac.
In certain cells, the in-folding’s of plasma membrane form pocket or chamber-like structures with which mitochondria also become associated. These may release energy for the transport of material across the plasma membrane.
2. Intercellular Spaces:
The plasma membranes of adjacent cells are not united but are separated by intercellular spaces or gaps. In between the plasma membrane of adjacent cells of myocardial tissue there is an intercellular gap of 20 Å wide. These gaps seem to be hexagonal in tangential section and these are called gap junctions.
3. Interdigitations:
Sometimes the plasma membranes of two adjacent cells give out finger—like outgrowths at some places. These are called inter-digitations.
4. Desmosomes and Associated Zones:
These are the thickened regions of plasma membranes with many filaments radiated towards the inner surface of cell. These filaments are called tonofibrils (tonofilaments) and the thickened regions of the plasma membranes are called desmosomes or maculae adhaerens.
The intercellular space between the desmosomes contains a coating material by which adjacent cells adhere with each other. Desmosomes seem to be present in vertebrate tissues. Generally, the lateral plasma membrane exhibits three parts in an apical-basal direction—a tight region or zonula occludens, an intermediate junction or zonula adhaerens and macula adhaerens or desmosome.
(а) Zonula Occludens:
The tight region starts at the junction of apical and lateral cell borders and is 2000-5000 Å wide. It seems to form a continuous belt-like attachment around the cell. It is characterized by the fusion of the adjacent plasma membranes resulting in the obliteration of intercellular space and thus acts as a barrier to diffusion of the large sized molecules. The zonula occludens is found in between epithelial cells and brain cells. It permits intercellular electrical communication and chemical communication of inorganic ions.
(b) Zonula Adhaerens:
Arises as a continuation of the previous zone and extends over 3000-5000A with a straight or wavy course. The membranes have a distinct tripartite structure and are thickened, and immediate cytyoplasm has a conspicuous densification of tightly matted fibrils. Both the membranes are separated by amorphous intercellular zone of about 200A° thickness.
(c) Macula Adhaerens:
It is located at a distance of 200 Å from the basal end of zonula adhaerens. It is an oval or circular area of 2000-3000 Å in length and occurs in pairs. Its outer part is of tonofibrils (fine filaments) measuring about 75 Å in diameter.
In Hydra and some other invertebrates, dense transverse bars join the adjacent desmosomes giving it a ladder-like appearance. Such desmosomes are called septate. In septate desmosomes, the tonofibrils are lacking.
4. Dannelli Model:
According to this model:
(i) Lipid and intrinsic proteins are present in a mosaic arrangement and
(ii) Biological membranes are semifluid so that lipids as well as intrinsic proteins are able to make movements within the bilayer.
This concept of fluidity implies that lipids, proteins and oligosaccharides are held in their positions by means of non-covalent interactions. On this basis, it can be stated that components can be dispersed by solvents or detergents without breaking any bonds. Intrinsic proteins are also intercalated to greater or lesser extent into a continuous lipid bilayer. Such proteins may be in contact with an aqueous solvent on both sides of the membrane.