Evolution of the plasma membrane models
A brief history of how our understanding of the cell membrane evolved.
The human cell on an average, is about 100µ in diameter, and is covered by a thin plasma membrane, about 75Å thick. We know from the series of experiments conducted in the past few decades that this membrane is essentially composed of a phospholipid bilayer with proteins embedded within it. Now let’s do a quick thought experiment! Imagine you were in the early 1900s; What basic experiments would you conduct to decipher the structure and composition of this thin membrane, given that some basic information on the membrane was available by then such as: The membrane must have lipids in it, as lipid soluble factors easily enter the cells, and that lipid molecules were amphiphilic with polar heads and non-polar tails?
Now that this thinking exercise has gotten you wondering about different experimental assays, let’s look at how Groter and Grendel figured certain aspects of the cell membrane way back in 1925.
Groter and Grendel, 1925
Membranes have a lipid bilayer
In order to study the cell membrane, Groter and Grendel extracted RBCs from different animals and isolated the lipids from the cells. They further calculated the surface area of the cells using the magnifying power of the microscope and some basic mathematical calculations. Using a method that was previously described by Langmuir and Adam, they suspended the extracted lipid molecules into a monolayer by dissolving the lipids in benzene and adding few drops of the solution into a large surface of water. They then calculated the surface area occupied by this layer and established that this area was twice the surface area of the cells under study. They hence concluded that the cell membrane had a bilayer of lipid moieties, with their polar heads facing outwards (either towards the outside or towards the inside of the cell) and their non-polar fatty acid tails facing each other. The theory however, did have its pitfalls, for example, if the membrane was purely made of lipids, how did molecules insoluble in lipids, get into the cell? Also, studies showed that the surface tension of the membrane was much lower that that of oil droplets, indicating that there must be some molecule associated to the membrane that lowers the surface tension. Studies proved this molecule to be a protein.
The Danielli – Davson Model, 1935
Not just lipids, membranes have proteins too!
To clarify the existing criticism, Danielli and Davson proposed that cells were covered by a thin film of ‘lipoidal’ substance. By lipoidal, they meant a substance that was more soluble in hydrocarbons than in water. They suggested that the lipoidal substance had a lipoidal core bordered by monolayers of lipids and adsorbed onto this were protein monolayers. Essentially the added element in this theory was the presence of proteins in the membrane, which tried to address the permeability and surface tension issue. This theory received prominence when the first electron microscope images of the cell membrane was produced in the 1950's, which showed using appropriate staining, the membrane to be a triple layered structure, made up of 2 dense regions (presumed to be made up of the hydrophilic heads of lipids and the hydrophilic protein coating) surrounding a light central region (presumed to be made up of the hydrophobic tails of the lipids).
The unit membrane model
Not just lipids and proteins, membranes have carbohydrates as well
Research rapidly boomed in this area due to the now available electron microscope. Series of experiments were conducted on a spectrum of cells and all these experiments suggested one thing – that all biological membranes were made of a similar single bilayer – the unit membrane. In other words, it suggested the universality of the lipid bilayer. However, the unit membrane model also suggested one more very important thing – asymmetry. Electron microscopy images of cell membranes had shown a slight difference in the staining pattern between the outer and the inner surfaces and biochemical and staining experiments proved that the outer layer had carbohydrates associated to them. Hence the unit membrane model also suggested that the cell membrane was asymmetrical, consisting of two dissimilar layers (One with carbohydrates and one without).
You might be beginning to think that we are coming down to the perfect model here…but not yet! This model too had to face its share of criticism. Florescence based experiments showed that the proteins in the membrane did not essentially occupy the same position all the time – they moved! Not just this; freeze fracture etch electron microscopy – a method where in a frozen sample was fractured to study the internal structures revealed that there existed considerably large sized particles ranging in size 50 Å – 100 Å within the cell membrane. Proteins were not just present on the outside, they transversed the membrane too.
The fluid mosaic model of Singer and Nicolson, 1972
The cell membrane has fluidity
Taking into consideration all the previous considerations the fluid mosaic model suggested by Singer and Nicolson proposed that the cell membrane has fluidity, in that they are not rigid, and the components can move. They also proposed that the membrane was not really bordered by protein molecules as suggested by previous models but rather contained protein moieties dispersed in the membrane just like icebergs floating in the water. They proposed that proteins were both peripheral as well as integral, some spanning the membrane -transmembrane proteins. Some proteins on the outer surface had carbohydrate moieties liked to them.
This model holds to great extent even today. Of course, the string of research that ensued after 1972 has revealed a plethora of details. An example for this would be that the membrane is not as fluid as proposed. In fact, some proteins in the membrane are tethered to cellular cytoskeleton on the inside and the extracellular matrix on the outside making them immovable. The cell membrane as we know now is not just a structure that separates the inside of the cell from the outside, but indeed partakes in transport, attachment and signalling, making it one of the most important components of the cell.
It is indeed intriguing to look back and see how over the past 100 years, our understanding of the cell membrane has transformed step by step, largely owing to the diligence and patience of the scientists and the supporting technological advancements. It shows that it takes decades and sometimes centuries to paint the complete picture and we can never actually be sure if the picture is indeed ‘complete’! Perhaps, that’s the beauty of science.