Cell membrane also known as a cytoplasmic membrane or plasma membrane is a biological membrane that protects the interior of cells from its surrounding. The main function of a cell membrane is to protect the cell from its outer environment. The cell membrane controls the movement of all the substances that come in or out of organelles and cells.
Charles Overton was the first one to suggest that cell membrane is made up of lipids. He suggested this when he observed that stuff which is lipid soluble enters the cell much faster than stuff that’s water soluble. After that Gorter and Grendel suggested that the cell has two layers just like a sandwich.
The next development came in the year 1935 when James Danielli and Hugh Davson stated that some protein molecules are also sprinkled on the surface of the membrane. This theory is famously called the Sandwich Theory or more technically called the Lamellar Theory.
The phospholipids molecules are set at right angles to the surface of the cell membrane. But the proteins are globular, so it was assumed that the carbohydrates and the proteins are held together by weak Vander Waal forces. So as you can see there is an issue in the theory because that is not what a cell membrane looks like. There was also no explanation for the phenomenon like the movement of water and water-soluble stuff across the membrane. So then the question was how does it happen if all the globular proteins are sitting on the surface? And there is also no explanation for something like active transport, so what is active transport?
Later on, David Robertson came up with the Robertson model. Named after the proposer David Robertson, his model was based on the observation of a red blood cell. He found that it had 3 layers: 2dark staining layers and 1 light staining layer in between. So the outer layers were assumed to be the protein layers and the inner one, the lipid bilayer. Therefore this supported the Lamellar or Sandwich Theory. But David Robertson went one step further in the protein front and suggested that proteins are not necessarily globular, but they can be extended. So he went on to put a hypothesis.
According to the hypothesis, there is a membrane consisting of a phospholipid bilayer sandwiched between two protein monolayers. David Robertson also said that all the various membranes that are in a cell are of just two types: unit membranes or multiples of these unit membranes. Examples of unit membranes are Endoplasmic Reticulum, the Golgi, the Lysosomes, etc. and examples of double unit membranes are Mitochondria, Plastid, etc.
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So, now that you know how a cell membrane looks… Let’s understand the evolution of the cell membrane… it actually happened in steps…
So, Charles Overton was the first one to suggest that cell membrane is made up of lipids and he suggested this when he observed that stuff which is lipid soluble enters the cell much faster than stuff that’s water soluble… interesting and very smart, right? Then what happened..? Gorter and Grendel suggested that the cell has two layers like this… like a sandwich… Next development sprinkled in the protein molecules, much like pepper on the surface… A model by Danielli and Davson, famously called the Sandwich Theory… more technically called the Lamellar Theory… Ok ? So the Sandwich Theory, Lamellar Theory are one and the same.
So the phospholipids here are set at right angles to the surface… that’s okay, the proteins are globular, that’s where the problem was… And it was assumed that the proteins and carbohydrates layers are held together by weak vander waal forces… So as you must have guessed, there’s an issue with this theory… this is not what the cell membrane looks like… There was no explanation for phenomena like… movement of water and water soluble stuff across the membrane… How does it happen if all the globular proteins are sitting on the surface, right? So… and there’s also no explanation for something like active transport… What’s active transport? Hold on for that… we’re going to learn about it in just a bit… fine ? That’s very very interesting…
So this model also assumed that the membrane was very very stable… it won’t… there’s no variation… there’s no variability…which is again wrong… We know that now… at that time, this was the most accurate thing they actually had. So after this, we had the Robertson Model, Ok ?
Named after the proposer David Robertson, his model was based on the observation of a red blood cell… And he found that it had three layers… Two dark staining layers and one light staining layer in between. So the outer layers are assumed to be the protein layers… the inner one, the lipid bilayer. So this supported the Lamellar or Sandwich Theory… But he went one step further in the protein front… saying that proteins are not necessarily globular, but they can be extended… So he went on to put a hypothesis… that’s what you do when you have a lot of ideas… A hypothesis that says that we have a membrane consisting of a phospholipid bilayer sandwiched between two protein monolayers… He also added a very simple point, that all the various membranes that you meet in a cell are of just two types… Unit membranes or multiples of these unit membranes… So you actually have examples and that’s the best way to learn… What would the examples of both of these types be… the Endoplasmic Reticullum, the Golgi, the Lysosomes…. they are all unit membranes… but, Mitochondria, Plastid, … they are all double unit membranes… which means that there are two layers, right? You must have also guessed the name of this hypothesis, because it’s all about unit membranes… It’s called the Unite Membrane Hypothesis… See how simple it is, right? It’s very very easy…
And what he did was, he’ll give sizes to these layers, Ok? The unit membrane is around 75A (amstrongs) with the peripheral protein layer being 20A around, and the central phospholipid bilayer being around 35A… it’s really really really tiny, right?