Welcome back to my channel, in today’s video we are going to look at the structure of the
cell membrane and how models of the cell membrane changed over time leading to the current model,
the fluid mosaic model.
Models are theoretical representations of objects and structures, models are used a
lot in science especially in describing structures that are microscopic such as the cell membrane.
One of the first proposed models of the cell membrane as a bilayer came from research done
in the 1920’s by Gorter and Grendel.
They were investigating the structure of cell membranes.
They theorized that if the cell membrane was composed of a lipid bilayer then the number
of lipids extracted from a cell and lined up would measure twice the surface area of
that cell.
They looked at red blood cells and found that the ratio of the single layer of lipids compared
to the surface area of the cell was 2:1.
This supported the hypothesis and led to the proposal that the cell membrane structure
consists of a bilayer of lipids with the polar hydrophilic heads on the outside and the hydrophobic
tails facing inwards.
Other models for the cell membrane have been proposed over the years.
One of these was proposed by Hugh Davson and James Danielli in 1935.
It was called the Davson-Danielli model or the lipo-protein sandwich model.
This was based on images from electron microscopy, in these images, the cell membrane appeared
as two dark lines and a light area inside.
On electron micrographs, proteins often appear dark, whilst lipids lighter.
This lead to their model of the phospholipid bilayer having two layers of proteins on each
side of the lipid bilayer.
This Davson-Danielli model was falsified.
This is because the understanding of the structure of the cell membrane has changed over time
because tools and technologies have improved over time.
One technique that helped falsify the Davson-Danielli model was freeze-fracturing.
In this case, cells were rapidly frozen, then the frozen cell was broken apart, fractured,
to open up the cell membrane, and electron micrographs were taken of them.
What was found when they split open the membrane were rough irregular surfaces within the membrane.
This indicated that the proteins were also embedded within the membrane and not solely
found on the outside of the membrane as thought in the Davson-Danielli model.
Another technique that helped in understanding the structure of the cell membrane and in
falsifying the Davson-Danielli model was fluorescent tagging and cell fusion.
These experiments involved taking proteins from two different cells being tagged with
fluorescent antibodies and then fusing them together.
Larry Frye and Michael Edidin conducted such experiments in 1970, where they tagged mouse
cells and human cells with fluorescent antibodies.
They tagged some of the proteins in the mouse cell, and some of the proteins in the human
cell with a different coloured fluorescent dyes, then used a virus to get the two cells
to fuse.
At first the different colour labelled proteins were segregated into separate halves of the
fused cell, but over time, these labels became mixed.
This gave evidence towards the cell membrane being fluid, and that the proteins in the
membrane moved around within the membrane.
Another technique that helped falsify the Davson-Danielli model was protein purification
With the gained ability to extract proteins from the cell membrane scientists were able
to look at their properties.
They found that the proteins were varied in shape and size, and they also found that some
of the proteins had hydrophobic regions in them.
This is in contrast to the Davson-Danielli model which had the proteins be mostly uniform
in shape and size bordering the membrane, and also had them being hydrophilic around
this edge.
In 1972 Singer and Nicolson incorporated some of this evidence about the structure of the
cell membrane into their model, where the proteins were found embedded within and through
the membrane.
This model is called the fluid-mosaic model.
The fluid mosaic model is a model of the lipid bilayer of the cell membrane.
There are proteins embedded in the phospholipid bilayer which creates a sort of mosaic appearance,
which is where the mosaic in the model name comes from.
The fluid part of the model names comes from the membrane being dynamic, the phospholipids
can move and the embedded proteins move around the phospholipids.
In this model, you have the phospholipid bilayer, consisting of phospholipids, with the heads
facing the opposite direction.
Phospholipids consist of a phosphate-glycerol head and two fatty acid tails.
These phosphate heads facing outwards in the bilayer are hydrophilic and the tails are
hydrophobic facing inwards.
Hydrophilic in simple terms means ‘water-loving’.
Hydrophilic molecules are those that attract water, such as polar and charged molecules.
Hydrophobic in simple terms means ‘water-fearing’.
Hydrophobic molecules are those that repel water, such as nonpolar molecules, or nonpolar
regions of a molecule.
Amphipathic is the term used when a molecule has both hydrophilic and hydrophobic parts
to it.
Phospholipids are considered amphipathic as they have both hydrophobic and hydrophilic
parts.
The head is polar and hydrophilic whilst the tails are non-polar and hydrophobic.
Because they are amphipathic, phospholipids form bilayers when they are in water.
This is because there is water both inside the cell and outside of the cell.
So the hydrophobic tails arrange themselves to face away from the water by facing against
another set of tails of another phospholipid, creating a polar internal region of the bilayer
whilst the heads of the phospholipids arrange facing the water creating a phospholipid bilayer
where all the hydrophobic tails are facing inwards protected by the hydrophilic heads.
Within the phospholipid membrane, there are different kinds of proteins.
There are integral proteins that are embedded within the membrane.
There are also peripheral proteins that are found at the surface of the membrane.
Some of the proteins found in the membrane are protein channels, these proteins have
a pore running through them.
Some embedded proteins are glycoproteins that have a protruding branching, antenna-like
carbohydrate.
For more types of proteins found in the membrane check out my video on membrane proteins.
Cholesterol molecules are also found, they are found between the tails of the phospholipids
within the inner hydrophobic region of the membrane.
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