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Cytology

Protein layer
Hydrophilic phospholipid head

Hydrophobic phospholipid tail

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Phospholipid
bilayer

Figure 1.6: Model of plasma membrane as proposed by Danielli and Davson

The model explains the presence of The Fluid Mosaic Model
polar head (hydrophilic) and non-polar The model was proposed by Seymour
tail (hydrophobic) in the phospholipids. J. Singer and Garth L. Nicolson in
The proteins bilayers are continuous 1972. According to this model, protein
and are of the same size. However, the molecules are dispersed and inserted into
model does not explain how material a phospholipid bilayer. The phospholipid
enters or leaves the cell. The model also bilayer comprises hydrophilic heads
indicates that the membrane is static, it that face outwards towards the acquous
never changes its structure and water environment and hydrophobic tails
passes freely between the adjacent protein that face inward (Figure 1.7). The
molecules. The protein bilayers are on the hydrophilic heads interact with water
outside, while phospholipids bilayers are whereas hydrophobic tails are shilded
at the middle part. The model explains the from water within the membrane. Due to
presence of proteins and phospholipids. the fluid nature of phospholipids and the
Moreover, this model does not explain arrangement pattern of protein molecules
how cell recognises external signals. Due in the phospholipid bilayer, the model is
to these weaknesses, further studies were referred to as the Fuid Mosaic Model.
conducted and a new model called Fluid
Mosaic Model was developed.

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