Plasma membrane

The fluid mosaic model

The structure of the plasma membrane is basically the same as other membranes inside the cell. so, its structure can be called the "unit membrane". It is made of proteins and phospholipids.

There are two important features of the plasma membrane. Firstly, the plasma membrane is made of two layers of phospholipid molecules. The phosphate group makes one side of the molecule much polar. Therefore, the phospholipid molecule should have one polar end and one non-polar end. It is denoted as follow.

The membrane is made of two continuous layers of phospholipid molecules. These molecules have their non-polar ends facing each others, and with their polar ends facing out. This makes the surfaces (inner and outer) of the membrane much hydrophilic.


The bilipid layer	Electromicrograph of the membrane

Feature 1 : The property of a fluid , This bilipid layer has the property of a fluid, because the molecules are mobile. They can flow here and there, and, the lamella arrangement of the molecules can be re-arranged. So, the orientation of the membrane can be changed. This enables the recombination of the plasma membrane after the penetration of particles.

Feature 2 : Embedded with proteins, Another feature of the plasma membrane is that there are much proteins included. Some stick on the outer surface, some on inner surface, some penetrate the whole bilipid layer, and some are extending out from the bilipid layer. So, these proteins forms a discontinuous covering on the bilipid layer. It is said to be a mosaic appearance. So, this model of the structure of the plasma membrane is called the fluid mosaic model.

The fluid mosaic model for the structure of the plasma membrane.

The proteins in the plasma membrane serve for the following purposes.

(1)	The carrier enzymes, that brings materials across the plasma membrane into the cell or out of the cell.
(2)	The channel proteins, that forms the passage of a certain material and control the pass of this material, as the calcium channel, sodium channel etc.
(3)	The receptor proteins, that can bind with some materials passing by, and then, trigger on some types of reactions inside the cell, as the receptor for hormones, receptors for antigens etc.
(4)	The identity proteins (histocompetible proteins), that shows the identity of the cell, as the group of living things it belongs to, the type of cells it belongs to, etc. By this histocompetible proteins, we can identify the human cells from the monkey cells, liver cells from the kidney cells etc.

(28.08.2006)

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