Membranes

A membrane is roughly 75% lipids, 20% proteins, and 5% carbohydrates.. 

The fluid mosaic model proposes that a membrane is a lipid bilayer with proteins embedded in it. 
This model names lipids, proteins, and carbohydrates as the three major components of the bilayer.  
                                           
Lipids are amphipathic and arranged in a sphere. 
A phospholipid bilayer stabilizes the membrane.  Its fatty acids make the membrane fluid. 
                                                                
 Phospholipids move laterally in the membrane bilayer but are reluctant to flip across the lipid bilayer.  
This reluctance can contribute to asymmetry.
Asymmetry is defined as different lipid compositions on each side of the bilayer.

The fluidity of a membrane is influenced by:
the composition of phospholipid tail groups, cholesterol, and temperature.
To increase membrane fluidity a cell changes its composition in
favor of unsaturated fatty acids and shorter hydrocarbon chain links. 

Two types of proteins can be found in membranes: 
integral membrane proteins and peripheral membrane proteins. 
Integral membrane proteins penetrate the lipid bilayer and come in two forms: 
transmembrane proteins and partially embedded proteins. 

Transmembrane proteins are completely embedded in the lipid bilayer and
each end of the protein may perform a different job.  
Partially embedded proteins are partially embedded in the lipid bilayer.  
Peripheral membrane proteins are not embedded in the lipid bilayer but remain in association with it.

The more fluid the membrane, the easier it is for a protein to change its conformation.

Some membrane proteins are free to diffuse throughout the entire
cell membrane whereas some membrane proteins are not free to migrate.

              To test the movement of certain membrane proteins, scientists combined a mouse cell and a human cell.

      Some proteins, however, are localized to specific spots within cell membranes. 
The acetylcholine receptor is an example of such proteins.

Carbohydrates make up only about 5-10% of the membrane.  
They are always located on the external surface of the membrane and are attached to lipids or proteins.    

A carbohydrate attached to a lipid is called a glycolipid and is involved in intracellular signaling.
A carbohydrate attached to a protein is called a glycoprotein, and it recognizes foreign surfaces within the cell. 

Carbohydrates sort out the proteins in a cell and are useful in intercellular signaling. 
They are good signaling molecules because they can be assembled in a variety of different ways. 

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