Cell Membranes

Cell membranes regulate what enters and exits a cell through its construents and their respective properties.They are constructed from lipids, proteins, and carbohydrates. These three complement eachother to give to the cell's function of semi permeability(allows what enters and exits). In cell membranes there are sides, a hydrophilic( ability to bond with water) and hydrophobic(inability to bond with water).

In a cell membrane there is a lipid layer and a protein layer as stated earlier. The lipid layer (phospholipids)symmetrically face eachother with the fatty acids lined up to create a hydrophobic zone. There are two models that describe membranes and they differ in where the proteins or hydrophilic zones are located.

The first model of the membrane is the Davson-Danielli model proposed in 1935. It is a sandwhiched phospholipid bilayer between two protein layers. This model was widely accepted until 1970. The following is the Davson-Danielli Model:

In 1972 S.Singer and Nicolson created a new model for the membrane proposing that proteins were dispersed and individually inserted into the phospholipid bilayer with only their hydrophilic portions being exposed to water. It is called the fluid Mosaic model.

This model gives to the reason why certain molecules enter the cell which are not permeable to the phospholipid bilayer.