The Fluid Mosaic Model

According to the fluid mosaic model, the cell membrane is a “mosaic” of protein molecules bobbing in a fluid phospholipid bilayer

The cell membrane (and membranes of various organelles) have their own unique collection of proteins
There are two major types of proteins in the cell membrane (see previous figure)
Integral Membrane Proteins penetrate far enough into the cell membrane for their hydrophobic regions to be surrounded by hydrophobic tails of phospholipids. Function as transporters
Peripheral Proteins are not embedded in the lipid bilayer. They are appendages attached to the surface of the membrane. Function as support
Carbohydrates also have a role in the cell membrane
Carbohydrates play a role in cell-cell recognition (a cell’s ability to distinguish one type of neighbouring cell from another). This is important for example when sorting cells into tissues and organs during embryo development
An example of a carbohydrate marker include the four human blood groups (A, B, AB, and O – reflect variation in saccharides on red blood cells)
•Cholesterol also plays a role in cell membranes to add more stability to to the structure of the membrane.
Membranes embedded with more cholesterol are generally firmer at room temperature. At lower temps, cholesterol breaks down, making the membrane more fluid at lower temperature.

The cell membrane is a selectively permeable membrane – this means it allows the passage of some substances through the membrane
The hydrophobic core of the membrane stops the transport of ions and polarmolecules which are hydrophilic. Hydrophobic molecules like oxygen and hydrocarbons can diffuse through the membrane and cross it with ease
Very small molecules (like water and carbon dioxide) can also pass through the membrane as they aresmall enough to squeeze through the lipids of the membrane
Cell membrane proteins (e.g. integral proteins) play a role in transporting substances through the membrane