Plasma membrane

A plasma membrane encapsulates each and every cell, and is also known as the cell membrane. It acts as a selective barrier which is necessary to allow different concentrations of substances to be maintained between the cell and it's environment. Eukaryotic cells have internal membranes which surround organelles, allowing different constituent concentrations within the cell^[1]. 

The plasma membrane is a lipid bilayer embedded with proteins and cholesterol which is 5nm thick. Lipid bilayers are composed of phospholipids, the most common being phosphatidylcholine. The head part of which is made up of a phosphate and a hydrophilic choline, the tail component of the phospholipid is comprised of two fatty acid chains, long hydrocarbon structures with a chemically active COOH. Since the head is Hydrophilic and the tails are hydrophobic, the phospholipid is said to be amphipathic. The structure of the bilayer is such that the tails are shielded from contact with water, creating a selectively-permeable barrier which will be impermeable to most hydrophilic molecules. 

The plasma membrane has many functions and is a fundamental part of the cell. It communicates with the environment and other cells by means of a glycocalyx, a system of protein-linked and lipid-linked carbohydrates. It allows passage of selective molecules in and out of the cell via carrier or channel proteins embedded within the membrane, and it regulates cell growth, shape, movement and division. 

The membrane is associated with proteins, both integral and peripheral which act as either:

• transporters
• linkers
• receptors  
• enzymes
  

The lipids within the bilayer can freely move, they can diffuse laterally along the membrane (across the same layer), or rotate along their axis, but move occasionally between layers (transverse diffusion). Therefore, plasma membranes are highly assymetric, as protein and lipid composition on intracelullar and extracellular faces of membranes varies^[2].

Lipids are said to be fluid within the system.This fluidity is dependent on:

• the length of the phospholipid tail
• whether the tail is saturated or not
• the presence of cholesterol.
  

References

1. ↑ Alberts B., Johnson A., Lewis J., Raff M., Roberts K., Walkter P., (2008) Molecular Biology Of The Cell, 5th edition, Newy York: Garland Science
2. ↑ Voet D., Voet J. (2011): Biochemistry, 4th Edition, Hoboken: John Wiley and Sons, Inc. p410