A cell is the base unit for life, containing the genetic information of the organism to which it contributes. The cell contains organelles (structures within the cell that carry out specific functions, much like organs within a body), cytoplasm, and a diverse assortment of proteins. What allows the cell to retain its contents and keep itself protected is perhaps the most key portion of the cell-the cell membrane. The cell membrane performs a multitude of life-supporting functions, from protection and separation to diffusion and osmosis.
One of the most emphasized rules for life support and maintenance is to keep the external environment separate from the internal environment. Where humans possess skin to perform this function, the cell has its membrane. The membrane is composed of two layers of phospholipids (contains phosphate as well as glycerol). Each layer of the phospholipid bi-layer contains two opposite ends, one being hydrophilic (attracted to water) and the other being hydrophobic (repelled by water). The hydrophobic portion of both layers of phospholipids move toward each other, leaving the hydrophilic portion of the outer layer exposed to the fluid external environment (the hydrophilic portion of the other layer faces the cytoplasm). This construction gives the cell its structural integrity.
The organization of phospholipids in a cell membrane is crucial to the various operations which must be carried out by the cell (function is dependent upon structure, after all). The membrane is pliable, but not easily invaded. The cell membrane exhibits the characteristic called selective permeability, that is to say that the only external objects able to permeate the membrane must be of a certain size or electrical charge (usually the neutral, smaller molecules gain access). For instance, water molecules are easily able to diffuse across the membrane in the process of Osmosis. The solution with high concentration of water (hypertonic) moves into the area of low concentration (hypotonic) via facilitation from a potential gradient. Similar processes occur with other molecules, and are vital to the survival of the cell and the organism.
Without the cell membrane it is highly unlikely that cells, and therefore complex organisms, could survive. The formation and maintenence of such a complex structure as the cell membrane, with its channels and performing proteins, is truly a natural miracle. The processes that take place on, in, and around the structure can only be viewed with wonder. The cell membrane is the perfect example of the reliance of function upon structure, and of the smallest pieces working together for the benefit of the whole.