Final answer:
The 'fluid mosaic model' depicts the cell membrane as a flexible structure composed of various proteins and lipids that can move within its layer, providing necessary fluidity for cellular functions.
Step-by-step explanation:
The reason the cell membrane is referred to as the 'fluid mosaic model' is because this model describes the structure of the plasma membrane as a mosaic of components such as phospholipids, cholesterol, glycoproteins, and glycolipids that are able to flow and change positions within the membrane. This fluidity is necessary for the functioning of various enzymes and transport molecules. Membrane proteins and lipids are not static; they can move around, much like buoys on water, creating a dynamic 'mosaic' pattern that is essential for the membrane's function.