Final answer:
The plasma membrane is described as a 'fluid mosaic' because it is a dynamic structure with various molecules in constant motion. The components of the membrane can move fluidly within the plane of the membrane, allowing for various functions.
Step-by-step explanation:
The plasma membrane is described as a 'fluid mosaic' because it is a dynamic structure with various molecules in constant motion. The components of the plasma membrane, including phospholipids, cholesterol, proteins, and carbohydrates, can move fluidly within the plane of the membrane, giving it a fluid character. This movement allows for various functions of the membrane, such as the diffusion of molecules and the activities of enzymes and transport molecules.