Final answer:
The principal fibrous protein in the cortex of red blood cells is spectrin. It forms a critical part of the cytoskeleton that determines the mechanical properties and durability of red blood cells.
Step-by-step explanation:
The principal fibrous protein in the cortex of the red blood cell is spectrin. Red blood cells have a characteristic biconcave shape, providing a large surface-to-volume ratio conducive to gas exchange. The spectrin protein plays a crucial role in maintaining this shape and providing the structural support necessary for the elasticity and stability of the red blood cell membrane.
While other proteins such as actin, tubulin, and various attachment proteins are also important in a cell's structure and function, it is spectrin that forms the lattice-like network on the cytoplasmic side of the red blood cell plasma membrane. This spectrin network, along with other proteins like ankyrin, form the cytoskeleton of red blood cells, which determines their mechanical properties and ensures their durability as they travel through the circulatory system.