Fibrous proteins are not typically found in cell membranes. Cell membranes are primarily composed of phospholipids, which form a bilayer structure, and proteins that are embedded within or associated with this bilayer.

Here's why fibrous proteins aren't commonly found in membranes:

* Structure: Fibrous proteins, like collagen and keratin, are elongated and often form long, rigid structures. This shape is not well-suited for the fluid and dynamic nature of cell membranes.

* Function: Fibrous proteins primarily function in providing structural support and protection within tissues and organs. They are important for things like skin, bone, and connective tissues.

* Membrane Proteins: The proteins embedded in cell membranes are mostly globular proteins, which are more compact and have diverse shapes and functions. These proteins are involved in transport, signaling, and cell-cell communication.

However, there are some exceptions:

* Peripheral membrane proteins: Some fibrous proteins can associate with the membrane surface, often interacting with other membrane proteins. These interactions help in anchoring the membrane to the cytoskeleton or other cellular structures.

* Extracellular matrix: Fibrous proteins like collagen and elastin are found in the extracellular matrix, which surrounds cells and provides structural support and organization to tissues.

Therefore, while fibrous proteins don't directly form part of the cell membrane itself, they play important roles in supporting and connecting the membrane with other cellular components and the surrounding environment.