Protein charged molecules cannot diffuse. Membrane proteins contain internal channels that allow such molecules to enter and exit the cell. Many ion channel proteins are specialized to select for only a particular ion; for example, potassium and sodium channels often discriminate for only one of the two ions. Structural proteins confer stiffness and rigidity to otherwise-fluid biological components. Most structural proteins are fibrous proteins; for example, collagen and elastin are critical components of connective tissue such as cartilage, and keratin is found in hard or filamentous structures such as hair, nails, feathers, hooves, and some animal shells. Some globular proteins can
Keratin used as fluorescent markers to distinguish subsets of TECs in genetic studies of the thymus. Additionally, the baleen plates of filter-feeding whales are made of keratin. Keratins (also described as cytokeratins) are polymers of type I and type II intermediate filaments, which have only been found in the genomes of chordates (vertebrates, Amphioxus, urochordates). Nematodes and many other non-chordate animals seem to only have type VI intermediate filaments, lamins, which have a long rod domain (vs. a short rod domain for the keratins). The human genome encodes 54 functional keratin genes which are located in two clusters on chromosomes 12